How the chlorine math works
One ppm is one milligram per litre, so raising a pool by Δ ppm needs Δ × litres milligrams of available chlorine. Each product carries a different amount per gram or millilitre:
| Product | Per 1 ppm in 10,000 gal | Side effect per 1 ppm FC |
|---|---|---|
| 10% liquid chlorine | 13 fl oz | about +1.6 ppm salt (harmless) |
| 12.5% liquid chlorine | 10 fl oz | about +1.6 ppm salt |
| 6% bleach | 21 fl oz | about +1.6 ppm salt |
| 65% cal-hypo | 2.1 oz | +0.71 ppm calcium |
| 56% dichlor | 2.4 oz | +0.91 ppm CYA |
| 90% trichlor | 1.5 oz | +0.61 ppm CYA, lowers TA |
Pool liquid chlorine at 10% or more is labelled in "trade percent" (grams of chlorine per 100 mL). Household bleach below 10% is labelled by weight, so its real strength per millilitre is a little lower than the number suggests. The calculator handles both conventions.
Worked example
A 15,000 gallon pool with CYA 40 reads 3 ppm. The target at CYA 40 is 6 ppm, so you need 3 ppm: 0.45 gal (1.7 L) of 10% liquid chlorine. Add it at dusk in front of a return jet with the pump running.
Choosing your target
Stabilizer (CYA) holds most of your chlorine in reserve, so the more CYA you have, the more free chlorine you need for the same sanitizing power. The minimum is about 7.5% of CYA; we set the target about 3 ppm above that. Salt water pools, which add chlorine all day, run lower: a 5% minimum and a 7.5% target. The FC/CYA calculator lists every level.
Common mistakes
- Using a fixed target such as 3 ppm whatever the CYA. At CYA 60 that is below the minimum.
- Old chlorine. Liquid chlorine loses strength in a hot garage, so a dose that used to work stops working.
- Reading the wrong percentage. Pool chlorine (10% and up) is sold by trade percent and household bleach by weight. A jug labelled "sodium hypochlorite 12.5%" that lists "available chlorine 11.9%" is about 14% stronger than 12.5% pool chlorine, so enter about 14.2% for it here (in the app, choose "Active chlorine by weight" and enter 11.9).
- Guessing the pool volume. A 30% error in gallons is a 30% error in every dose.
- Adding chlorine at noon in full sun, where much of it is lost before evening.
- Pouring chlorine and acid together. Add one, wait 30 minutes, then the other.