Chlorine Dosage Calculator

The arithmetic is cleaner than it looks. One gallon of 12.5% liquid chlorine raises 10,000 gallons by 12.5 ppm — the percentage and the ppm line up, which is a coincidence of units worth remembering because it makes everything else mental arithmetic.

Halve the pool, double the effect. Halve the strength, halve the effect. A gallon of 10% in the same 10,000 gallons gives 10 ppm. Two ppm on a 10,000 gallon pool is about 20 fl oz, a little over a pint.

The coincidence survives the move to metric, because the trade percentage is grams per 100 mL. One liter of 12.5% raises 10,000 liters by 12.5 ppm, so two ppm in 10,000 liters is 160 mL.

For granular products the numbers are less friendly. 73% cal hypo takes about 1.8 oz by weight per 10,000 gallons per ppm, or about 14 g per 10,000 liters, which is why it gets sold by the pound and dosed by the scoop, and why pre-dissolving matters — undissolved granules sitting on the floor bleach vinyl and stain plaster.

The dose is only half the decision. What matters over a season is the passenger that comes with it, because none of it leaves when the chlorine is used up.

Trichlor adds about 0.6 ppm of cyanuric acid for every 1 ppm of chlorine. That is not a rounding error over a summer: a pool using 2 ppm a day gains roughly 36 ppm of stabilizer a month, so a season on tablets alone takes a pool from 30 ppm to past 100.

Dichlor is worse per dose at about 0.9 ppm of CYA per ppm of chlorine, which is why it belongs in a hot tub that gets drained quarterly rather than in a pool.

Cal hypo adds about 0.7 ppm of calcium hardness per ppm of chlorine, and that ratio holds whether the bag says 65% or 73%. A 30 ppm shock puts roughly 20 ppm of hardness in. Fine on soft fill water, a slow problem in Phoenix.

Liquid chlorine adds a little salt and nothing else, which is the whole argument for it as the daily dose on an established pool. The four products compared properly covers the trade-offs.

The number on a shock bag assumes a pool with no stabilizer in it, and almost no outdoor pool is that.

Cyanuric acid protects chlorine from sunlight by binding most of it, which means the more stabilizer you carry, the more total chlorine you need for the same sanitizing power. Shock level is about 40% of your CYA reading — 16 ppm at 40 ppm CYA, 28 ppm at 70.

That scaling is why a neglected pool is expensive to clear. At 100 ppm CYA you are chasing 40 ppm of free chlorine and holding it for days, which on a 20,000 gallon pool is over six gallons of 12.5% before you have topped it up once. Partial drainage is often genuinely cheaper than shocking through it.

Test CYA before shocking rather than after. It is the input that sets everything else, and guessing it is how people shock a pool three times and wonder why the algae came back.

This calculator gives you one dose. If you are clearing a bloom rather than topping up, the shock calculator works out the whole job — the repeat doses, the total cost, and whether diluting the stabilizer first would be cheaper.

The jug is not what it says. Liquid chlorine degrades from the day it is made, faster in heat and sunlight, and a jug that spent August in a hot shed can be well down on its label strength. Dosing 12.5% arithmetic from a jug that is now nearer 9% under-delivers by a quarter. Buy it fresh and often, and treat a slow-moving reading as a clue about the jug.

You tested too soon. A dose needs a full turnover to distribute — six to ten hours on a variable-speed pump at filtration speed. Testing at two hours measures the water near the returns.

Something is eating it. If chlorine disappears overnight rather than over a week, you have demand rather than a dosing problem: algae, high combined chlorine, or organic load. The overnight chlorine loss test separates those, and dosing harder without diagnosing is how people spend a season on it.

You dosed at noon. Unstabilized chlorine in full summer sun has a half-life measured in tens of minutes. Add it at dusk and it has all night to work.