Pool chlorine and shock calculator
How much chlorine do I add? Choose your product, read its strength off the label and pick what you want to do: top up free chlorine, reach breakpoint or give the shock dose on the label. You get the amount in US and metric units, the math behind it and what else the product adds to your water.
Safety first
- NEVER MIX CHEMICALS. Never combine pool chemicals with each other, in a bucket, a scoop, a feeder or the skimmer. Add one product at a time, directly to the pool water as its label directs.
- FOLLOW THE PRODUCT LABEL. The label is the law for dose, protective equipment and how long to wait before swimming. If this calculator and the label disagree, follow the label.
- KEEP OUT OF REACH OF CHILDREN. Store pool chemicals in their original, closed containers, locked away from children and pets.
More safety rules
- Never mix chlorine and acid. Mixing chlorine products with muriatic (hydrochloric) acid or sodium bisulfate can release toxic chlorine gas. Add them on different occasions and let the pump circulate in between.
- Never mix different chlorine products. Calcium hypochlorite (cal-hypo shock) and trichlor or dichlor can start a fire or explosion, even in small amounts. Use separate, clean, dry scoops and never put cal-hypo in a tablet feeder.
- Always add chemical to water, never water to chemical. If the label tells you to pre-dissolve, fill a clean plastic bucket with pool water first, then add the product.
- ONE CHEMICAL AT A TIME. Open one container at a time, close it before opening the next, and retest before adding anything else.
- Protect yourself. Wear chemical-splash goggles and chemical-resistant gloves. Read the product label and Safety Data Sheet before handling.
- Wait before swimming. Do not swim until free chlorine has dropped to the level allowed on your product label (4 ppm or lower on the labels we checked). Measure free chlorine again with your test kit before anyone gets in the pool.
- Liquid chlorine loses strength. An old or warm jug can be much weaker than its label. Retest after dosing instead of adding more blindly.
- STORE SAFELY. Keep pool chemicals in their original, closed containers in a cool, dry, ventilated place, apart from each other and out of reach of children and pets.
Sources: CDC Pool Chemical Safety poster and toolkit; EPA-registered labels 550-198, 748-311 and 1258-1237; CCOHS.
Add about
How we got this
Never mix chemicals. Add one product at a time, as the label directs, and retest before adding anything else. Safety rules
The three modes
Raise free chlorine to a target. The everyday case: your test kit shows a free chlorine (FC) level and you want a higher one. The calculator adds the difference. Stabilized pools need more free chlorine: a peer-reviewed study (Falk et al. 2019, Water 11(6):1314) uses a ratio of no more than 20 ppm cyanuric acid per 1 ppm free chlorine. If you enter your CYA, the calculator warns when your result is above that ratio.
Breakpoint. Combined chlorine (CC) is total chlorine minus free chlorine. CDC's Model Aquatic Health Code describes breakpoint chlorination as adding about 10 times the combined chlorine level. With total chlorine at 1.5 ppm and free chlorine at 1.0 ppm, that is 5.0 ppm to add.
Shock dose from your label. Shock directions differ between products. The liquid chlorine label we checked, EPA label 550-198 (a 12.5 % product registered for institutional and industrial use), gives 5 to 10 ppm. Enter the ppm your own label gives and the calculator converts it into an amount for your pool. Do not swim until free chlorine is back down to the level on your label.
How much product per 1 ppm
For a 10,000 US gallon pool, raising free chlorine by 1 ppm takes 37.9 grams of available chlorine. How much product that is depends on its strength:
| Product (strength on the label) | Per 1 ppm in 10,000 US gal | Per 1 ppm in 50,000 L |
|---|---|---|
| Cal-hypo 65 % | 2.05 oz | 77 g |
| Cal-hypo 75 % | 1.78 oz | 67 g |
| Dichlor 56 % | 2.38 oz | 89 g |
| Trichlor 90 % | 1.48 oz | 56 g |
| Liquid chlorine 10 %, trade-% | 12.80 fl oz | 500 mL |
| Liquid chlorine 12.5 %, trade-% | 10.24 fl oz | 400 mL |
| Liquid chlorine 12.5 %, weight-% | 8.96 fl oz | 350 mL |
What else the product adds
Chlorine products are not only chlorine. Trichlor and dichlor contain cyanuric acid (stabilizer), and cal-hypo adds calcium. From the chemical formulas, every 1 ppm of free chlorine from trichlor adds about 0.61 ppm CYA, dichlor adds 0.91 ppm CYA, and cal-hypo adds 0.71 ppm calcium hardness (as CaCO3). Cal-hypo products also contain other calcium salts, so treat the calcium figure as an estimate. Over a season these side effects add up, which is why the calculator shows them with every dose.
Where the sources disagree
- Liquid chlorine labels. EPA label 550-198 lists 12.5 % sodium hypochlorite by weight, but its own shock directions (52 fl oz for 5 ppm in 10,000 gallons) match the trade-% strength. With the weight-% the label states, we calculate 44.8 fl oz, 14 % less than the label's dose. With trade-% we calculate 51.2 fl oz. Follow your label and retest after dosing.
- Rounded dosing tables. A state health department table (Indiana DOH, adapted from the NSPF Pool & Spa Operator Handbook) gives doses that are rounded: for dichlor 56 % and trichlor its 10 ppm values are about 6 % off its own 1 ppm values × 10.
- A simplified formula sheet uses one divisor for “10 %” liquid chlorine that sits between the two bases. It calls the number a “usable approximation”.
Every case we test, including these disagreements, is listed on the methodology page. This calculator covers home chlorine pools only: no spas, hot tubs or bromine.
Disclaimer: GoodToSwim.com gives estimates for home (residential) chlorine pools based on published primary sources. It does not cover spas, hot tubs or bromine, and it does not report water quality at beaches, lakes, rivers or other open water. It never tells you that your water is safe to swim in. Results depend on your test accuracy, your pool volume and the real strength of your product. They do not replace the product label, local health regulations or a qualified pool professional. Public and commercial pools must follow local code.