Last updated: April 1, 2026 — all product specs and prices verified today
Pool shock is the most misused product in pool chemistry. It gets blamed for problems it did not cause, credited for fixes it did not make, and added to pools at the wrong time, in the wrong concentration, at the wrong pH. This guide covers every type of shock, when each one is the right call, the exact math for dosing, and the mistakes that turn a $15 bag of chemicals into a wasted afternoon.
QUICK ANSWERS
Q: What is pool shock and what does it actually do?
A: Pool shock rapidly raises free chlorine to a level high enough to destroy chloramines (the byproducts of normal chlorine use), kill algae that has established in the water, and sanitize after a heavy bather load or storm. Regular chlorine tablets maintain 1-4 ppm for prevention. Shock raises free chlorine to 10-30 ppm for a short period to handle problems maintenance chlorine cannot address.
Q: When should I shock my pool?
A: Shock after heavy bather load or a pool party, after a rainstorm, when free chlorine reads zero, when combined chlorine (chloramines) reads above 0.5 ppm per PHTA guidelines, when algae is visible, when water is hazy despite normal chemistry readings, at opening, and at closing. For heavily used pools in summer, weekly shocking is not excessive.
Q: How much shock do I need per 10,000 gallons?
A: For routine maintenance shocking with no visible algae: 1 lb of cal-hypo (68%) per 10,000 gallons raises free chlorine by approximately 5-7 ppm. For green water with visible algae: 3-5 lbs per 10,000 gallons. For severe neglect with dark green or black-green water: 6+ lbs per 10,000 gallons plus follow-up algaecide.
Q: Should I shock my pool during the day or at night?
A: Always shock after sunset. Cal-hypo shock contains no cyanuric acid stabilizer. UV radiation from direct sunlight destroys unstabilized free chlorine at a rate of up to 90% within 2 hours. Shocking at 9 AM in full summer sun means the shock has largely stopped working by 11 AM. Shock after sunset, run the pump overnight for minimum 8 hours.
Q: What is the difference between cal-hypo, dichlor, and non-chlorine shock?
A: Cal-hypo (calcium hypochlorite) is the standard for most shocking situations — highest available chlorine per dollar, no CYA added. Dichlor (sodium dichloro) dissolves completely with no residue, best for vinyl liner pools — but adds CYA. Non-chlorine shock (potassium monopersulfate) is an oxidizer not a sanitizer — it cannot kill algae or bacteria but allows swimming 15 minutes after adding. Use the right tool for the job.
Q: How long after shocking can I swim?
A: Wait until free chlorine drops back to 1-4 ppm before swimming. Per PHTA guidelines, bathers should not enter until the tested chlorine level is at 4.0 ppm or less. At opening shock doses of 10-30 ppm, most pools are safe to swim in 24-48 hours. Test before swimming — do not rely on time alone.
Q: Why does my pool stay green after shocking?
A: Three causes in order of likelihood: (1) CYA above 80 ppm — chlorine is present but substantially locked up and ineffective. Test CYA first. If elevated, partial drain and refill before re-shocking. (2) pH too high when you shocked — less effective chlorine at pH 8.0. Lower pH to 7.2-7.4 then re-shock. (3) Shock dose was too low for the algae load — brush the pool first, then re-shock at 3-5x the standard dose.
Key Terms Defined
Pool Shock: A concentrated dose of chlorine or oxidizer used to rapidly raise free chlorine above the normal maintenance range. The goal is to reach a level high enough to destroy chloramines, kill algae, or sanitize after contamination. Standard shock doses target 10-30 ppm free chlorine, well above the 1-4 ppm maintenance range.
Cal-Hypo (Calcium Hypochlorite): The standard pool shock. A 68% calcium hypochlorite formulation yields 65% available chlorine. A 73% formulation yields 70% available chlorine. Adds no cyanuric acid. Must be pre-dissolved before adding to the pool.
