Breakpoint Chlorination Calculator
Pool shock dose and breakpoint from CC readings.
What it does
Chlorine smell means too little free chlorine, not too much: combined chloramines cause the sting while breakpoint multiples destroy them. Ten times the CC reading sets the shock target.
The tool above takes combined and free chlorine with pool gallons. It returns shock ounces of 65% cal hypo, ppm to raise, and the breakpoint target.
The default 20,000 gal pool at CC 0.4 with FC 1.0 needs 12.3 oz to rise 3 ppm toward breakpoint 4. The heavy case at CC 1.0 needs 19.5 oz.
Aquatic health codes anchor breakpoint near ten times combined chlorine with circulation and retesting. Operator courses teach the same factor with product specific dose conversions.
Shock at dusk with pumps running, brush the walls, and keep covers off overnight. Retest the trio after 24 hours; green or ammonia loaded pools earn repeat rounds.
How to use
- Type combined chlorine.
- Type free chlorine.
- Type pool gallons.
- Read shock ounces first.
- Read ppm to raise beside it.
- Retest after 24 h.
Formula
- CC
- Combined chlorine in ppm.
- FC
- Free chlorine in ppm.
- gal
- Pool volume in gallons.
CDC Model Aquatic Health Code frames breakpoint near ten times combined chlorine for chloramine destruction. Pool operator training repeats the same 10x target with retesting after circulation. The 2.05 oz factor converts ppm to 65% calcium hypochlorite per 10,000 gallons.
Worked example
Example: shock a 20k pool
Defaults: CC 0.4, FC 1.0, 20,000 gal.
- Pool: 20,000 gal, CC 0.4, FC 1.0.
- Target: 10 x 0.4 = 4 ppm breakpoint.
- Raise: 4 minus 1 = 3 ppm.
- Dose: 3 x 2 x 2.05 = 12.3 oz of 65% cal hypo.
- Retest: after 24 h of circulation.
- Tool shows 12.3 oz of 65% cal hypo, Raise 3 ppm, Breakpoint 4 ppm.
Reference table
| Pool and CC | Target | Dose |
|---|---|---|
| 20k gal, CC 0.4, FC 1.0 | 4 ppm | 12.3 oz |
| 10k gal, CC 1.0, FC 0.5 | 10 ppm | 19.5 oz |
| 15k gal, CC 0.2, FC 2.0 | 2 ppm | 0 oz |
| 25k gal, CC 0.8, FC 2.0 | 8 ppm | 30.8 oz |
Limits
- 65% cal hypo assumed.
- Routine maintenance, not swamps.
- Cyanuric acid unread here.
- Test error moves targets.
Common mistakes
- Diluting smelly pools.
- Shocking at noon sun.
- Dosing liquid by cal hypo math.
- Skipping the 24 h retest.
- Swimming into fresh shock.
Try it yourself
Quick checks. Answers sit tucked under each question.
CC 1.0, FC 0.5, 10,000 gal?
Target 10, raise 9.5, dose 19.5 oz in 10,000 gal.
CC 0.2, FC 2.0, 15,000 gal?
Target 2, raise 0: 0 oz, already past breakpoint.
CC minus 1?
Negative chlorine is untestable: enter CC from 0 up.
Questions
Breakpoint rule?
Ten times combined chlorine. Chloramines break near that multiple, freeing the pool.
Combined chlorine?
CC from sweat and urine plus chlorine. Smell and sting signal them, not too much chlorine.
Dose factor?
2.05 oz of 65% cal hypo per 10k gallons per ppm. Liquid and dichlor differ.
Shock when?
Dusk, circulation running, brush after. Sunlight eats unstabilized shock fast.
Wait to swim?
Yes, 24 hours of filtering with the cover off, then retest all three readings.
Green pools same?
No, algae and ammonia loads demand higher or repeated shocks with brushing.