How to raise free chlorine in a pool (and how much to add)
Raising free chlorine is simple arithmetic: pool size times the ppm you need, divided by the product's strength. The harder part is knowing what level to aim for, and working out why chlorine keeps dropping if it won't stay up.
What level to aim for
The CDC's minimum for pools is 1 ppm free chlorine without stabilizer and 2 ppm with stabilizer (cyanuric acid, CYA). Stabilizer holds chlorine in reserve, so many pool owners also follow a guideline of keeping free chlorine at least 7.5% of CYA:
| Stabilizer (CYA) | Minimum free chlorine |
|---|---|
| 0–20 ppm | 2 ppm (CDC minimum with stabilizer; 1 ppm with none) |
| 40 ppm | 3 ppm |
| 60 ppm | 4.5 ppm |
| 80 ppm | 6 ppm |
That's a floor. Aim a couple of ppm above it, so a sunny day or a busy afternoon doesn't take you below. The chlorine and stabilizer guide explains why the level depends on CYA.
How much chlorine to add
To raise 10,000 gallons by 1 ppm of free chlorine, the pool chlorine calculator gives:
| Product | Per 10,000 gal per 1 ppm | Side effect |
|---|---|---|
| Liquid chlorine 12.5% | 10.2 fl oz | Adds a little salt (harmless) |
| Household bleach 6% | 21.3 fl oz | Adds a little salt (harmless) |
| Cal-hypo 65% | 2.1 oz | Raises calcium hardness about 0.7 ppm per ppm of chlorine |
| Dichlor 56% | 2.4 oz | Raises stabilizer about 0.9 ppm per ppm of chlorine |
Multiply by the ppm you need and by your volume in 10,000s of gallons. Product strength varies, especially for bleach, which loses strength as it ages. Check the label for the percentage and use unscented bleach without additives.
Worked example
A 15,000-gallon pool with 40 ppm stabilizer reads 1 ppm free chlorine. The minimum at 40 ppm CYA is 3 ppm, so the owner aims for 4 ppm, a rise of 3 ppm. The chlorine calculator gives:
- Liquid chlorine 12.5%: about 46.1 fl oz
- Bleach 6%: about 96 fl oz (three-quarters of a gallon)
- Cal-hypo 65%: about 9.2 oz, which also adds about 2.1 ppm of calcium hardness
- Dichlor 56%: about 10.7 oz, which also adds about 2.7 ppm of stabilizer
For a single dose the side effects are small. Repeated every day for a season, they add up, which is why many owners use liquid chlorine for daily top-ups. The chlorine cost calculator shows a month's worth.
How to add it
- Liquid chlorine and bleach: pour slowly in front of a return jet with the pump running. Evening is best, so the sun doesn't burn off part of it right away.
- Cal-hypo: dissolve in a bucket of pool water first (add the product to the water, never water to the product), then pour it around the pool. Undissolved granules can bleach vinyl liners.
- Dichlor: dissolves quickly and can be broadcast or pre-dissolved. Follow the label.
- Trichlor tablets dissolve slowly in a floater or feeder. They maintain chlorine over days, but they're not a way to raise it quickly.
Never mix chlorine products together, or with acid. See pool chemical safety.
Why won't my chlorine go up?
If you add chlorine and the reading barely moves, or it's gone by the next morning, something is using it up:
- Algae or organic debris. Even before the water turns green, algae uses up chlorine. Test overnight: add chlorine at dusk, test at dusk and again at dawn. A loss of more than about 1 ppm overnight, when the sun isn't burning it off, points to algae. The fix is to shock and hold: see how to shock a pool.
- Combined chlorine. If combined chlorine is above 0.5 ppm, chlorine is being spent on ammonia compounds. See strong chlorine smell.
- Sunlight with low stabilizer. Without CYA, sunlight can destroy most of the chlorine in a few hours. Test CYA and add stabilizer if it's low.
- Weak product. Old bleach or liquid chlorine stored in heat loses strength. Buy fresh, store it cool and shaded.
- Test problems. Expired reagents, or a DPD test bleaching out at high levels, can give false readings. See how to test pool water.
- Salt cell. In a saltwater pool, check the salt level with your own test, the output setting, pump run time, water temperature and the cell for scale.
Frequently asked questions
How long after adding chlorine can I swim?
Follow the product label. For a normal top-up, many owners wait until it has circulated. After a shock dose, see when you can swim after shocking.
Is household bleach the same as pool chlorine?
Both are sodium hypochlorite. Pool liquid chlorine is usually stronger (often 10–12.5%) than household bleach (often 6–8.25%). Use plain, unscented bleach and adjust the dose for its strength.
Should I use shock to raise chlorine?
"Shock" products are usually cal-hypo, dichlor or a strong liquid chlorine. Any of them raises chlorine. What makes it a shock is the level you raise it to, not the product.