Breakpoint Chlorination: Raising free chlorine high enough to completely and permanently destroy all chloramines and organic contaminants in a single treatment. The standard is to reach a free chlorine level 10 times the combined chlorine reading. Half-dosing leaves behind resistant chloramines and algae.
Combined Chlorine (Chloramines): The chemical byproducts of chlorine reacting with ammonia, nitrogen compounds, and organic waste from bathers. Chloramines cause the ‘strong chlorine smell’ at pools, eye and skin irritation, and reduced sanitizing effectiveness. The PHTA recommends combined chlorine stay below 0.5 ppm.
Free Chlorine (FC): The active, unbound chlorine available to sanitize pool water. Per PHTA guidelines, the target range for residential pools is 1.0-4.0 ppm for maintenance and 4.0 ppm maximum for safe swimming reentry.
Potassium Monopersulfate (MPS): The active ingredient in non-chlorine shock. An oxidizer that burns off organic compounds and chloramines without adding any chlorine. Does not kill algae or bacteria. Safe to swim within 15-30 minutes of adding.
The Four Types of Pool Shock: What Each One Actually Does
These four products are not interchangeable. Each has a specific application where it outperforms the others. Using the wrong type wastes money and often fails to solve the actual problem.
| Type | Active Ingredient | Available Chlorine | Adds CYA? | Best Application |
| Cal-Hypo (68%) | Calcium hypochlorite (68%) | 65% available chlorine | No | General shocking, algae treatment, opening, closing |
| Cal-Hypo (73%) | Calcium hypochlorite (73%) | 70% available chlorine | No | Heavy algae, commercial pools, maximum chlorine boost needed |
| Di-Chlor | Sodium dichloro-s-triazinetrione | 56-62% available chlorine | Yes (~6-7 ppm per lb per 10k gal — same rate as trichlor) | Vinyl liner pools, spas, situations requiring complete dissolution |
| Non-Chlorine Shock (MPS) | Potassium monopersulfate | Not a chlorine product — oxidizer only | No | Pre-swim oxidation, biguanide pools, swim-ready oxidizing treatment |
| Liquid Chlorine | Sodium hypochlorite (10-12%) | 10-12% available chlorine | No | Saltwater pools, routine chlorine boosting, vinyl liner safe |
Source: PHTA Cal-Hypo Fact Sheet — 68% cal-hypo formulation, available chlorine standards
Source: PHTA Liquid Chlorine Fact Sheet — sodium hypochlorite characteristics
Cal-Hypo (Calcium Hypochlorite) — The Standard
Cal-hypo at 68% formulation delivers 65% available chlorine — more sanitizing power per dollar than any other shock type. It adds no CYA, which matters in summer when CYA already climbs from tablet use. The 73% formulation delivers 70% available chlorine and dissolves faster.
Cal-hypo is the right call for: opening shock, green pool treatment, post-storm treatment, weekly maintenance shocking, and closing shock.
⚠ WARNING: NEVER add cal-hypo directly to the pool in powder form. Pre-dissolve each bag in a 5-gallon bucket of pool water first. Undissolved cal-hypo bleaches vinyl liners on contact, stains plaster, and creates white residue that can take days to filter out. This is a product label requirement, not optional advice.
Di-Chlor (Sodium Dichloro) — For Vinyl and Spas
Di-chlor dissolves completely with no residue. It has a near-neutral pH of approximately 6.5-7.0, meaning it causes less pH swing than cal-hypo when added. It is the preferred shock for vinyl liner pools and spas where complete dissolution before the chemical contacts a surface is important.
The limitation: every pound of di-chlor adds approximately 6-7 ppm of CYA to a 10,000-gallon pool — essentially the same rate as trichlor. Per Taylor Technologies and Orenda Technologies, both trichlor and dichlor raise CYA by 6-7 ppm per lb per 10,000 gallons. Use situationally, not as your primary season-long shock. A pool owner shocking weekly with di-chlor will hit the PHTA CYA maximum of 100 ppm within a few months.
