How much shock for a 20,000-gallon pool?

To raise free chlorine by 10 ppm you need about 2.29 lb of 73% cal-hypo (20,000 × 10 × 8.34 × 10⁻⁶ = 1.668 lb available, ÷ 0.73), or about 214 fl oz of 12.5% liquid chlorine.

How often should I shock my pool?

Shock when combined chlorine climbs, after heavy use or rain, or when water turns cloudy or green. There is no fixed calendar — it depends on load — so test and shock on demand rather than by date.

Can high cyanuric acid stop shock from working?

Yes. High CYA raises the free chlorine required to reach breakpoint, so a heavily stabilized pool may resist shocking. If CYA is very high, partial draining and dilution is often more effective than adding still more chlorine.

How long after shocking can I swim?

Wait until free chlorine falls back into the normal range — typically a few hours to overnight, depending on the dose, sunlight and circulation. Always re-test before the first swim, because water can look perfectly clear while still carrying a swim-unsafe chlorine level.

How much shock does my pool need? Breakpoint explained

Q: What is breakpoint chlorination?

It is the free-chlorine threshold — about 10 times the combined chlorine — above which chlorine destroys chloramines instead of forming more of them. Shocking below breakpoint can make the chlorine smell worse.

Shocking is just a big chlorine dose aimed at a target. Here is how to size it and what “breakpoint” actually means.

By Francesco Zinghinì

Safety: dosing figures are estimates. Follow the product label, add one chemical at a time, never mix pool chemicals, add chemical to water (never water to chemical), keep chemicals away from children, and re-test before swimming. Add about ¾ of a dose, re-test, then top up. Not a substitute for professional pool service.

Shocking a pool means raising free chlorine sharply to burn off combined chloramines, kill algae and clear cloudy water. The math is the same as ordinary chlorine dosing — you are just aiming at a much higher free-chlorine target for a short time.

What is breakpoint chlorination?

When chlorine reacts with sweat, sunscreen and other contaminants it forms combined chlorine (chloramines) — the “chlorine smell” of a poorly maintained pool. To destroy those chloramines you have to push free chlorine past a threshold called the breakpoint, generally taken as roughly 10 times the combined chlorine reading. Below that threshold you make more chloramine; above it, you break it apart. That is why a half-hearted shock can make a pool smell worse.

The formula

Pick a target free chlorine (a preset shock level, or 10× your combined chlorine), then use the chlorine dosing math:

available chlorine (lb) = gallons × (target FC − current FC) × 8.34 × 10⁻⁶

product (lb) = available chlorine ÷ strength

Strength is the product fraction — 0.73 for 73% cal-hypo, 0.125 for 12.5% liquid, and so on. It is exactly the chlorine calculation, run at a bigger Δppm.

Worked example

Raise a 20,000-gallon pool by 10 ppm with 73% cal-hypo:

available = 20,000 × 10 × 8.34 × 10⁻⁶ = 1.668 lb, then 1.668 ÷ 0.73 ≈ 2.285 lb of cal-hypo shock.

With 12.5% liquid chlorine, the same 20,000-gallon, +10 ppm shock is (20,000 ÷ 10,000) × 10 × 10.7 = 214 fl oz, about 1.7 gallons.

How to shock well

Test combined chlorine first. Combined = total chlorine − free chlorine. Multiply by ~10 to find your breakpoint target, or use a standard shock level if you are clearing algae.
Shock at dusk so sunlight does not destroy the chlorine before it finishes the job, and run the pump for several hours.
Mind your cyanuric acid. A high CYA level raises the free chlorine you need to reach breakpoint; badly over-stabilized water may need dilution instead.
Wait to swim until free chlorine falls back into the normal range and re-test before anyone gets in.

Liquid chlorine and cal-hypo are common shock choices; dichlor works but adds cyanuric acid every time, so it is a poor pick for frequent shocking. Whatever you use, dose to the target, never mix products, and add the chemical to water rather than water to chemical.

Shocking for algae vs chloramines vs heavy use

“Shock” is one action with several purposes, and the target dose depends on why you are doing it:

Clearing algae: visible green needs a high free-chlorine target held for a while, plus brushing and continuous filtration until the water clears. Mustard and black algae are more stubborn and may need repeated shocking and thorough brushing.
Destroying chloramines: a strong “chlorine smell” and rising combined chlorine call for a breakpoint shock — aim at roughly ten times the combined-chlorine reading.
After heavy load: a big party, a rainstorm or a hot, crowded weekend dumps contaminants and burns through chlorine, so a preventive shock restores a healthy residual.

There is also non-chlorine shock (potassium monopersulfate, or MPS), which oxidizes contaminants and lets you swim sooner, but it does not sanitize or kill algae — so it complements chlorine rather than replacing it. Whatever the reason, the workflow is the same: shock at dusk, run the filter for hours, brush and vacuum as needed, and do not let anyone swim until free chlorine has fallen back into the normal range. Re-test before the first swim; a pool can look clear while still carrying a swim-unsafe chlorine level.

The bottom line

Shocking is just chlorine dosing aimed at a high target, so the same two-step math applies — available chlorine for your Δppm, divided by product strength. What changes is the target: use roughly ten times the combined chlorine to hit breakpoint and clear that chlorine smell, or a high held level to kill algae. Test combined chlorine first so you shock hard enough; a half dose below breakpoint makes the problem worse, not better. Shock at dusk, run the filter for hours, brush and vacuum for algae, and watch your cyanuric acid, since a heavily stabilized pool resists shocking and may need dilution instead. Above all, do not swim until free chlorine has dropped back to normal and you have re-tested — clear water can still be over-chlorinated.

Size a shock for your exact numbers on the shock calculator, handle routine dosing with the chlorine calculator, and check whether stabilizer is working against you with the cyanuric acid tool.