Pool Chlorine Calculator
Find how much liquid or granular chlorine to add to raise free chlorine by a target ppm.
The baseline: about 10.7 fluid ounces of 12.5% liquid chlorine per 10,000 gallons lifts free chlorine by roughly 1 ppm. Granular cal-hypo is figured near 2 ounces by weight per 10,000 gallons per ppm at 65%. We scale both to your pool size, target, and product strength. Add in the evening and retest before swimming.
How it works
Free chlorine is the sanitizer actually doing the work, and it gets used up by swimmers, sunlight, and debris. To bump it up, you add a measured amount of chlorine scaled to how much water you have.
The industry baseline is about 10.7 fluid ounces of 12.5% liquid chlorine per 10,000 gallons to raise free chlorine by roughly 1 ppm. Granular cal-hypo runs near 2 ounces by weight per 10,000 gallons per ppm at 65% strength. We scale both by your pool size, your target rise, and the strength printed on the jug.
So a 15,000-gallon pool that needs 2 more ppm of free chlorine takes about 32 fluid ounces of 12.5% liquid. Add it in the evening with the pump running, then retest before anyone swims.
Frequently asked questions
Liquid or granular — which should I use?
Liquid chlorine adds no extra calcium or stabilizer, which keeps your water balanced, but it's weaker and doesn't store well. Granular cal-hypo is concentrated and shelf-stable but adds calcium hardness over time. Many owners use liquid for regular dosing.
Why add chlorine at night?
Sunlight burns off unstabilized chlorine fast. Dosing in the evening lets it circulate and sanitize overnight instead of being destroyed by the afternoon sun before it can work.
My chlorine won't hold — what's wrong?
Usually it's cyanuric acid (stabilizer) that's too low, letting the sun eat the chlorine, or a hidden demand from algae or contaminants. Check your stabilizer level and consider a shock treatment if the water looks off.