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How Much Chlorine to Add to a Pool
Your stabilizer (CYA) sets how much free chlorine the water needs. Three questions give the dose.
1 of 3: your gallons
Your dose appears here after the last question.
- 4 to 6 ppm
- free chlorine with 40 ppm CYA
- 2 to 4 ppm
- free chlorine with no stabilizer
- 16 ppm
- shock level at 40 ppm CYA
Working out how much chlorine to add to your pool comes down to 3 numbers: how many gallons your pool holds, your stabilizer level, and what your free chlorine reads right now. One ppm (part per million, the number your test kit reads) of free chlorine comes from 10 fl oz of the 12.5% liquid in each 10,000 gallons of water.
- To lift free chlorine 1 ppm: 10 fl oz of the 12.5% liquid per 10,000 gallons
- To shock, lifting free chlorine 10 ppm: 102 fl oz of the 12.5% liquid per 10,000 gallons
- Aim for 3 to 5 ppm free chlorine at 30 ppm stabilizer
- Aim for 4 to 6 ppm free chlorine at 40 ppm stabilizer
The free chlorine your pool needs
How much stabilizer you have decides how much free chlorine the pool should carry. Test kits often label it CYA, short for cyanuric acid. It protects chlorine from sunlight, and the more you have, the more free chlorine you need.
| CYA level | Free chlorine to hold |
|---|---|
| 30 ppm | 3–5 ppm |
| 40 ppm | 4–6 ppm |
That abbreviation, ppm, means parts per million. It measures how much chemical is dissolved in the water, and it's the number your test strip or kit gives you. Most pools are happiest with stabilizer between 30 and 50 ppm, or 70 to 80 ppm if your pool runs on a salt chlorine generator.
Telling free chlorine from total chlorine
Plenty of kits give you both readings. The targets and doses here all refer to free chlorine, so go by that number when you decide what to pour in.
Chlorine doses by pool size
Pick the size closest to your own. Double the dose for a pool twice as big, or cut it in half for one half the size.
| Pool size | Liquid chlorine 12.5% to lift 1 ppm | Cal-hypo 65% to lift 1 ppm | Liquid chlorine 12.5% for a 10 ppm shock |
|---|---|---|---|
| 5,000 gallons | 5 fl oz | around 1 oz, or 30 g | 51 fl oz |
| 10,000 gallons | 10 fl oz, about 305 mL | around 2.1 oz, or 60 g | 102 fl oz, about 3.03 L |
| 20,000 gallons | 20 fl oz | about 120 g | double 102 fl oz |
Cal-hypo is a dry, granular form of chlorine. It's stronger than liquid, so you need far less of it. Check the percentage on the label, since these amounts assume 12.5% liquid and 65% cal-hypo.
Here's a worked example. Your test shows 2 ppm free chlorine, you want 5 ppm, and your pool holds 10,000 gallons. That means 30 fl oz of the 12.5% liquid.
How many gallons does your pool hold?
Each dose depends on how much water you're treating. Here are typical sizes, with water averaging 4.5 ft deep:
| Pool size | Rough volume |
|---|---|
| 12 x 24 ft | about 9,690 gallons |
| 16 x 32 ft | about 17,200 gallons |
| 20 x 40 ft | about 26,900 gallons |
Got a sloped floor? Then average the depth by splitting the difference between the shallow and deep ends.
Adding chlorine safely
- 1. Keep the pump running so the chlorine moves through the whole pool.
- 2. Pour it near a return jet, the outlet where filtered water flows back into the pool.
- 3. One chemical at a time, and never combine acid with chlorine.
- 4. Retest before you dose again, so you know where the level sits.
At ordinary doses, liquid chlorine is fine to swim with after about 30 minutes. After shocking, hold off until free chlorine drops under your shock target.
The math behind it
With a rectangular pool, take the surface area in square feet, multiply by the average depth in feet, then multiply by 7.48 to get gallons. For round and oval pools, swap 7.48 for 5.9. Doses then scale with gallons from the 10,000-gallon baseline.
Questions people ask
What's the chlorine dose for a pool measured in litres?
A US gallon equals 3.785 litres. In a 10,000-gallon pool, 305 mL of the 12.5% liquid gains you 1 ppm of free chlorine.
What about a 1,000-litre pool?
To get its size in gallons, divide 1,000 litres by 3.785. To gain 1 ppm of free chlorine, it takes roughly 8 mL of the 12.5% liquid.
Does shocking lift free chlorine?
It does. Shocking just means adding a big slug of chlorine. Adding 102 fl oz of the 12.5% liquid to 10,000 gallons lifts free chlorine by 10 ppm.
Is pH a factor when chlorinating?
This site targets pH 7.4 to 7.6, though anything from 7.2 to 7.8 works. If you use acid to bring pH down, keep it apart from the chlorine and retest pH 30 minutes later.