Pool shock calculator
“One bag per 10,000 gallons” ignores your stabilizer. Enter your test and why you’re shocking, and get the chlorine level to hit and the amount of product to reach it.
Raise free chlorine to
Product needed
| Product | Amount | Side effect |
|---|---|---|
| Liquid chlorine 12.5% | 1.26 gal | none |
| Liquid chlorine 10% | 1.58 gal | none |
| Household bleach 8.25% | 1.79 gal | none |
| Household bleach 6% | 2.55 gal | none |
| Cal-hypo 73% | 28.8 oz | +7.4 ppm calcium |
| Cal-hypo 65% | 2 lb | +7.4 ppm calcium |
| Dichlor 56% | 2.3 lb | +9.5 ppm CYA |
| Lithium hypochlorite 35% | 3.8 lb | none |
Trichlor tabs are left out: they dissolve too slowly to shock and would add a lot of stabilizer. Dichlor adds about 0.9 ppm CYA per ppm FC, so avoid it for repeated shocking.
Chemical safety: read the product label first and follow it where it differs from this page. Add acid to water, never water to acid. Never mix chlorine products with acid or with each other, not even in a bucket. Add one chemical at a time with the pump running, wait at least 30 minutes (acid and chlorine: until the next test), and retest before topping up.
How to shock a pool until it’s actually done
- Get pH to 7.2–7.5 first. Chlorine works faster at lower pH.
- At dusk, add the amount above with the pump running. Brush walls and floor.
- Test every few hours while awake. Whenever free chlorine drops, top it back up to the shock level.
- Keep the filter running and clean it when pressure climbs 8–10 psi above clean.
- You’re finished when the water is clear, combined chlorine is 0.5 ppm or less, and free chlorine drops no more than 1 ppm overnight (test at dusk and again before sunrise).
Shock levels by stabilizer
| CYA | Shock FC | Mustard algae FC |
|---|---|---|
| 0 | 10 | 12 |
| 20 | 10 | 12 |
| 30 | 12 | 18 |
| 40 | 16 | 24 |
| 50 | 20 | 30 |
| 60 | 24 | 36 |
| 70 | 28 | 42 |
| 80 | 32 | 48 |
Above about 80 ppm CYA, shock levels become impractical. Partial draining is cheaper than the chlorine you’d burn.
Questions pool owners ask
How much shock do I need for my pool?
Shock means raising free chlorine to a level high enough to clear algae or chloramines, which depends on your stabilizer. A common guide is about 40% of your CYA (so 16 ppm at 40 CYA), with at least 10 ppm. For clearing chloramines, CDC’s breakpoint guidance is about 10 × your combined chlorine. This calculator uses the higher of the two.
How many bags of shock per 10,000 gallons?
A 1 lb bag of 73% cal-hypo raises free chlorine about 11–12 ppm in 10,000 gallons (65% cal-hypo about 10 ppm). Labels that say “1 bag per 10,000 gallons” assume little or no stabilizer; with CYA at 50 ppm or more you may need two or three.
Should I shock at night?
Yes. Sunlight destroys unstabilized chlorine quickly, so adding shock around dusk lets it work overnight. Run the pump for at least 8 hours afterwards and brush the walls.
How long after shocking can I swim?
Wait until free chlorine drops back to your normal target, and never swim above 10 ppm, the level at which CDC advises staying out. After a heavy shock this can take a day or more.
Why does my pool stay green after shocking?
Usually the chlorine level wasn’t held high enough for long enough. Algae consumes chlorine, so you need to keep testing and topping up to the shock level until the water is clear, combined chlorine is 0.5 ppm or below, and you lose no more than about 1 ppm of chlorine overnight.