Non-Chlorine Shock (Potassium Monopersulfate) — For Oxidation Only
Non-chlorine shock is an oxidizer, not a sanitizer. It cannot kill bacteria or algae. What it does is oxidize organic compounds — swimmer waste, sunscreen, body oils, ammonia — that consume chlorine and create chloramines.
The main advantage: you can swim 15-30 minutes after adding it. The correct use case is pre-swim oxidation before a pool party — reduce the organic load in advance so your regular chlorine can focus on sanitizing during heavy use.
⚠ WARNING: Non-chlorine shock will NOT clear a green pool. It will NOT kill algae. It will NOT substitute for chlorine-based shock when bacteria or algae are present. If your pool is green, cloudy, or has a combined chlorine reading above 0.5 ppm, use cal-hypo shock.
Liquid Chlorine (Sodium Hypochlorite) — For Saltwater and Vinyl
At 10-12% available chlorine, liquid chlorine requires significantly more volume than cal-hypo to achieve the same free chlorine boost. The advantages: pre-dissolved and immediately active, adds no calcium or CYA, and is gentle on liners and pool surfaces. It is the preferred shocking agent for saltwater pools and is safe for all pool surface types per PHTA documentation.
Storage limitation: liquid chlorine loses strength over time, especially in warm conditions. The PHTA Liquid Chlorine Fact Sheet notes that strength declines based on temperature, UV exposure, and initial concentration. Buy what you will use within 60-90 days.
Why You Must Shock at Night: The UV Math
Cal-hypo shock contains no cyanuric acid stabilizer. In direct summer sunlight, UV radiation destroys unstabilized free chlorine rapidly. Multiple sources including the PHTA and pool chemistry researchers confirm up to 90% of unstabilized chlorine can be lost within 2 hours of direct UV exposure.
| Scenario | Chlorine Remaining After 2 Hours of Direct Sun |
| Unstabilized chlorine (cal-hypo shock), no CYA in pool | Approximately 10% — 90% destroyed by UV |
| Stabilized chlorine (trichlor tablets), CYA at 30-50 ppm | Much higher retention — CYA provides UV protection |
| Shock added after sunset, pump run overnight | Full effectiveness maintained through overnight treatment window |
✓ KEY FACT: Shock after sunset. Run the pump overnight for a minimum of 8 hours. A pool shocked at 9 AM in full summer sun has largely stopped working by 11 AM. The overnight window is when shock does its actual work.
Source: UV destruction of unstabilized chlorine — Engineer Fix citing chemistry research
How Much Shock Do You Need: Dosing by Pool Size and Condition
Routine maintenance shocking (no visible algae):
| Pool Size (gallons) | Cal-Hypo Dose (68%) | Expected Free Chlorine Boost | Target After Overnight |
| 10,000 | 1 lb | ~5-7 ppm boost | 1-4 ppm (safe to swim after test confirms) |
| 15,000 | 1.5 lbs | ~5-7 ppm boost | 1-4 ppm |
| 20,000 | 2 lbs | ~5-7 ppm boost | 1-4 ppm |
| 25,000 | 2.5 lbs | ~5-7 ppm boost | 1-4 ppm |
| 30,000 | 3 lbs | ~5-7 ppm boost | 1-4 ppm |
Dosing Formula: Routine Shocking
Standard dose: 1 lb cal-hypo (68%) per 10,000 gallons
Example: 22,000-gallon inground pool, routine weekly shock
Calculation: 22,000 / 10,000 = 2.2 lbs
Round up: Use 2.5 lbs for a complete treatment
Pre-dissolve each lb separately in a 5-gallon bucket.
Add after sunset. Run pump overnight minimum 8 hours.
