Chlorine dose calculator
Four products, four different amounts for the same result. Put in your volume, the free-chlorine reading off the kit and the number you want, and this gives you the pour.
Enter the pool volume and the free-chlorine reading from your kit. WHO puts the working range at 1–3 mg/L for a public pool — the same number your test kit prints as ppm — and the bar below marks it.
Add
54.1fl oz
of Liquid chlorine 12.5% to move 2.4 ppm in 22,000 gallons.
Free chlorine after the dose
3ppm
What it measures
Free chlorine is the sanitiser still available to kill things. The WHO guideline for public and semi-public pools is blunt about the top of the range — free chlorine “should not exceed 3 mg/l” — and just as blunt about the bottom: a conventional pool with sound hydraulics disinfects adequately at about 1 mg/L held throughout the water. Everything this calculator does is arithmetic on that gap.
How to use it
- Enter the pool volume in gallons. A guessed volume gives a guessed dose, so measure it once and keep the number.
- Pick the product you are actually going to pour — the same gap needs a very different amount of each.
- Type the free-chlorine reading off your test kit and the reading you want. The bar shows where that target sits against the WHO 1–3 ppm band; the chemistry basics guide covers the other readings to take at the same time. A kit that reads total chlorine only cannot give you this number; the best pool water test kit comparison shows which kits read free chlorine.
- Read the dose. If you chose dichlor or trichlor, read the stabiliser line under it as well — that is the part that builds up week after week.
Worked example
A 15,000 gallon pool reads 1.0 ppm free chlorine and you want it at 3.0 ppm. These are the numbers the calculator returns for each product:
| Step | Working | Result |
|---|---|---|
| Gap to close | 3.0 − 1.0 ppm | 2 ppm |
| Chlorine needed | 15,000 gal × 3.785 L × 2 mg | 113.6 g |
| Liquid chlorine 12.5% | 113.6 g ÷ 0.125 g/mL = 908 mL | 30.7 fl oz |
| Cal-hypo 73% | 113.6 g ÷ 0.73 | 5.5 oz |
| Dichlor 56% | 113.6 g ÷ 0.56; CYA 2 × 0.9 | 7.2 oz, +1.8 ppm CYA |
| Trichlor 90% | 113.6 g ÷ 0.9; CYA 2 × 0.6 | 4.5 oz, +1.2 ppm CYA |
The stabiliser looks small on one dose. Fifteen weekly doses of dichlor at that size add 27 ppm, which is why the liquid versus tablets comparison counts stabiliser alongside price.
The four products, honestly
| Product | Available Cl | Per ppm / 10k gal | What it does to the water |
|---|---|---|---|
| Liquid chlorine 12.5% | 12.5% | 10.2 fl oz | Raises pH briefly, adds nothing else, loses strength in storage |
| Cal-hypo 73% | 73% | 1.8 oz | Adds calcium hardness — watch it in hard-water regions |
| Dichlor 56% | 56% | 2.4 oz | Adds ~0.9 ppm CYA for every ppm of chlorine |
| Trichlor 90% | 90% | 1.5 oz | Adds ~0.6 ppm CYA per ppm, and is strongly acidic |
Every figure above is computed from trade strength, not copied from a bucket label.
The tablet trap
A floater of trichlor is the easiest way to chlorinate and the fastest way to a pool that will not hold chlorine by August. Watch stabiliser climb and switch to liquid before it passes 100 ppm.
Dosage notes
Dose at dusk on an outdoor pool: unstabilised chlorine added at midday can be half gone in a couple of hours. Pour liquid slowly over a return jet with the pump running, never into the skimmer while a tablet sits in it. If the reading will not move at all, you are probably not short of chlorine — you are short of a proper SLAM or holding too much stabiliser.