Breakpoint chlorination calculator
Estimates for typical pool water. Always follow the product label, your state/local code and your certified pool operator. Add about three-quarters of a pH or alkalinity dose first, circulate, and re-test before adding the rest. Never mix chemicals or add acid and chlorine at the same time.
Quick answer
Breakpoint chlorination target = 10 × combined chlorine, where combined chlorine = total chlorine − free chlorine. Raise free chlorine by the target minus your current free chlorine. Example: FC 1.5 and TC 2.1 give CC 0.6, a 6.0 ppm target and a 4.5 ppm increase — about 2.7 gallons of 12.5% liquid chlorine in a 75,000-gallon pool.
- CC = TC − FC. Treat when CC reaches 0.4 ppm (MAHC).
- Breakpoint target ≈ 10 × CC.
- Use unstabilized chlorine and add it in one dose.
- Close to bathers if the target exceeds your code maximum (10 ppm MAHC).
Breakpoint chlorination formula
- Combined chlorine: CC = TC − FC.
- Breakpoint target: FC target = CC × 10.
- Chlorine to add: ppm to add = FC target − current FC.
- Product: ppm to add × volume ÷ 10,000 × product per ppm (10.2 fl oz of 12.5% liquid chlorine).
| FC | TC | CC | Target | Add (ppm) | 12.5% liquid / 50,000 gal |
|---|---|---|---|---|---|
| 1.5 | 2.1 | 0.6 | 6.0 | 4.5 | 1.8 gal |
| 2.0 | 2.5 | 0.5 | 5.0 | 3.0 | 1.2 gal |
| 3.0 | 3.8 | 0.8 | 8.0 | 5.0 | 2.0 gal |
| 2.0 | 3.2 | 1.2 | 12.0 | 10.0 | 4.0 gal — close pool |
Getting a breakpoint treatment to work
- Hit the full target in one addition — partial doses can make chloramines worse before they get better.
- Keep pH in the 7.2–7.6 range so chlorine works efficiently.
- Run circulation and, indoors, maximize fresh-air exchange.
- Re-test FC and TC after 1–2 hours; repeat if CC is still 0.4 ppm or higher.
- Record each treatment — inspectors look for corrective action when CC is high. See combined chlorine & breakpoint chlorination.
How to breakpoint chlorinate a commercial pool
- Test FC and TC. Use a FAS-DPD kit for accurate free and total chlorine.
- Calculate the target. CC = TC − FC; target = 10 × CC; add = target − FC.
- Add unstabilized chlorine. Add the full dose of liquid chlorine or cal-hypo at once with the pump running.
- Re-test and log. Re-test after 1–2 hours, repeat if CC is still 0.4 ppm or more, and record each step.
Frequently asked questions
How do you calculate breakpoint chlorination?
Subtract free chlorine from total chlorine to get combined chlorine, multiply combined chlorine by 10 to get the breakpoint target, then subtract current free chlorine to find how many ppm to add.
When should a commercial pool be breakpoint chlorinated?
When combined chlorine reaches 0.4 ppm or higher, the MAHC threshold. Many operators act at 0.2–0.3 ppm to stay ahead of chloramine odor and eye complaints.
Can you swim during breakpoint chlorination?
Not if free chlorine exceeds your code maximum — 10 ppm under the MAHC. Many breakpoint treatments stay under 10 ppm and can be done after hours.
Why does my combined chlorine stay high after shocking?
The dose may have been too small to reach breakpoint, pH may be too high, or bather load and poor showering keep adding waste. Indoor pools also need good air handling to remove chloramines.
This page is an educational summary, not legal advice. Pool and spa codes differ by state and county and change over time — always confirm requirements with the health department that inspects your facility.