What free chlorine should you aim for?
The CDC’s minimum for pools is 1 ppm of free chlorine, or 2 ppm if the pool uses stabilizer (cyanuric acid, CYA). Those are floors, not targets. Stabilizer holds most of the chlorine in reserve, so the more CYA you have, the higher free chlorine has to be to do the same job. The widely used Trouble Free Pool chart puts the target at roughly 12% of CYA:
Recommended free chlorine by stabilizer level
Show the numbers
| CYA (ppm) | Minimum FC | Target FC | Shock FC |
|---|---|---|---|
| 0 | 1 | 3 | 10 |
| 10 | 2 | 3 | 10 |
| 20 | 2 | 3 | 10 |
| 30 | 2 | 4 | 12 |
| 40 | 3 | 5 | 16 |
| 50 | 4 | 6 | 20 |
| 60 | 5 | 7 | 24 |
| 70 | 5 | 8 | 28 |
| 80 | 6 | 9 | 31 |
| 90 | 7 | 10 | 35 |
| 100 | 7 | 12 | 39 |
The formula
Available chlorine needed (g) = rise in ppm × litres ÷ 1,000
Product = available chlorine ÷ product strength
For liquids we read the label percentage as grams of available chlorine per 100 mL. For powders and tablets it’s the percentage of available chlorine by weight.In US units, 1 ppm in 10,000 gallons is 37.9 grams of available chlorine, or 1.34 oz. Every product is that amount divided by its strength.
How much each product takes
To raise free chlorine by 1 ppm in 10,000 gallons:
| Product | Amount | Also adds |
|---|---|---|
| Liquid chlorine, 12.5% | 10.2 fl oz | About 1.65 ppm salt |
| Liquid chlorine, 10% | 12.8 fl oz | About 1.65 ppm salt |
| Unscented bleach, 8.25% | 15.5 fl oz | About 1.65 ppm salt |
| Unscented bleach, 6% | 21.3 fl oz | About 1.65 ppm salt |
| Cal-hypo, 65% | 2.05 oz | 0.71 ppm calcium |
| Cal-hypo, 73% | 1.83 oz | 0.71 ppm calcium |
| Dichlor, 56% | 2.38 oz | 0.91 ppm CYA |
| Trichlor, 90% | 1.48 oz | 0.61 ppm CYA |
| Lithium hypochlorite, 35% | 3.82 oz | Nothing that builds up |
The “also adds” column comes straight from the molecules. Trichlor carries three chlorines per cyanuric acid, so each ppm of chlorine brings 0.61 ppm of CYA; dichlor carries two, so it brings 0.91 ppm. That matches Orenda’s figure of about 6 to 6.5 ppm CYA from each pound of trichlor in 10,000 gallons.
Worked example
15,000 gallons, FC 1 ppm, CYA 40, using 12.5% liquid chlorine
- Target for 40 ppm CYA is 5 ppm, so raise by 4 ppm
- Available chlorine = 4 × 56,781 L ÷ 1,000 = 227.1 g
- Liquid chlorine = 227.1 ÷ 0.125 = 1,817 mL = 61 fl oz (about half a gallon)
Which chlorine to use
- Liquid chlorine or bleach adds nothing that builds up except a little salt. It’s the easiest way to keep CYA and calcium under control.
- Trichlor tablets are convenient but add CYA every week. Expect to drain some water each season if you rely on them.
- Dichlor dissolves fast and suits hot tubs and small pools, but it raises CYA faster than trichlor.
- Cal-hypo adds calcium, which is fine in a plaster pool with low calcium and a problem where calcium is already high.
Liquid chlorine loses strength in heat and sunlight. A jug that has sat in a hot garage for a month may be well under the label strength, so if your doses come up short, that’s the likely reason. For shocking algae or clearing cloudy water, use the pool shock calculator.
Frequently asked questions
How much liquid chlorine do I add to raise chlorine by 1 ppm?
About 10.2 fl oz of 12.5% liquid chlorine per 10,000 gallons, or 12.8 fl oz of 10%.
How much bleach do I put in my pool?
About 15.5 fl oz of 8.25% unscented bleach raises 10,000 gallons by 1 ppm, or 21.3 fl oz of 6% bleach. Use plain bleach without scents, splash-less additives or thickeners.
What should my free chlorine level be?
It depends on your stabilizer. The CDC minimum is 1 ppm without stabilizer and 2 ppm with it. With 40 ppm CYA, aim for about 5 ppm and don’t let it fall below 3.
Why does my chlorine keep dropping?
Sunlight, warm water, swimmers and algae all use it up. Low CYA lets the sun burn it off within hours. If it drops more than about 1 ppm overnight with the sun down, something in the water is consuming it, and a shock is the next step.
Is pool chlorine stronger than bleach?
Usually. Pool liquid chlorine is 10 to 12.5%, while household bleach is 6 to 8.25%. It’s the same chemical, sodium hypochlorite, so you just need more bleach.
Can I swim right after adding chlorine?
Once it has mixed in and free chlorine is at or below your target. With liquid chlorine and the pump running, that’s usually within an hour. Follow the product label.
Sources
- Home Pool and Hot Tub Water Treatment and Testing. U.S. Centers for Disease Control and Prevention (CDC), reviewed May 10, 2024.
- Model Aquatic Health Code, 5th edition (2024). CDC.
- Understanding the Chlorine/CYA Chart. Trouble Free Pool.
- What increases cyanuric acid (CYA) in a swimming pool?. Orenda Technologies.
Reviewed November 7, 2026. We check the formulas against the sources above and re-run the worked examples whenever we update this page. Found something wrong? Tell us.