Pool Chlorine Dose Calculator

The math for raising free chlorine in a pool is straightforward once you know the strength of what you are pouring in. The same one ppm of free chlorine costs different amounts of liquid, granular, or tablet depending on the product. People constantly overdose because they assume “shock is shock.”

Start from the water rather than from a table. Ten thousand gallons weighs 83,450 lb, so one part per million of that is 0.0834 lb, which is 1.34 oz of actual available chlorine. Divide by the strength printed on the tub and you have the dose:

• Cal-hypo granular (65% calcium hypochlorite): 1.34 ÷ 0.65 ≈ 2.0 oz by weight
• Dichlor granular (56% available chlorine): 1.34 ÷ 0.56 ≈ 2.4 oz by weight
• Trichlor tablet (90% available chlorine): 1.34 ÷ 0.90 ≈ 1.5 oz by weight

Those are rounded, and deliberately so. The percentage on the tub is a nominal figure that drifts as the product ages in a hot shed, so a second decimal place would be false precision.

Liquid is sold by volume, not weight, and the meaning of the percentage on the jug is not the same in every country. Those two come from the published figures rather than from the division above:

• Liquid bleach (10% sodium hypochlorite, pool-strength): 10.7 fl oz
• Liquid bleach (8.25% sodium hypochlorite, household): 13 fl oz

If you have ever wondered why granular feels like it does nothing, this is usually why. A tub of dichlor is only just over half chlorine by weight, so the scoop has to be nearly twice the size the number in your head says.

Multiply linearly with both pool size and the ppm you want to raise. A 20,000-gallon pool that reads 1 ppm when you want 4 ppm needs three ppm of correction across double the volume, which is six times the per-10k-per-ppm number.

A few practical notes that keep people out of trouble. Always test before you dose. The single biggest cause of green pools is people dosing on schedule without testing first; the single biggest cause of bleached liners is the opposite. A reliable test kit (Taylor K-2006 or any decent DPD kit) takes 90 seconds and saves you hundreds of dollars over a season.

Cyanuric acid changes the math. If your CYA is over 50 ppm, the effective chlorine is much lower than the test number suggests, and the standard advice is to keep free chlorine at roughly 7.5% of CYA. Trichlor and dichlor both add CYA every time you use them, which is why long-time pool owners stop using them and switch to liquid for routine dosing.

Pour liquid in front of a return jet, with the pump running, never near skimmers. Granular goes through a brush, and if your pool has a vinyl liner, pre-dissolve it in a bucket of pool water first. Cal-hypo landing directly on a liner will bleach a permanent pale patch into it. And do not mix two chlorine types in the same bucket. Trichlor and cal-hypo, when combined dry, can react vigorously enough to start a fire. This sounds like internet hyperbole, and it is not.