How much chlorine does a pool need?
Chlorine is what keeps pool water safe: it kills germs and oxidises the sweat, sunscreen and organic matter swimmers bring in. The level that matters is free chlorine (FC), the chlorine still available to do that job, measured in parts per million (ppm) with a DPD or FAS-DPD test kit.
The US Centers for Disease Control and Prevention (CDC) recommends at least 1 ppm free chlorine in pools, at least 2 ppm if the pool uses cyanuric acid (stabilizer, including dichlor and trichlor products), and at least 3 ppm in hot tubs, with pH between 7.0 and 7.8. Many outdoor pool owners keep a little above the minimum so the level does not drop below it before the next test. Never exceed the maximum on your product label.
Active chlorine needed (g) = ppm increase × litres ÷ 1,000
Product amount = active chlorine ÷ product strength
1 ppm in 10,000 US gallons = 37.9 g of available chlorine
How much of each product raises chlorine by 1 ppm in 10,000 gallons
| Product | Amount for +1 ppm / 10,000 gal | Side effect |
|---|---|---|
| Liquid chlorine 12.5% | 10.2 fl oz (303 mL) | Raises pH slightly at first; adds salt over time |
| Liquid chlorine 10% | 12.8 fl oz (379 mL) | Same as above |
| Plain bleach 6% | 20.4 fl oz (602 mL) | Use unscented bleach with no additives |
| Cal-hypo 65% | 2.1 oz (58 g) | Adds about 0.7 ppm calcium hardness |
| Dichlor 56% | 2.4 oz (68 g) | Adds about 0.9 ppm cyanuric acid |
| Trichlor 90% | 1.5 oz (42 g) | Adds about 0.6 ppm cyanuric acid; very acidic |
Liquid strengths are trade percentages (grams of available chlorine per 100 mL) for pool chlorine and percent sodium hypochlorite for household bleach, with an assumed density of 1.1. Solid strengths are percent available chlorine by weight, as printed on the label. If your product's strength differs, the label wins.
Worked examples
- 15,000-gallon pool from 1 to 4 ppm (a 3 ppm rise): you need 3 × 56,781 ÷ 1,000 = 170 g of available chlorine. That is 46.1 fl oz (1.36 L) of 12.5% liquid chlorine, or 91.7 fl oz (2.71 L) of 6% bleach, or 9.2 oz (262 g) of 65% cal-hypo.
- Same pool with dichlor: 10.7 oz (304 g), but it also adds about 2.7 ppm of cyanuric acid. Used every day, dichlor builds stabilizer quickly, which is why many owners switch to liquid chlorine once stabilizer reaches its target.
- 50,000-litre pool from 1 to 3 ppm: 800 mL of 12.5% liquid chlorine, or 154 g of 65% calcium hypochlorite.
Choosing a chlorine product
- Liquid chlorine (sodium hypochlorite) adds nothing except chlorine and a little salt. It loses strength in storage, especially when warm, so buy fresh and keep it shaded.
- Cal-hypo stores well and dissolves fast, but every dose adds calcium. Pre-dissolve it in a bucket of water as the label directs, and never mix it with other chemicals.
- Trichlor tablets are convenient in a floater or feeder, but add stabilizer and acid continuously. Watch cyanuric acid and alkalinity.
- Dichlor dissolves quickly and is pH-neutral, but adds stabilizer. The CDC advises against stabilizer in hot tubs.
- Salt chlorinators make chlorine from salt in the cell; use the pool salt calculator for the salt dose.
Chlorine and stabilizer go together
In outdoor pools, sunlight destroys unprotected chlorine within hours. Cyanuric acid (stabilizer) protects it, but also slows down how fast chlorine kills germs. That is why the CDC's minimum doubles to 2 ppm when stabilizer is used, and why pool professionals scale the chlorine target with the stabilizer level. See the pool stabilizer calculator.
Safe dosing
- Add chlorine with the pump running, ideally in the evening, and pour slowly in front of a return jet.
- Never mix chlorine products with each other or with acid. Add them separately, hours apart, straight into the pool water.
- Retest after the water has circulated for at least an hour. Adjust if needed rather than adding everything at once.
- Follow the label for when swimmers can re-enter after a large dose.
Common mistakes
- Testing total chlorine instead of free chlorine. Total includes combined chlorine (chloramines) that no longer sanitises well.
- Old liquid chlorine. A jug stored for months in the sun may be well below the strength on the label, so doses come out too small.
- Wrong volume. Check it with the pool volume calculator.
Estimates for general information. Always read and follow the product label, and test the water before swimmers return.
Frequently asked questions
How much liquid chlorine do I add to raise 1 ppm in 10,000 gallons?
About 10.2 fl oz (303 mL) of 12.5% liquid chlorine, or about 20.4 fl oz of 6% plain bleach.
What should free chlorine be in a pool?
The CDC recommends at least 1 ppm in pools, at least 2 ppm when cyanuric acid (stabilizer) is used, and at least 3 ppm in hot tubs, with pH 7.0–7.8.
How much chlorine for a 15,000 gallon pool?
To raise free chlorine by 3 ppm (for example 1 to 4 ppm): about 46 fl oz of 12.5% liquid chlorine or about 9.2 oz of 65% cal-hypo.
Does dichlor or trichlor add stabilizer?
Yes. Each 1 ppm of chlorine from dichlor adds about 0.9 ppm cyanuric acid; from trichlor about 0.6 ppm.
Sources
- CDC: Home pool and hot tub water treatment and testing (free chlorine and pH recommendations).
- CDC Model Aquatic Health Code.
- Product strengths: typical label values for sodium hypochlorite, calcium hypochlorite, sodium dichloroisocyanurate and trichloroisocyanuric acid.
Last reviewed 28 October 2026. Results are estimates; see our disclaimer.