Guide
How much chlorine to add to a pool
Your chlorine target depends on your stabilizer (CYA). Here is how to find it, how much liquid chlorine, bleach or granules to add, and the mistakes that waste chlorine.
Updated · 5 min read
The right amount of chlorine depends on two numbers: how much free chlorine (FC) the water has now, and how much stabilizer (CYA) it holds. Stabilizer shields chlorine from sunlight, but it also slows chlorine down, so a pool with more CYA needs more free chlorine to stay clean.
If you only want the number, use the chlorine calculator. It does everything on this page for your pool and your product.
Find your target from CYA
Test FC and CYA first. A drop-count kit is the most reliable way to read both; strips are rougher. Then find your CYA in the table:
| CYA (ppm) | Never below | Target FC (added by hand) | Target FC (salt cell) |
|---|---|---|---|
| 30 | 2 | 4–6 | 3–6 |
| 40 | 3 | 5–7 | 3–7 |
| 50 | 4 | 6–8 | 3–8 |
| 60 | 5 | 7–9 | 4–9 |
| 70 | 5 | 8–10 | 5–10 |
| 80 | 6 | 9–11 | 6–11 |
Aim for the middle of the range. Salt cells make chlorine all day, so salt pools can sit lower in the range than pools dosed by hand once a day.
Work out the dose
Subtract what you have from what you want. That difference, in ppm, times your pool's volume, gives the dose. For 10% liquid chlorine, raising a 10,000-gallon pool by 1 ppm takes about 1½ cups (12.8 fl oz).
A worked example: a 15,000-gallon pool at CYA 40 reads FC 3. The target is 5 to 7, so aim for 6.
- Increase: 6 − 3 = 3 ppm
- Dose: 3 ppm × 15,000 gal ÷ 781.8 = 57.6 fl oz of 10% liquid chlorine
- In the units you buy: 7 cups, about half of a 1-gallon jug
Dose per 1 ppm, by product
The amount that raises free chlorine by 1 ppm. Multiply by the ppm you need, then by your volume divided by 10,000.
| Product | Per 10,000 US gal | Per 10,000 litres | Also adds, per 1 ppm of chlorine |
|---|---|---|---|
| 12.5% liquid chlorine | 10.2 fl oz | 80 mL | nothing else |
| 10% liquid chlorine | 12.8 fl oz | 100 mL | nothing else |
| 8.25% bleach (unscented) | 14.8 fl oz | 116 mL | nothing else |
| 6% bleach (unscented) | 20.8 fl oz | 162 mL | nothing else |
| Cal-hypo 73% | 1.84 oz | 13.8 g | 0.7 ppm calcium |
| Cal-hypo 65% | 2.07 oz | 15.5 g | 0.7 ppm calcium |
| Dichlor 56% | 2.41 oz | 18.1 g | 0.9 ppm CYA |
| Trichlor 90% | 1.46 oz | 10.9 g | 0.6 ppm CYA |
In metric, the easy one to remember: 100 mL of 10% liquid chlorine per 10,000 litres raises FC by 1 ppm, and 80 mL of 12.5% does the same.
Weaker products need more. Household bleach at 6% needs about two thirds more than 10% liquid chlorine for the same change; check the strength on the label, because bleach loses strength as it ages.
Which chlorine to use
- Liquid chlorine or plain bleach adds nothing but chlorine (and a little salt). It's the easiest to dose exactly.
- Cal-hypo granules add calcium with every dose. Fine if your calcium is low, a problem if it's already high.
- Dichlor granules add stabilizer with every dose: about 9 ppm of CYA for every 10 ppm of chlorine. Useful to build CYA up, then switch.
- Trichlor tablets add stabilizer and lower pH. Convenient in a floater, but CYA creeps up all season, and chlorine needs rise with it.
How to add chlorine, step by step
- Test free chlorine and CYA. They set both the target and the dose.
- Find your target. Use the table above or the calculator, and aim for the middle of the range.
- Measure the dose for your product. Use the calculator, and check the strength on the label.
- Add it separately, with the pump running. Pour liquid chlorine slowly in front of a return jet; dissolve granules in a bucket of pool water first. Never mix chlorine with acid or any other product.
- Retest after 30 minutes. Test again the next evening to see how much the sun used, and adjust tomorrow's dose.
Common mistakes
Dosing by habit. "A jug a day" is right for one pool in one week and wrong for most others. The amount depends on volume, CYA and weather.
Ignoring CYA. A pool at CYA 80 with FC 3 is under-chlorinated even though 3 ppm sounds fine. That is how algae starts in pools that "have chlorine".
Shocking with the wrong product. Clearing algae means holding chlorine at shock level (40% of your CYA) until the water clears. Doing that with dichlor or trichlor adds so much stabilizer that the next shock needs even more chlorine.
Adding chlorine and acid together. Add one, let it circulate, then add the other. Mixing them releases chlorine gas.
Questions people ask
Is it safe to swim right after adding chlorine?
Once it has mixed, yes, as long as free chlorine is between the minimum for your CYA and the shock level (about 40% of CYA) and you can see the bottom. Above the shock level, add nothing and let it drift down before swimming.
I added too much. How do I lower chlorine?
Usually you do nothing. Chlorine above the target but at or below the shock level comes down on its own, and sunlight does most of the work. Retest before the next swim.
Why did my chlorine disappear overnight?
Losing more than 1 ppm between dusk and dawn means something in the water is using it up, often algae getting started. Check combined chlorine too: above 0.5 ppm, or more than 1 ppm lost overnight, it's time to shock and hold. The green pool guide covers the routine.
Can I use a bag of shock instead?
Bagged shock is usually cal-hypo or dichlor. Both raise chlorine, and both also add calcium or CYA every time, which adds up. For everyday top-ups, liquid chlorine adds nothing else to the water.
Do salt pools need chlorine added?
The salt cell makes it. If free chlorine sits below the target, raise the cell's output by about 10% or run the pump longer; keep a jug of liquid chlorine for catching up. Salt pools also run lower in the range at the same CYA, as the table shows.
When in doubt, put your numbers into the chlorine calculator, or into the water check to get an ordered plan for pH and everything else too.