How much chlorine to add to a pool

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Dry products are ounces by weight; liquids are fluid ounces. Percentages are the strength printed on the bottle or bucket — check yours, because 10% and 12.5% liquid chlorine are both sold as "pool shock" and the dose differs by a quarter. Cal-hypo is sold at several strengths and the difference is real: 2.1 oz of 65% does the same job as 1.8 oz of 73%.

Worked example

A 15,000 gallon pool tests at 1 ppm free chlorine. You want 4 ppm, so you need to raise it by 3 ppm.

From the chart, 12.5% liquid chlorine is 15.4 fl oz per ppm. 15.4 × 3 = about 46 fl oz, or 5.8 cups.

Pour it slowly around the deep end with the pump running, then wait 30 minutes and retest before deciding you need more. Chlorine needs a full turnover to read accurately, and the most common way people overshoot is testing too early and adding twice.

Nothing raises only chlorine

Every chlorine product brings something else with it. Over a season those extras accumulate, and they are the usual reason a pool that "tests fine" stops responding to chlorine at all.

• Dichlor adds about 0.9 ppm cyanuric acid for every 1 ppm of chlorine. Raise chlorine by 3 ppm daily for a month and you have added roughly 80 ppm of CYA — enough to lock up your chlorine. CYA does not evaporate or burn off; the only way down is draining and refilling.
• Cal-hypo adds about 0.7 ppm calcium hardness per 1 ppm of chlorine at 65% strength — 0.6 ppm if you buy the 73% product, because it takes less of it. In hard-water areas this is what eventually scales your heater and clouds the water.
• Liquid chlorine adds about 0.8 ppm salt per 1 ppm of chlorine and nothing else. That is why it is the workhorse for routine dosing — but it loses strength on the shelf, so an old jug doses weaker than the label says.
• Trichlor tablets add cyanuric acid too, and they are acidic — they pull pH down and take 0.9 ppm of alkalinity with every ppm of chlorine as they dissolve.

The practical rule: use stabilized chlorine (dichlor, trichlor) to establish CYA early in the season, then switch to liquid chlorine or cal-hypo for day-to-day dosing once CYA is where you want it.

A 400 gallon spa needs roughly 1/25th of the 10,000 gallon dose — well under a teaspoon of dichlor per ppm. At that scale a casual scoop is an enormous overdose, which is why spa water so often swings between "no chlorine" and "burns your eyes."

Hot water also burns chlorine off far faster than a pool, so spas are dosed little and often rather than in weekly slugs.

• If the product label and this page disagree, follow the label.
• Never mix chemicals with each other — especially chlorine products with acid.
• Add chemicals to water, never water to chemicals.
• Add one thing at a time, circulate, and retest before adding the next.
• When in doubt, add less. You can always add more; you cannot take it out.

These figures are general pool chemistry for typical residential water. They are not a substitute for your product labels or for a test kit.

The chart is generated by the same dosing engine that runs inside PoolDose, so the app and this page cannot drift apart. The coefficients are ingredient-based rather than brand-based and are checked against published dosing tables; the app's test suite pins the expected values, so a change to the chemistry has to change the tests too.

In the app you enter your actual pool volume and the strength printed on your own product, and it splits the dose when a single addition would be too large.