Pool Chlorine Calculator
How much chlorine product raises a pool’s free chlorine to your target, from its volume and the available chlorine on the product label.
Result
Enter available chlorine on the label to see the result.
https://calcgallery.com/home-systems/pool-chlorine-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use the Pool Chlorine Calculator
- Choose the dose. Everyday chlorine and shock differ only in the level you are raising the water to; salt is for a salt chlorine generator.
- Enter the pool's volume, the free chlorine your test kit reads now, and the level you want.
- Choose the product and enter the available chlorine printed on its label. There is no default, because products differ several-fold.
- Read how much product to add. Follow the label's own instructions, and test again after the water has circulated.
- For salt, enter the salt reading now and the level your generator's manual asks for, and a bag size if you want a bag count.
The arithmetic
A part per million is by weight: one pound of chlorine in a million pounds of water. A US gallon of water weighs about 8.345 lb, so:
chlorine (lb) = gallons × (target − current) × 8.345 ÷ 1,000,000product = chlorine ÷ available chlorine on the labelShock and salt
Shocking uses exactly the same parts-per-million sum as an everyday dose; only the target is higher. It has its own field so the everyday target keeps its tighter range and the shock level is always one you entered.
A salt chlorine generator makes chlorine from dissolved salt, and the maker names the salt level it runs at. The sum is the same one again, with salt in place of chlorine and no label strength, because the salt is counted as pure:
salt (lb) = gallons × (target − current salt) × 8.345 ÷ 1,000,000bags = salt ÷ bag weight, rounded upSalt does not get used up the way chlorine does. It leaves the pool with water that splashes out, drains off a cover or is backwashed, and rain dilutes it, so a top-up is usually a few bags rather than a fresh start.
What else the products add
Stabilised chlorine, dichlor and trichlor, also adds cyanuric acid, which protects chlorine from sunlight but builds up with every dose and slows chlorine down at high levels. Calcium hypochlorite adds calcium hardness, which also accumulates. Neither shows up in a single dose, which is why they catch people out over a season. Test for them as well as for chlorine.
Test, do not trust the arithmetic
The calculation assumes the chlorine stays in the water. In practice sunlight, swimmers and debris use it up, sometimes quickly. Dose, let the pump circulate the water, then test again before swimming.
With your numbers
- Chlorine = volume × (target − current) ppm × 8.345 × 10⁻⁶ lb per gallon; product = chlorine ÷ label strength
Quick reference
An estimate from your figures: the product label governs, and a test after dosing tells you where you are.
| Job | Result |
|---|---|
| 21,000 gal, 1 to 3 ppm, liquid 12.5% | Product to add: 2.8 lb |
| Same, cal-hypo 65% | Product to add: 0.54 lb |
| Same, dichlor 56% | Product to add: 0.63 lb |
| Already at 3 ppm | Product to add: 0 lb |
| 80,000 L, 1 to 3 ppm, liquid 12.5% | Product to add: 1.28 kg |
| Shock 21,000 gal from 1 to 10 ppm, cal-hypo 65% | Product to add: 2.43 lb |
| Salt 21,000 gal from 0 to 3,200 ppm, 40 lb bags | Salt to add: 560.8 lb |
| Salt 80,000 L from 0 to 3,000 ppm, 25 kg bags | Salt to add: 240 kg |
| 10,000 gal, 0 to 2 ppm, trichlor 90% | Product to add: 0.19 lb |
Where the numbers come from
The fixed figures this calculator relies on, and the published source each one was checked against. How the calculators are built and checked
1 source
NIST Chemistry WebBook, water at 4 °C and 0.101325 MPa: 999.9749 kg/m³ (IAPWS-95), https://webbook.nist.gov/cgi/fluid.cgi?ID=C7732185, rounded to the conventional 1000 kg/m³ of NIST SP 811 (2008) Appendix B, centimeter of water, conventional = 98.0665 Pa.
Used for
- The conventional density of water, 1000 kg/m³. Pure water at 4 °C, its densest, is 999.97 kg/m³.
Common questions about the Pool Chlorine Calculator
- How much liquid chlorine do I need to raise my pool by 1 ppm?
- One part per million of 10,000 US gallons is about 0.083 lb, or 1.3 oz, of chlorine itself. How much product that means depends on its strength: at 12.5% available chlorine, divide by 0.125, so about 0.67 lb of product per 10,000 gallons for each ppm. Always check the label, which may give its own dosing figure.
- Why does the page need the product’s strength?
- Because the same amount of chlorine comes in very different packages. A liquid chlorine at 12.5% available chlorine and calcium hypochlorite granules at 65%, for example, differ by a factor of five: for the same pool and target, the liquid needs about five times the weight. A page that assumed one would tell you to add several times too much of the other.
- What is combined chlorine?
- Chlorine that has already reacted with contaminants, mostly from swimmers, into chloramines, which sanitise poorly and cause the typical pool smell. It is total chlorine minus free chlorine. Enter your total chlorine reading to see it.
- How much shock does my pool need?
- Enough chlorine to lift the free chlorine from where it is now to the shock level you are aiming for, divided by the strength on the label. Shocking is the same arithmetic as an everyday dose with a higher target, so the shock mode only gives the level its own field, which accepts levels above 20 ppm. The level itself is yours to choose from the product label or the guidance you follow.
- How much salt do I add to a pool?
- The difference between the salt level you read now and the level your generator asks for, as parts per million of the water. For 21,000 US gallons, raising salt from 0 to 3,200 ppm takes about 561 lb, or 15 bags of 40 lb. Most of a top-up is much smaller, and adding it in stages and testing between them avoids overshooting, which only more water can undo.
- Does the salt calculation allow for impurities in the salt?
- No. It counts the whole bag as sodium chloride. Generator makers name the salt they accept, usually a high-purity pool salt, and say what to avoid. Use the salt they specify.
- What target level should I use?
- This page does not recommend one. Use the guidance that comes with your test kit, the product label, or your local health rules for public pools. The CDC’s Model Aquatic Health Code is the reference many public codes draw on.