How much chlorine to add to your pool: the formula

The formula in one line

Chlorine dosing is plain arithmetic, because 1 ppm in water is the same as 1 mg per litre. To raise free chlorine (FC) by a given number of ppm you need that many milligrams of chlorine for every litre of pool water. The only extra piece is the strength printed on your bottle or bucket.

For liquid chlorine (sodium hypochlorite) labelled with a trade percent X, which means X grams of available chlorine per 100 mL:

• mL of liquid chlorine = ΔFC (ppm) × pool volume (litres) ÷ (10 × X)

For dry products the same formula gives grams instead of millilitres, with X being the available chlorine percentage on the label (for example 65 for cal-hypo 65 %).

If you think in US gallons, multiply gallons by 3.785 to get litres, and divide millilitres by 29.57 to get fluid ounces.

Worked examples

Metric: 50 m³ pool, 12.5 % liquid chlorine

You tested 2 ppm FC and want 6 ppm. ΔFC = 4 ppm, volume = 50,000 L.

4 × 50,000 ÷ (10 × 12.5) = 1,600 mL of 12.5 % liquid chlorine.

Imperial: 15,000 gallon pool, 10 % liquid chlorine

Same jump of 4 ppm. 15,000 gal × 3.785 = 56,775 L.

4 × 56,775 ÷ (10 × 10) ≈ 2,271 mL ≈ 77 fl oz, a little over half a gallon.

Quick reference: 1 ppm in 10,000 US gallons

Product	Amount for +1 ppm FC	Side effect
Liquid chlorine 10 %	≈ 379 mL (12.8 fl oz)	None besides a little salt
Liquid chlorine 12.5 %	≈ 303 mL (10.2 fl oz)	None besides a little salt
Cal-hypo 65 %	≈ 58 g (2.1 oz)	+0.7 ppm calcium hardness
Dichlor 56 %	≈ 68 g (2.4 oz)	+0.9 ppm CYA
Trichlor 90 %	≈ 42 g (1.5 oz)	+0.6 ppm CYA

The side effects column matters more than it looks. Every ppm of chlorine you add with dichlor or trichlor leaves 0.6–0.9 ppm of stabilizer behind, and stabilizer does not break down or evaporate. A summer of tablets can push CYA from 30 to 80 or more, and then you need much more chlorine to get the same sanitizing effect. Cal-hypo does the same with calcium. Liquid chlorine adds neither, which is why many experienced pool owners use it for daily dosing.

How much is "enough"? Your target depends on CYA

The formula tells you how much to add; it does not tell you where to aim. Public guidance gives floors: the CDC recommends at least 1 ppm free chlorine in pools, at least 2 ppm in pools that use stabilizer, and at least 3 ppm in hot tubs.

For a home pool with stabilizer, those floors are not enough. Cyanuric acid (CYA) binds most of the free chlorine and holds it in reserve, so the share that is actively sanitizing shrinks as CYA goes up. That is why Trouble Free Pool sets chlorine targets as a fraction of CYA: minimum around 7.5 % of CYA, a target around 11.5 %, and a shock (SLAM) level around 40 %. Some typical values from their chart:

CYA, ppm	Minimum FC	Target FC	SLAM FC
30	2	4–6	12
40	3	5–7	16
50	4	6–8	20
70	5	8–10	28

So the realistic workflow is: test FC and CYA, look up the target range for your CYA, aim for the middle of that range, and dose the difference. A deeper explanation is in our article on the FC/CYA ratio.

Step by step

1. Know your volume. For a rectangle: length × width × average depth (metres gives m³, multiply by 1,000 for litres). Round pools: 3.14 × radius² × average depth. An error of 20 % in volume is an error of 20 % in every dose.
2. Test FC and CYA. A drop-based FAS-DPD kit is far more precise than a strip at the higher chlorine levels you will see with stabilizer.
3. Pick the target from the FC/CYA chart, not from the bottle label.
4. Calculate with the formula above, using the strength printed on your product.
5. Add it right. Pour liquid chlorine slowly in front of a return jet with the pump running. Never mix chlorine with acid or with another chlorine product, not even in the same empty bucket.
6. Re-test. After the water has circulated, check that you landed where the maths said. If you are consistently lower, your bottle is weaker than the label or your volume estimate is off.

Why the label amount is often wrong

A label like "one cup per 5,000 gallons" assumes an average starting level, an average target and fresh product. Your pool has none of those. Three real-world corrections:

• Bleach loses strength. Sodium hypochlorite slowly decomposes, faster when it is warm. A jug that sat in a hot garage all summer may be well below its printed strength, so your doses come up short.
• Trade percent versus weight percent. Pool chlorine is often labelled in trade percent (grams per 100 mL); household bleach is labelled by weight. The formula above uses trade percent; if your label shows weight percent, the result will be slightly off. Re-testing after dosing tells you how much.
• Daily loss. Sunlight and swimmers consume chlorine every day. A pool that loses 3 ppm a day needs more than just "topping up to target" once a week; dose so you stay above the minimum until the next test.

Hot tubs are a different case

Hot tubs hold little water, run hot, and carry a heavy bather load per litre. The same formula works, but the volume is tiny, so measuring precision matters: in a 1,500 L spa, 15 mL of 10 % liquid chlorine already adds 1 ppm. Note also that the CDC recommends not using cyanuric acid or stabilized chlorine products in hot tubs.

Doing it without the arithmetic

If you would rather not run the formula by hand every time, PoolDose does it on your iPhone: you enter your test numbers once, choose the chlorine product and strength you actually own, and it shows the dose in millilitres or fluid ounces for your exact volume, with the target taken from the FC/CYA chart. It is a calculator, not a replacement for the product label, so the safety directions on the bottle still apply.