Two reasons to shock, two ways to size it
1. Algae, cloudy water, or opening a pool. Here, the goal is to hold free chlorine high enough, for long enough, to kill everything growing. How high depends on stabilizer. CYA binds chlorine, so a pool with 70 ppm CYA needs far more free chlorine than one with 30 ppm to get the same killing power. The calculator uses a shock level of about 40% of CYA, with a floor of 10 ppm.
2. Chloramines (combined chlorine). A strong “chlorine smell” or red eyes usually means combined chlorine, not too much chlorine. Combined chlorine is total chlorine minus free chlorine. The traditional breakpoint rule is to add free chlorine equal to about 10 times the combined chlorine.
| CYA (ppm) | Shock level (ppm FC) |
|---|---|
| 0–25 | 10 |
| 30 | 12 |
| 40 | 16 |
| 50 | 20 |
| 60 | 24 |
| 70 | 28 |
| 80 | 32 |
Shocking is a process, not a single dose
Algae and organic debris keep using up chlorine, so one dose usually isn’t enough. The approach the calculator assumes:
- Raise free chlorine to the shock level, preferably in the evening.
- Brush the pool and keep the pump running.
- Retest every few hours and top up to the shock level.
- Stop when the water is clear, combined chlorine is 0.5 ppm or less, and free chlorine drops by no more than 1 ppm overnight.
When CYA is too high
At 80+ ppm CYA, the shock level runs to 30+ ppm. That’s expensive, and it’s hard to measure because high chlorine bleaches most test reagents. Replacing part of the water first is often cheaper and faster. The CYA calculator shows how much to replace.
Assumptions and limitations
- The shock level of about 40% of CYA is homeowner guidance from Trouble Free Pool; the 10 × combined chlorine breakpoint rule is a long-standing rule of thumb.
- The result is the dose to reach the shock level once. Algae consumes chlorine, so expect to retest and top up repeatedly.
- Very high chlorine bleaches many test reagents. A FAS-DPD test gives the most reliable readings during a shock.
- A fecal accident needs CDC's specific response protocol, not this calculator.
Key terms
Common questions
How many bags of shock do I need?
It depends on your CYA, not just pool size. One pound of 65% cal-hypo raises 10,000 gallons by about 7.8 ppm. At 40 ppm CYA the shock level is 16 ppm, so a 20,000-gallon pool starting at 1 ppm needs about 3.9 lb. Bag sizes vary, so the calculator gives weight.
Is "shock" different from chlorine?
Most products sold as shock are simply concentrated chlorine (usually cal-hypo or dichlor). Liquid chlorine works just as well for shocking and doesn't add calcium or stabilizer. "Non-chlorine shock" (potassium monopersulfate) oxidizes contaminants but does not kill algae.
Should I shock once a week?
Not by default. If your free chlorine stays in range and combined chlorine stays at or below 0.5 ppm, routine shocking isn't necessary. Shock when you see algae, cloudy water, high combined chlorine, or after heavy use.
Sources and methodology references
- Pool School (chlorine/CYA chart and shocking method), Trouble Free PoolShock level of roughly 40% of CYA, held until three clear-water criteria are met.
- Home pool and hot tub water treatment and testing, US Centers for Disease Control and Prevention
Published October 2, 2026. Results are estimates. See how our calculators work.
Spotted a wrong result or an outdated source? Report a problem with this calculator.
Related tools
- Chlorine calculatorSee how much chlorine to add to reach the right free chlorine level for your pool's stabilizer, in ounces, gallons or pounds of the product you actually have.
- CYA (stabilizer) calculatorGet the stabilizer amount to raise CYA, or the share of water to replace to lower it, plus the free chlorine level your new CYA calls for.
- Pool opening plannerTurn your first test of the season into an ordered start-up checklist with balancing and chlorine doses filled in.
- Pool closing plannerAnswer a few questions and get a printable closing checklist with your shock and balancing doses already calculated.
Related guides
- How to Shock a Pool (and How Much Shock You Actually Need)How to shock a pool so it actually works. Find the right chlorine level for your stabilizer, choose a product, and hold the level until the job is done.Read the guide (4 min)
- Cyanuric Acid (Pool Stabilizer): How Much You Need and Why It MattersStabilizer stops sunlight from destroying chlorine, but it also raises the chlorine level your pool needs. How CYA works, good levels, and how to raise or lower it.Read the guide (3 min)
- How to Close an Inground Pool for WinterWhen to close, how much chlorine to add, how far to lower the water, and how to protect pipes and equipment from freezing, in the order the work actually happens.Read the guide (4 min)
- How to Lower (or Raise) Pool pH SafelyWhy pool pH drifts up, how to bring it down with muriatic or dry acid, how to raise it, and why your alkalinity decides how much acid you actually need.Read the guide (3 min)