Green pool shocking (algae treatment):
✓ KEY FACT: Brush the pool walls, floor, and steps before adding shock. Algae attached to surfaces is not fully exposed to the water. Brushing breaks it loose and exposes it to the chlorine. Skipping this step means you may clear the water but leave algae on surfaces that regrows within days.
| Pool Condition | Dose per 10,000 Gallons | Total for 20,000-Gal Pool | Expected Timeline to Clear |
| Slightly hazy, faint green tint | 2 lbs cal-hypo (68%) | 4 lbs | 1-2 overnight treatment cycles |
| Clearly green, visible algae on surfaces | 3-4 lbs cal-hypo (68%) | 6-8 lbs | 2-3 overnight cycles |
| Dark green, heavy algae on walls and floor | 5-6 lbs cal-hypo (68%) | 10-12 lbs | 3-5 overnight cycles |
| Black-green, severe neglect, pool fully green | 6+ lbs cal-hypo + algaecide next morning | 12+ lbs | 5-7+ overnight cycles, filter backwashing daily |
Worked Example: Treating a 20,000-Gallon Green Pool
Pool volume: 20,000 gallons
Condition: Clearly green, visible algae on walls and steps
Dose needed: 3 lbs cal-hypo (68%) per 10,000 gallons
Calculation: (20,000 / 10,000) x 3 = 6 lbs total
Step 1: Brush all walls, floor, and steps first.
Step 2: Pre-dissolve 2 lbs in a 5-gallon bucket of pool water.
Step 3: Walk the perimeter, broadcasting dissolved solution evenly.
Step 4: Repeat with remaining 4 lbs in two more buckets.
Step 5: Run pump continuously — do not turn it off.
Step 6: Backwash or clean filter every 24 hours.
Step 7: Retest free chlorine in the morning.
If reading is zero: re-shock at the same dose.
If reading is above 1 ppm: continue filtering.
Step 8: Add polyquat 60 algaecide once FC drops below 4 ppm.
Bottom Line: Half-treating a green pool is worse than not treating it at all. Surviving algae is exposed to chlorine stress and can develop resistance. If you are going to shock a green pool, use the full dose for the condition you see — not the minimum.
pH Before You Shock: The Step Everyone Skips
In pools without cyanuric acid, the percentage of chlorine in its active form (hypochlorous acid) drops dramatically as pH rises. At pH 8.0 in an unstabilized pool, approximately 20% of your chlorine is in its active form. At pH 7.2, approximately 63% is active.
✓ KEY FACT: Test and adjust pH before every shock treatment. Target pH 7.2-7.4 for maximum shock effectiveness. A pool that drifted to pH 8.0+ over winter needs pH correction before opening shock — otherwise most of your chemical investment is wasted.
| pH at Time of Shocking | Approx. Active Chlorine % (unstabilized) | Effect on Shock Dose |
| 7.0-7.2 | 63-73% | Maximum effectiveness — ideal range for shocking |
| 7.4 | ~48% | Good — slight reduction in effectiveness |
| 7.6 | ~36% | Marginal — shock dose should be increased slightly |
| 7.8 | ~27% | Poor — a significant portion of your shock is wasted |
| 8.0+ | ~20% or less | Ineffective — correct pH first before adding any shock |
⚠ IMPORTANT: These percentages apply to unstabilized pools — pools without cyanuric acid. In residential pools using trichlor tablets, CYA is always present. In stabilized pools, the FC:CYA ratio governs chlorine effectiveness more than pH does. However, maintaining pH in the 7.2-7.8 range is still recommended by the PHTA for equipment protection, LSI balance, and swimmer comfort.
Source: PHTA Liquid Chlorine Fact Sheet — pH 7.5 = 50% HOCl in unstabilized pools
Source: Pool Help (Pickens 2017) — CYA effect on pH/chlorine relationship in stabilized pools
How to Read Your Shock Results the Morning After
| Free Chlorine Reading Next Morning | What It Means | What to Do |
| 0 ppm | Extremely high chlorine demand consumed everything — heavy algae load, high CYA, or severely contaminated water | Test CYA before re-shocking. If CYA is above 80 ppm, partial drain first. Then re-shock at full dose after brushing. |
| 1-4 ppm | Normal drop — chlorine did its work overnight. Pool is likely clearing. | Allow to continue dropping. Test again in 4-6 hours. Swim once below 4 ppm. |
| 5-9 ppm | Chlorine demand was lower than expected — light algae load or pool was relatively clean | Continue filtering. Do not swim yet. Test again in 4-6 hours. |
| 10+ ppm | Chlorine dropped very little — unusual. Could indicate test kit error or very light demand. | Retest with a fresh test kit. If confirmed, continue filtering. Allow to drop before swimming. |
Bottom Line: If free chlorine reads zero the morning after shocking at the correct dose, the problem is almost always high CYA above 80 ppm consuming the effectiveness of the shock before it could work. Test CYA before adding more chemicals.
