What is breakpoint chlorination?

The point where added chlorine oxidizes chloramines instead of just adding to them. Below the breakpoint you make more combined chlorine; above it, the chloramines are destroyed. The 10× combined chlorine rule is the standard way to be sure you clear it in one dose.

Can I just dump in a whole bag of shock?

You can, but you may badly over-shoot and have to wait a long time to swim. Dose to the breakpoint target, circulate, and retest. If combined chlorine is still high, shock again rather than guessing high the first time.

When is it safe to swim after shocking?

When free chlorine falls back to the normal 1–4 ppm range and combined chlorine is near zero — test with a kit, do not go by the clock. Always follow the product label's re-entry guidance.

Pool Shock Calculator

Get the breakpoint shock dose to clear chloramines — free chlorine driven to ~10× your combined-chlorine reading, from your pool volume and product.

Inputs

Pool volume — gallons

gal

Current FC

ppm

Combined chlorine

ppm

Product — 65% available chlorine (calcium hypochlorite)

Results update as you type — no submit

Shock to add

12.3 oz

≈ 349 g

Breakpoint target (10× CC)6 ppm FC

FC change4 ppm

ProductCal-hypo 65%

Formula

raise FC by 4 ppm in 15,000 gal
shock = 12.3 oz ≈ 349 g

Add chemicals to water, never water to chemicals. Broadcast over the deep end with the pump running, then retest before adding more.

Doses are estimates from published product concentrations — always follow the product label, add chemicals to water (never the reverse), test with a kit, and retest after dosing.

MeasureShedmeasureshed.com/calculator/pool-shock-calculator

Pool shock — dose sheetUNITS: US GAL · ppm

Inputs

Pool volume15,000 gal

Current FC2 ppm

Combined chlorine0.6 ppm → target 6 ppm

ProductCal-hypo 65%

Results

shock to add12.3 oz

Other units349 g

FC change4 FC ppm

Safe dosing: add product to water (never the reverse) · broadcast with the pump running · wait and retest before re-dosing · keep chemicals apart and dry.

Shopping list

☐ 12.3 oz (349 g) — Cal-hypo 65%

Formula

raise FC by 4 ppm in 15,000 gal

shock = 12.3 oz (349 g)

Doses are estimates from published product concentrations — always follow the product label, add chemicals to water (never the reverse), test with a kit, and retest after dosing.SHEET 1 OF 1

How it works

Shocking a pool means raising free chlorine high enough to break apart combined chlorine — the chloramines that make eyes sting and give a pool that "chlorine smell." The published target is **breakpoint chlorination**: drive free chlorine to about `10×` your combined-chlorine reading. If combined chlorine tests at `0.6 ppm`, the breakpoint target is `6 ppm` of free chlorine.

The calculator takes your current free chlorine and combined-chlorine readings, computes the breakpoint target, and then works out how much product it takes to make up the difference across your pool's volume — using each product's published available-chlorine strength. Cal-hypo at `65%` and dichlor at `56%` are common shock products; liquid chlorine works too and adds no calcium or stabilizer.

Shock late in the day so sunlight does not burn off the chlorine before it finishes the job, run the pump for several hours, and **retest before swimming** — most labels want free chlorine back under `4 ppm` first. Add product to water, never the reverse, and keep it clear of your skin and eyes.

Worked example

Volume `15,000 gal` · current FC `2 ppm` · combined chlorine `0.6 ppm`
breakpoint target = 10 × 0.6 = `6 ppm` → need `+4 ppm`
cal-hypo `65%` = 4 × 15,000 × 8.34 ÷ 1,000,000 ÷ 0.65 = `0.77 lb` = `12 oz`

Questions about shocking a pool

Enough to reach 10× your combined-chlorine reading. For a 15,000 gal pool that needs to go from 2 to 6 ppm free chlorine, that is about 12 oz of 65% cal-hypo or roughly a half-gallon of 12.5% liquid chlorine. Read combined chlorine on your test kit as total minus free.

SOURCES — APSP-11 / CDC MAHC breakpoint chlorination (10× combined chlorine) · product available-chlorine %

Doses are estimates from published product concentrations — always follow the product label, add chemicals to water (never the reverse), test with a kit, and retest after dosing.