Pool Shock Calculator
Calculate the exact amount of pool shock needed to reach breakpoint chlorination. Works for calcium hypochlorite, dichlor, and liquid shock.
Safety
Shock at sunset for best results — UV destroys chlorine. Do not swim until FC drops below 5 ppm (typically 8–24 hours). Pre-dissolve granular shock in a bucket of water before adding to the pool. Never mix different shock products.
Shock raises free chlorine to your breakpoint — CYA × 0.4, with a floor of 10 ppm. With cal-hypo 65% that is roughly 2 oz per 10,000 gallons for each 1 ppm of rise. Wait until FC falls back below 5 ppm before swimming, usually 8–24 hours. Enter your volume and CYA above for the exact dose.
Shocking Is Two Different Jobs
"Shock" is one word for two treatments that share a chemical and nothing else. One is breakpoint chlorination — a single overwhelming dose that destroys the combined chlorine responsible for the smell and the stinging eyes. The other is holding free chlorine at an elevated level for days to kill algae. They need different targets, and treating them as the same job is why a lot of shock gets wasted.
Both start from the same reading, though: you cannot dose either one without knowing your volume and your CYA. A pool at CYA 80 needs roughly twice the chlorine of a pool at CYA 40 to reach the same effective sanitizer strength — same water, same algae, different number.
Breakpoint: free chlorine at 10× combined
Combined chlorine (chloramines) is chlorine that has already reacted with sweat, urine and other organics and is no longer sanitizing. It does not decay quietly — it has to be broken apart, and that only happens once free chlorine reaches about ten times the combined chlorine reading. A pool reading CC 0.5 ppm needs FC 5 ppm to break it.
This is the one place in pool chemistry where a partial dose is worse than none. Stop halfway and you have produced more chloramine than you destroyed, the smell gets stronger, and the natural conclusion — that the pool needs more chlorine — is for once correct. Dose the whole amount at once.
Algae: the target is set by your CYA
Cyanuric acid protects chlorine from sunlight by binding most of it into a reserve. That reserve is not sanitizing while it is bound, so the more CYA in the water, the higher free chlorine has to sit for the same killing power. The working rule is a shock level of CYA × 0.4, with a floor of 10 ppm — the figure this calculator uses. Reach it, then hold it, testing and re-dosing until the water clears and chlorine stops disappearing overnight.
Shock Level Chart
Target FC by stabilizer level, with the dose to get there from zero in 10,000 gallons. Scale the dose to your own volume — 20,000 gallons is double, 15,000 is one and a half.
| CYA | Shock to | Cal-hypo 65% | Liquid 12.5% |
|---|---|---|---|
| 20 ppm | 10 ppm FC | 20.0 oz | 102.4 fl oz |
| 30 ppm | 12 ppm FC | 24.0 oz | 122.9 fl oz |
| 40 ppm | 16 ppm FC | 32.0 oz | 163.8 fl oz |
| 50 ppm | 20 ppm FC | 40.0 oz | 204.8 fl oz |
| 60 ppm | 24 ppm FC | 48.0 oz | 245.8 fl oz |
| 70 ppm | 28 ppm FC | 56.0 oz | 286.7 fl oz |
| 80 ppm | 32 ppm FC | 64.0 oz | 327.7 fl oz |
| 90 ppm | 36 ppm FC | 72.0 oz | 368.6 fl oz |
Assumes free chlorine starts at 0. If you already hold 3 ppm, dose for the gap, not the target — which is what the calculator above does with your real reading. Above CYA 100 ppm chlorine becomes largely ineffective whatever you add; drain and dilute first.
Every shock leaves something behind
The three products deliver the same chlorine and different side effects, and at shock doses those side effects are large enough to matter.
- Cal-hypo (65%) adds roughly 7 ppm calcium hardness for every 10 ppm of FC. Cheap, fast, and the default choice — until your calcium is already near 350 ppm, where it is the one to avoid. Pre-dissolve it in a bucket rather than broadcasting granules over a vinyl liner.
- Liquid chlorine (12.5%) adds no calcium and no CYA, which makes it the only product you can shock with repeatedly without changing anything else. It loses strength on the shelf, so buy it fresh and store it out of the sun.
- Dichlor (56%) adds about 9 ppm CYA for every 10 ppm of FC. Shock a pool to 20 ppm with dichlor and you have added roughly 18 ppm of permanent stabilizer — which raises the shock target for every treatment after this one. Fine once at start-up, wrong as a habit.
Shock at sunset
Unbound chlorine is destroyed by UV within hours, and a shock dose is deliberately far past what your CYA can hold in reserve — so most of a midday dose is gone before it has done the work. Add it after the sun is off the water, with the pump running so it circulates overnight, and brush the walls first if you are treating algae so the chlorine reaches what it is meant to kill.
Test before anyone swims — not by the clock. Free chlorine needs to fall back below about 5 ppm, which usually takes 8 to 24 hours. From there, work out your routine dose with the chlorine calculator, check where your stabilizer actually sits with the CYA calculator, and if the water is green rather than merely cloudy, follow the full sequence in how to fix a green pool.