The Mistakes That Make Shock Ineffective
Shocking at high pH
The single most common and most costly shock mistake. At pH 8.0 in an unstabilized pool, only about 20% of your chlorine is in its active form. Pool owners add shock, see no results, add more shock, still no results — all because they skipped the pH adjustment step before shocking. Correct pH to 7.2-7.4 before every shock treatment.
Not brushing before shocking a green pool
Algae attached to pool surfaces is not fully exposed to the water column where your chlorine is active. Brushing breaks the algae loose and suspends it in the water where the shock can reach it. Skipping this step means you may clear the water but leave algae on walls, steps, and the floor that regrows within days.
Adding shock directly to the skimmer
Concentrated cal-hypo flowing through plastic fittings, pump components, and filter media undiluted can damage these components and create dangerous concentrated chlorine deposits. Pre-dissolve shock in a bucket, then broadcast the dissolved solution around the pool perimeter while walking.
Shocking a cloudy pool without diagnosing why it is cloudy
Shock treats algae and oxidizes chloramines. It does not treat cloudiness caused by high calcium hardness, poor filtration, or fine particles in suspension after an algae kill. Adding shock to a cloudy pool without a correct diagnosis wastes the product and may not address the actual cause. Test all parameters before deciding to shock.
Using old or degraded shock
Cal-hypo stored improperly — in a hot garage, open to air, or exposed to moisture — degrades over time and loses available chlorine. If you are adding the recommended dose and seeing no free chlorine rise on your test kit, the product may be degraded. Buy fresh shock and compare results before assuming the pool chemistry is the problem.
Adding algaecide before chlorine drops below 4 ppm
Quat and polyquat algaecides are chemically deactivated by high free chlorine levels. Adding algaecide immediately after shocking — when free chlorine is at 10-30 ppm — means the algaecide is destroyed before it reaches any algae. Wait until free chlorine drops to 4 ppm or below, then add algaecide per label directions.
How Often to Shock: The Schedule That Actually Makes Sense
Shock when any of these conditions exist — not on a fixed calendar schedule alone:
- After a pool party or any unusually heavy bather load
- After a heavy rainstorm — runoff brings organic matter, phosphates, and algae spores
- When free chlorine tests at zero or below 1 ppm
- When combined chlorine reads above 0.5 ppm per PHTA guidelines
- When algae is visible anywhere in the pool
- When water is hazy despite chemistry readings appearing correct — likely elevated combined chlorine
- At pool opening
- At pool closing
For heavily used residential pools in summer, weekly shocking with 1 lb per 10,000 gallons of cal-hypo is standard practice and not excessive. For pools with light use, bi-weekly shocking or shocking after heavy use events is typically adequate.
Source: PHTA Cal-Hypo Fact Sheet — combined chlorine threshold and shock timing
If X Happens, Do Y: Troubleshooting Shock Problems
Why does my pool still smell like chlorine even after shocking?
The ‘strong chlorine smell’ at pools is not from free chlorine — it is from chloramines (combined chlorine), the byproducts of chlorine reacting with bather waste, ammonia, and organic matter. High chloramine levels indicate insufficient oxidation, not too much chlorine.
If the smell persists after shocking, combined chlorine is still elevated. Test total and free chlorine. The difference is combined chlorine. If combined chlorine is above 0.5 ppm, shock again at the same dose or higher. Non-chlorine shock (potassium monopersulfate) can also help oxidize the organic load causing chloramine formation.
