How much chlorine should you add?

Almost every answer to this question online gives you a fixed number — 1 to 3 ppm, usually — and then a dosing table. The table is fine. The fixed number is the problem, because chlorine’s strength is a ratio, not an amount.

Cyanuric acid binds most of the chlorine in your water and holds it in reserve. That is genuinely useful — without it, sunlight would strip an unprotected pool in hours. But bound chlorine is not working chlorine, so the more CYA you have, the more total chlorine you need to keep the same amount actually doing its job. At CYA 30 you need about 3.5 ppm. At CYA 100 you need around 11 ppm for the same effect.

This is why people dose “correctly” all season and still get algae in water that tests fine. They are hitting a number that was right in June and stopped being right as their tablets pushed CYA upward every week.

Sunlight destroys unstabilised chlorine fast, so a large dose added at midday loses a meaningful share of itself before it does any work. Add it after sundown with the pump running and you get the whole night to circulate.

Add liquid slowly around the perimeter of the deep end rather than in one spot, and never pour any chlorine product through the skimmer — if it meets anything acidic in the plumbing you have created a genuine hazard. Granular products should be pre-dissolved in a bucket of pool water unless the label says otherwise, because undissolved granules sitting on a vinyl liner bleach it.

Then re-test. A dose you did not verify is a guess with extra steps, and which form you used changes what else it left behind.

1. Demand is outrunning you. If algae is established or the water is carrying a lot of organics, chlorine is being consumed as fast as you add it. The test reads low not because the dose failed but because it was eaten. The fix is not a bigger single dose — it is holding shock level over days, re-testing and re-dosing, until the level finally holds overnight.

2. Your chlorine is old. Liquid chlorine degrades in storage, faster when warm, and a jug that has sat in a hot garage for a month can be meaningfully weaker than the label. If your usual dose stopped working and nothing else changed, suspect the jug. This is the main practical argument for buying it in smaller quantities more often.

3. You are dosing against the wrong target. Covered above, and it is the most common of the three. Before concluding anything is wrong with the pool or the product, confirm CYA with a drop test.

One thing that is not on this list: the pool being “too big for chlorine to work.” Volume changes the dose, never the outcome.

If more than one reading is out at once, correct them in order — the levels chart explains the sequence and why doing it out of order means chasing your tail.

The calculator above asks for your current reading rather than your daily loss, because that is what you can measure today. But the figure that decides how often you are back here dosing is daily chlorine loss, and it is largely a function of UV and season length — which is to say, geography.

A pool in the desert Southwest can lose double what a Pacific Northwest pool loses on the same day, and runs that demand for three times as many weeks a year. Use the regional figures below as a starting point, then measure your own: test in the evening, test again 24 hours later, take the difference.

Far less than people fear. The level used to clear an algae bloom is several times a normal target and is still considered swimmable, so an accidental double dose is a nuisance rather than an emergency. What you do about it depends on how far over you went.

Moderately over — say double your target. Do nothing. Take the cover off, run the pump, and let sunlight bring it down; an uncovered pool sheds chlorine quickly, especially at low CYA. It will usually be back in range within a day.

Substantially over. Same answer, more patience, and stay out until it comes down — not because a high reading is dangerous to touch, but because at very high levels chlorine is hard on swimwear, hair and vinyl. Nobody needs to drain a pool over this.

The real cost of overshooting is not safety, it is collateral chemistry. If you overshot with cal-hypo you have also added calcium you cannot remove, and if you overshot with tablets or dichlor you have added cyanuric acid you cannot remove. The chlorine sorts itself out in a day. The passenger doesn’t.

Test too soon after dosing and you are not measuring your pool, you are measuring the plume of chemical that hasn’t mixed yet. Give it at least one full turnover with the pump running — for most residential pools that is somewhere between four and eight hours — before you trust a number.

There is a second reason to wait that catches people during algae recovery. Very high chlorine bleaches DPD reagent: the sample flashes pink and then goes clear, which reads as zero. A reading of zero immediately after a heavy dose almost always means the opposite of what it appears to. Dilute the sample or wait, rather than adding more.

And take your reading at the same time of day when you are tracking a trend. Chlorine falls through the daylight hours and holds overnight, so an evening reading compared against a morning one will look like a loss that never happened.