How much chlorine does a pool need?
The right dose depends on four things: how many gallons your pool holds, your current free chlorine (FC) reading, the level you want to reach, and the strength of the product you are adding. A "one jug" rule cannot be right for every pool, because a 5,000 gallon above-ground pool and a 30,000 gallon in-ground pool need very different amounts.
How the calculator works
First it subtracts your current reading from your target to get the ppm increase you need. One ppm means one part of chlorine per million parts of water by weight. It then works out how much available chlorine that is for your volume and divides by the product's strength.
- Liquid chlorine and bleach are dosed by volume. Their strength is the percentage of available chlorine shown on the label.
- Dry products (cal-hypo, dichlor, trichlor) are dosed by weight, using the percentage of available chlorine on the label.
The formulas
These give the familiar rules of thumb: raising 10,000 gallons by 1 ppm takes about 10.2 fl oz of 12.5% liquid chlorine, 12.8 fl oz of 10% liquid, 21.3 fl oz of 6% bleach, about 2.1 oz of 65% cal-hypo, 2.4 oz of 56% dichlor or 1.5 oz of 90% trichlor.
Worked example
Your 20,000 gallon pool tests at 1 ppm and you want 4 ppm, a 3 ppm increase.
With 10% liquid chlorine: 3 × 20,000 × 128 ÷ (10 × 10,000) = 76.8 fl oz, about 0.6 gallons. With 65% cal-hypo instead: 3 × 20,000 × 8.345 ÷ 1,000,000 × 16 ÷ 0.65 ≈ 12.3 oz by weight.
Choosing a product
- Liquid chlorine (sodium hypochlorite) adds chlorine and a little salt. It loses strength over time, especially in heat, so buy it fresh and store it somewhere cool.
- Cal-hypo also adds calcium, which matters if your water is already hard.
- Dichlor and trichlor are stabilized. Each 1 ppm of chlorine from dichlor adds about 0.9 ppm of cyanuric acid (CYA), and trichlor adds about 0.6 ppm. CYA protects chlorine from sunlight, but too much makes chlorine less effective. The calculator shows how much CYA your dose adds.
Safety and good practice
- Never mix different chlorine products, or chlorine and acid, in the same container. Add each chemical to the pool separately.
- Pour liquid chlorine slowly in front of a return jet with the pump running. Pre-dissolve granular products in a bucket of pool water if the label says to.
- Retest after the water has circulated, usually 30 minutes to a few hours, before adding more.
- Always follow the product label. Labels can state strength differently, and the label is the final word on dosing and safety.
Frequently asked questions
How much liquid chlorine do I add to raise 1 ppm?
For 12.5% liquid chlorine, about 10.2 fluid ounces raises 10,000 gallons by 1 ppm. Multiply by your pool’s size in 10,000s of gallons and by the ppm increase you need.
Can I use household bleach in my pool?
Plain, unscented bleach is sodium hypochlorite, the same chemical as pool liquid chlorine, just weaker. Avoid scented, splash-less or thickened varieties, and check the percentage on the label.
What free chlorine level should I aim for?
Many pool owners aim for roughly 1 to 4 ppm for routine maintenance, but the right level depends on your cyanuric acid level: more stabilizer requires more chlorine. Shocking requires a much higher level for a short period; follow your product label or local guidelines.
Why does my chlorine drop so fast?
Sunlight, heat, heavy swimmer use, rain and organic debris all consume chlorine. Low stabilizer (CYA) in an outdoor pool is a common cause of chlorine disappearing on sunny days.
Does this calculator work for hot tubs?
The math works for any volume, so enter your spa’s gallons. Spas usually run at different chlorine levels and use smaller doses, so follow your spa manufacturer’s guidance.
Last reviewed November 2026. Spotted an error? Tell us.