Why did my water turn cloudy after shocking?
Temporary cloudiness after cal-hypo shocking is normal and typically clears in 12-24 hours as the filter removes killed algae, dead organic matter, and fine particles. If cloudiness persists beyond 24 hours:
- Check filter pressure — backwash or clean the cartridge if pressure is elevated above the clean baseline
- Test pH — cal-hypo raises pH. If pH is above 7.8, add pH decreaser
- Test calcium hardness — cal-hypo adds calcium. If hardness is above 400 ppm, the cloudiness may be calcium precipitation
- Add a clarifier — helps aggregate fine particles so the filter can catch them faster
Can I shock too much?
Yes, but the consequences are temporary. Extremely high free chlorine — above 20 ppm — can bleach pool surfaces, degrade equipment seals over extended exposure, and irritate anyone who contacts the water. The practical fix is simply time: run the pump and allow UV and normal chlorine demand to bring levels down. Test before swimming and wait until free chlorine is at or below 4 ppm per PHTA guidelines.
Why is my free chlorine zero the morning after shocking?
High chlorine demand consumed the entire shock dose. This happens for one of three reasons:
- CYA above 80 ppm — the shock was substantially less effective due to CYA interference. Test CYA first. If elevated, partial drain and refill before re-shocking.
- Extremely heavy algae or organic load — the pool needed more shock than you added. Re-shock at a higher dose after brushing all surfaces.
- Shock added during the day — UV destroyed most of it before it could work. Re-shock after sunset.
Does shock expire?
Cal-hypo shock in sealed, properly stored bags can maintain potency for multiple years. The degradation rate increases with heat, humidity, and air exposure. A bag stored in a hot garage over summer may lose 20-30% of its stated available chlorine. If you add the full recommended dose and see minimal free chlorine rise, test with a fresh bag from a new case to isolate whether the product or the pool chemistry is the issue.
10 Buyer Questions Answered
Is liquid chlorine better than granular shock for opening a pool?
For most residential pools, granular cal-hypo at 68% is the better opening choice. It delivers more available chlorine per dollar than liquid chlorine and a single 1-lb bag is easier to store and transport than gallons of liquid. Liquid chlorine is preferable for saltwater pools, vinyl liner pools where you want zero risk of granule contact with the liner, and pools where adding calcium from cal-hypo is a concern due to already-high calcium hardness.
Can I use non-chlorine shock to open my pool?
No. Non-chlorine shock (potassium monopersulfate) is an oxidizer, not a sanitizer. It cannot kill algae, bacteria, or other pathogens that may have established in a closed pool over winter. Opening always requires a chlorine-based shock. Cal-hypo at 68% is the standard recommendation for opening.
What is the difference between pool shock and chlorine tablets?
Tablets maintain a steady baseline of 1-4 ppm free chlorine for ongoing prevention. Shock raises free chlorine to 10-30 ppm for a short period to destroy problems — chloramines, algae, and contamination — that maintenance levels cannot address. Tablets dissolve slowly over days. Shock acts within hours. You need both: tablets for prevention, shock for treatment.
How many bags of shock do I need for a green pool?
For a clearly green 20,000-gallon pool with visible algae: approximately 6-8 lbs of cal-hypo (68%), applied after brushing. For dark green water with heavy algae: 10-12 lbs for the same size pool. Buy more than you think you need — a second treatment is common, and having product on hand avoids a mid-treatment trip to the store.
Can I shock a pool while people are swimming?
No. Shock immediately raises free chlorine to levels well above the safe swimming range of 1-4 ppm. Keep all people and pets out of the water until free chlorine drops back to 4 ppm or below and you have confirmed this with a test. Do not estimate based on time — test.
Does rain dilute the pool shock I just added?
