How to calculate how much chlorine to add
Chlorine is measured in parts per million (ppm) by weight. A gallon of water weighs about 8.34 lb, so raising a pool by one ppm takes a small, predictable weight of pure chlorine. A product only supplies part of that weight as chlorine, and its strength tells you how much: 10% liquid chlorine is 10% available chlorine.
Available chlorine (lb) = gallons × (target ppm − current ppm) × 8.34 ÷ 1,000,000
Product (lb) = available chlorine ÷ strength (0.10 for a 10% product)
Liquid (gallons) = product pounds ÷ pounds per gallon
Fluid ounces = gallons × 128
Liquid chlorine is converted from weight to volume with a density of 9.8 lb per gallon for 10% and 10.0 lb per gallon for 12.5%. Granular products are shown by weight, in ounces and pounds. If you do not know your pool volume, find it with the pool volume calculator.
Worked example: raising a 15,000 gallon pool from 1 to 3 ppm
First find the pounds of available chlorine: 15,000 × (3 − 1) × 8.34 ÷ 1,000,000 = 0.2502 lb. Then divide by the strength of your product.
- 10% liquid chlorine: 0.2502 ÷ 0.10 = 2.502 lb, which is 2.502 ÷ 9.8 = 0.255 gallons, or 32.7 fluid ounces
- 12.5% liquid chlorine: 0.2502 ÷ 0.125 = 2.002 lb, which is 2.002 ÷ 10.0 = 0.200 gallons, or 25.6 fluid ounces
- 65% cal-hypo: 0.2502 ÷ 0.65 = 0.385 lb, or 6.2 ounces by weight
To shock a pool, set the target to the shock level printed on your product label and use the same method.
Chlorine needed for 1 ppm in 10,000 gallons
Use this table as a quick check, and multiply by the number of ppm and by the pool volume divided by 10,000. For 1 ppm in 10,000 gallons, the available chlorine is 0.0834 lb.
| Product | Strength | Amount |
|---|---|---|
| Liquid chlorine | 10% | 10.9 fl oz |
| Liquid chlorine | 12.5% | 8.5 fl oz |
| Calcium hypochlorite (cal-hypo) | 65% | 2.05 oz by weight |
| Dichlor | 56% | 2.38 oz by weight |
| Trichlor | 90% | 1.48 oz by weight |
Choosing a chlorine product, and staying safe
Liquid chlorine and cal-hypo are unstabilized, so they do not add cyanuric acid (the sunscreen for chlorine). Dichlor and trichlor are stabilized and do add it, which builds up with repeated use. Trichlor is most often sold as slow-dissolving tablets, so check the label before using it any other way. Liquid chlorine loses strength as it ages, so the amount for a fresh jug can differ from an old one.
Safety: follow the product label for dosing, handling and storage. Never mix chlorine products with each other or with acid, keep them in their original containers away from children, wear eye protection and gloves, and retest after the water has circulated. For a saltwater pool, the pool salt calculator covers the salt side.
Frequently asked questions
How much liquid chlorine raises 1 ppm in 10,000 gallons?
About 10.9 fluid ounces of 10% liquid chlorine, or about 8.5 fluid ounces of 12.5% liquid chlorine. Scale the amount up or down for your pool volume and the number of ppm you need to add.
How much chlorine do I add to a 15,000 gallon pool?
To raise it from 1 ppm to 3 ppm you need 0.25 lb of available chlorine, which is about 32.7 fluid ounces of 10% liquid chlorine or 6.2 ounces of 65% cal-hypo. Enter your own readings in the calculator for other targets.
What is a good free chlorine level for a pool?
Public health guidance commonly cites a minimum of 1 ppm free chlorine for pools and 3 ppm for hot tubs, and many pool owners keep the level a little higher. Follow the advice for your type of pool and test kit.
What is the difference between 10% and 12.5% liquid chlorine?
The 12.5% product is stronger, so you need about a fifth less of it by volume for the same result. Both are sodium hypochlorite, and both lose strength as they age.
Does this calculator account for chlorine demand and sunlight?
No. It works out the product needed to raise the reading by the gap you enter. Sunlight, bather load, organic matter and the stabilizer level change how fast chlorine is used up, so retest after the water has circulated and adjust.
More free calculators
These calculators give estimates for planning only. Densities, bag sizes and coverage vary by supplier, so confirm quantities before you order.