How to use the pool chlorine calculator
Enter the pool's volume, your current free chlorine test result and the level you want. Choose the product and enter the strength from its label. The calculator gives the amount to add and how much of it is actually chlorine. Products differ a lot in strength: a pound of trichlor carries about 60% more chlorine than a pound of 56% dichlor.
For liquid chlorine and bleach, labels usually give sodium hypochlorite as a percent by weight. The calculator converts that to grams of available chlorine per liter with OxyChem's published tables, because a jug labeled "10%" holds about 110 g/L, not 100.
The formulas
available chlorine (lb) = gallons × ppm increase × 8.3454 ÷ 1,000,000
dry product (lb) = available chlorine ÷ (available % ÷ 100)
liquid (gal) = available chlorine ÷ (g/L × 0.0083454 lb per gal per g/L)
Worked example: 15,000 gallons from 1 to 4 ppm with 10% liquid chlorine
- Increase: 3 ppm. Available chlorine needed: 15,000 × 3 × 8.3454 ÷ 1,000,000 = 0.376 lb.
- A 10% (by weight) sodium hypochlorite solution holds about 109.8 g/L of available chlorine, which is 0.916 lb per gallon.
- Liquid needed: 0.376 ÷ 0.916 = 0.41 gal, about 52 fl oz.
- The same job with a 65% calcium hypochlorite shock takes 0.58 lb (9.3 oz).
Product strengths
| Product | Available chlorine | Side effect |
|---|---|---|
| Liquid chlorine / bleach | Label % sodium hypochlorite (by weight) or trade % | Raises pH slightly |
| Calcium hypochlorite (cal-hypo) | Label % calcium hypochlorite × 0.99 (US shocks about 52-73%) | Adds calcium hardness |
| Sodium dichlor, dihydrate | about 56% | Adds cyanuric acid |
| Sodium dichlor, anhydrous | 62% minimum | Adds cyanuric acid |
| Trichlor | 90% minimum | Adds cyanuric acid, lowers pH |
Dosing safely
- Retest first and after. Test free chlorine, add the dose with the pump running, circulate at least an hour, and test again before adding more.
- Never mix chemicals. Cal-hypo and trichlor can react violently. Add products separately, to the water, never water to the product.
- Store chlorine cool and dry, away from acids and out of the sun. Liquid chlorine loses strength quickly in heat.
- Target depends on stabilizer. The right free chlorine level rises with cyanuric acid; outdoor pools with stabilizer need more chlorine than the bare minimum. Follow your test kit's guidance.
Questions people ask
How much liquid chlorine do I add to raise free chlorine 1 ppm?
For every 10,000 gallons, about 0.083 lb of available chlorine raises it 1 ppm. A jug labeled 10% sodium hypochlorite holds about 110 g/L of available chlorine, so that's roughly 11.7 fluid ounces per 10,000 gallons per ppm.
How much shock for a 15,000 gallon pool?
It depends on how far you need to raise free chlorine. To raise it 10 ppm with a 65% calcium hypochlorite shock: 15,000 × 10 × 8.345 ÷ 1,000,000 ÷ (0.65 × 0.992) = 1.9 lb. Many shock bags are 1 lb, so that's two bags.
What does 12.5% liquid chlorine mean?
Pool-supply liquid chlorine is often sold by 'trade percent', which is grams of available chlorine per liter divided by 10. A 12.5% trade solution has 125 g/L, and one gallon raises 10,000 gallons by about 12.5 ppm. Retail jugs usually list sodium hypochlorite as a percent by weight instead, which the calculator converts using OxyChem's tables.
What is the difference between cal-hypo, dichlor and trichlor?
Cal-hypo (calcium hypochlorite) adds calcium; dichlor and trichlor add cyanuric acid (stabilizer) along with chlorine. Over a season, stabilized products can push cyanuric acid high enough to weaken chlorine, so many pool owners use liquid chlorine or cal-hypo for routine dosing.
How long after adding chlorine can I swim?
Follow the product label. Most say to wait until free chlorine has dropped back into the normal swimming range, which you confirm by testing.
Sources and assumptions
- OxyChem Sodium Hypochlorite Handbook: trade percent = g/L available chlorine ÷ 10; Table 2 converts wt % sodium hypochlorite to g/L (used for label percentages)
- OxyChem chlorinated isocyanurates: trichlor (ACL 90 PLUS) minimum 90%, sodium dichlor (ACL 60) minimum 62%, dichlor dihydrate (ACL 56) about 56% available chlorine
- HTH Pool Care Shock: 52% calcium hypochlorite (example US label)
- HTH calcium hypochlorite (commercial): about 70% available chlorine, 78% for shock grade
- US labels list calcium hypochlorite by weight. Each pound of Ca(OCl)₂ supplies 0.992 lb of available chlorine (2 × 70.906 ÷ 142.98, from molar masses), so the calculator multiplies the label % by 0.992.
- Pounds of available chlorine = gallons × ppm × 8.3454 ÷ 1,000,000. Chlorine weakens in storage, especially liquid in hot weather, so older product delivers less: that's another reason to retest.
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.