Light rain has minimal dilution effect on pool chemistry. Heavy sustained rainfall has two effects: it dilutes pool chemistry and introduces organic matter, phosphates, and potential algae spores. After any significant rainstorm, test free chlorine and combined chlorine. If combined chlorine is above 0.5 ppm or free chlorine is below 1 ppm, shock.
Why does my shock have a different color from last season?
Cal-hypo shock should be white to off-white. Any significant yellowing, caking, or strong ammonia-type odor indicates chemical degradation or moisture contamination. Yellowed or caked shock may still function but at reduced potency. Discard any shock that has been exposed to moisture — wet cal-hypo can generate heat and is a fire hazard.
⚠ WARNING: Never mix cal-hypo shock with trichlor tablets, any other chlorine product, acid, or any other pool chemical directly. The combination of cal-hypo and trichlor is a documented fire and explosion hazard per PHTA safety guidelines.
Source: PHTA/APSP Trichlor Fact Sheet — chemical incompatibility with other chlorinating agents
Is it worth buying shock in bulk?
Yes. The price-per-pound drops significantly with case quantities. A 24-bag case of 1-lb cal-hypo bags from a direct pool supply source typically costs $75-110 — about $3.10-4.60 per lb. Individual bags at a hardware store typically cost $6-9 each. For a pool owner shocking weekly through a 20-week season, a case purchased at opening saves $60-90 versus retail buying.
Can I use bleach (household sodium hypochlorite) instead of pool shock?
Technically yes, but it is impractical for shocking. Household bleach is 1.5-6% sodium hypochlorite. Pool-grade liquid chlorine is 10-12.5% sodium hypochlorite. To achieve the same free chlorine boost as 1 lb of cal-hypo shock in a 10,000-gallon pool, you would need approximately 2-3 gallons of household bleach — logistically and economically impractical. Use pool-grade shock products.
What is the shelf life of unopened pool shock bags?
Cal-hypo shock in sealed, properly stored bags maintains potency for multiple seasons. Key storage requirements: cool (below 95 degrees F), dry, away from direct sunlight, and segregated from all other pool chemicals, especially trichlor tablets. A bag left in a hot garage over summer will lose potency faster. Check the available chlorine percentage on the label against your test results — if you are dosing correctly and seeing no chlorine rise, the product may have degraded.
Where to Buy Pool Shock
Doheny’s carries cal-hypo shock at 68% in single 1-lb bags, 6-packs, and 24-bag cases. Family owned since 1967. Free shipping on orders over $50. Next-day delivery available on Doheny’s brand chemicals to most of the continental US. Buying a full case at opening drops the per-pound cost to roughly half of what individual retail bags cost. Shop Doheny’s pool shock →
See our Where to Buy page for direct links and current April 2026 pricing on all shock types including cal-hypo cases, di-chlor, and non-chlorine shock.
External References
PHTA Cal-Hypo Fact Sheet 2021 — available chlorine standards, FAC range 1.0-4.0 ppm, safe swimming reentry: https://www.phta.org/pub/?id=07fd3498-1866-daac-99fb-8824a8f3147b
PHTA Liquid Chlorine Fact Sheet — sodium hypochlorite, UV degradation, all pool surface types: https://www.phta.org/pub/?id=0905861a-1866-daac-99fb-b239bf43994b
PHTA Water Balance Fact Sheet — CYA standards, combined chlorine threshold: https://www.phta.org/pub/?id=50ffe77d-1866-daac-99fb-9719108d1367
PHTA/APSP Trichlor Fact Sheet — chemical incompatibility between shock types: https://www.phta.org/pub/?id=09389576-1866-daac-99fb-f58144f4f5df
Pool Help (Pickens 2017) — pH and chlorine efficacy in stabilized vs. unstabilized pools: https://www.poolhelp.com/home/onbalance-research/onbalance-research/the-true-science-about-ph-chlorine-efficacy/
Engineer Fix — unstabilized chlorine UV degradation rate: https://engineerfix.com/do-chlorine-tablets-have-cyanuric-acid/
US Poison Control Center: 1-800-222-1222 — https://www.poison.org