Pool Chlorine Dosage Calculator

Calculate how much chlorine to add to raise free chlorine from current to target ppm, for liquid bleach, cal-hypo, dichlor, or trichlor products.

Inputs

Water volume of the pool in US gallons.

Free chlorine reading now (ppm). Use 0 if starting from none.

Desired free chlorine (typically 1โ€“3 ppm; 3 ppm is a common target).

Available chlorine percentage of the product you are using.

Result

Loading calculatorโ€ฆ
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General calculation reads

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How to use this calculator

  • Enter the pool volume in US gallons (use a pool-volume calculator if unsure).
  • Enter the current free-chlorine reading and your target (commonly 3 ppm).
  • Pick the chlorine product you have โ€” its available-chlorine percentage is built in.
  • Read the weight (and liquid volume or tablet count) to add, then re-test after circulating.

About this calculator

This calculator tells you how much chlorine product to add to raise a pool's free chlorine from its current reading to your target, typically 1โ€“3 ppm. The chemistry rests on one fact about water: a million US gallons weighs about 8.345 million pounds, so raising the whole pool by 1 ppm (one part per million by weight) of pure chlorine requires 8.345 pounds of chlorine per million gallons. The tool scales that to your pool volume and the desired increase, then divides by the product's available-chlorine percentage to convert pure chlorine into the actual weight of your product โ€” whether liquid chlorine, household bleach, cal-hypo, dichlor, or trichlor. For liquids it also gives a fluid-ounce volume, and for trichlor an approximate tablet count. Always add chlorine in the evening, run the pump, and re-test before swimming.

How it works โ€” the formula

ฮ”ppm = max(0, Target โˆ’ Current) Pure chlorine (lb) = ฮ”ppm ร— (Volume รท 1,000,000) ร— 8.345 Product (lb) = Pure รท (Available% รท 100) Product (oz) = Product lb ร— 16

One ppm by weight in a million gallons is 8.345 lb because that is the weight of one-millionth of a million gallons of water. Dividing by the product's available-chlorine fraction converts the pure-chlorine requirement into the dose of the actual product.

Sources: USGS โ€” Water density (8.34 lb/gal) ยท CDC โ€” Healthy Swimming: chlorine & pH

Worked examples

Example 1
10,000 gal, raise 0โ†’2 ppm with 65% cal-hypo
Inputs:
volume=10000, currentPpm=0, targetPpm=2, product=cal-hypo-65
Output:
2 ร— 0.01 ร— 8.345 รท 0.65 = 0.257 lb โ‰ˆ 4.1 oz
Example 2
20,000 gal, raise 1โ†’3 ppm with 12.5% liquid
Inputs:
volume=20000, currentPpm=1, targetPpm=3, product=liquid-12.5
Output:
2 ร— 0.02 ร— 8.345 รท 0.125 = 2.67 lb โ‰ˆ 42.7 oz (โ‰ˆ 34 fl oz)
Example 3
15,000 gal, raise 0โ†’1 ppm with 90% trichlor
Inputs:
volume=15000, currentPpm=0, targetPpm=1, product=trichlor-90
Output:
1 ร— 0.015 ร— 8.345 รท 0.90 = 0.139 lb โ‰ˆ 2.2 oz

Limitations

  • Assumes the product's labelled available-chlorine percentage; old product loses strength.
  • Liquid fluid-ounce figures use typical product densities and are approximate.
  • Does not model chlorine demand from contaminants โ€” re-test after dosing.

Pool-chemistry estimate. Follow product label directions and standard safety precautions; never mix chemicals.

Frequently asked

How much chlorine raises a pool by 1 ppm?+
About 8.345 pounds of pure (100%) available chlorine per million gallons, because a million gallons of water weighs roughly 8.345 million pounds and 1 ppm is one part per million by weight. For a real product you divide by its available-chlorine percentage โ€” e.g. 65% cal-hypo needs 8.345 รท 0.65 โ‰ˆ 12.8 lb per million gallons per ppm.
What free chlorine level should I target?+
Most authorities recommend keeping free chlorine between 1 and 3 ppm for residential pools, with 3 ppm a common target. Higher levels (and "shocking" to 10+ ppm) are used to clear algae or after heavy use, then allowed to fall back into range before swimming.
What is the difference between the chlorine products?+
Liquid chlorine and bleach are sodium hypochlorite (8โ€“12.5% available chlorine), pH-neutral-ish and unstabilised. Cal-hypo (65โ€“73%) is granular and adds calcium hardness. Dichlor (56%) and trichlor (90%) are stabilised โ€” they add cyanuric acid โ€” and trichlor is acidic, lowering pH. Pick based on your water chemistry needs.
Why does the product percentage matter so much?+
The percentage is how much of the product is actually available chlorine. A 90% trichlor tablet delivers far more chlorine per pound than 8.25% household bleach, so you need much less of it. The calculator divides the pure-chlorine requirement by this percentage to give the real dose.
Can I add it all at once?+
For routine dosing within a couple of ppm, yes โ€” add it to the water with the pump running and brush to disperse. For large increases or shocking, add gradually and never mix different chlorine products together or combine with acid, which can release dangerous gas.
Will trichlor and dichlor over-stabilise my pool?+
They can. Each adds cyanuric acid (stabiliser), which protects chlorine from sunlight but, above ~50 ppm, increasingly blunts chlorine's killing power. If you rely on stabilised chlorine, monitor cyanuric acid and switch to liquid or cal-hypo if it climbs too high.

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