Cyanuric Acid Calculator

Cyanuric acid calculator for pool stabilizer: work out how much CYA to add to hit your target, or how much water to replace when the level is too high, with the chlorine targets it sets.

Quick answer

Cyanuric acid is sunscreen for chlorine, and its level sets how much chlorine your pool needs. Enter your volume and your measured reading: raising it gives a weight of granular stabilizer (13 oz per 10 ppm in 10,000 gal, or 379 g per 10 ppm in 37,854 L), and lowering it gives a share of the water to replace, because nothing removes cyanuric acid chemically. Aim for 30–50 ppm outdoors, 60–80 ppm with a salt-water generator, and zero indoors.

Your pool and your stabilizer reading

Pool volume

≈ 37,854 L. Not sure? Work it out from your pool’s shape.

Units

Which way are you moving it?

Stabilizer now (ppm)

A drop-dilution test reads 0 as “under 20”. Send a sample out if you need a real number down there.

Target stabilizer

Outdoor pools: 30–50 ppm. Salt-water generators run higher, 60–80 ppm. Indoors: 0.

Add granular stabilizer (cyanuric acid)

1.7 lb

US units

757 g

Metric

Takes stabilizer from 20 to 40 ppm. Granular stabilizer dissolves slowly, so do not tip it into the skimmer — it will sit in the filter and eat it. Put it in an old sock, hang the sock in front of a return jet, and squeeze it every few hours. Leave the pump running, do not backwash for a week, and do not retest for a week either: an early reading comes back low and that is how people end up double-dosed and draining.

What that stabilizer level asks of your chlorine

Higher stabilizer binds more of the chlorine, so the level a pool needs scales with it — this is why one fixed “keep it at 3 ppm” number cannot be right for every pool.

At 20 ppm of stabilizer, the everyday band and the algae level are held up by a floor instead of a percentage: this low, the percentage asks for less chlorine than a pool needs to work at all. The published chart never prints an everyday band under 4–6 ppm, and that floor governs every reading below 40 ppm of stabilizer. It never prints an algae level under 10 ppm either, and that one governs below 25. The minimum column is the exception at every reading and stays at the unrounded 7.5%.

1.5

Minimum FC (ppm)

4–6

Everyday band (ppm)

To clear algae (ppm)

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What Stabilizer Actually Does

Cyanuric acid — sold as stabilizer, conditioner, or already mixed into dichlor and trichlor — is sunscreen for chlorine. Without it, ultraviolet light tears free chlorine apart within hours: an unstabilized outdoor pool can lose most of its chlorine over one sunny afternoon. Cyanuric acid bonds reversibly with the hypochlorous acid doing the sanitising, parks most of it in a reserve sunlight cannot reach, and releases it as the working chlorine is used up.

That reserve is the whole trade. Chlorine held in the reserve is not sanitising while it is held, so the more stabilizer in the water, the more total chlorine you have to carry for the same killing power. That is not a rule of thumb; it is the reason the published FC/CYA chart is a set of percentages rather than a set of numbers.

Why More Is Not Better

At 30 ppm of cyanuric acid, a pool needs a minimum of 2.3 ppm free chlorine and takes 12 ppm to clear algae. At 80 ppm the minimum is 6 ppm — 2.6 times the chlorine for the same pool doing the same job — and clearing algae takes 32 ppm. Nothing about the water got safer; the bill got bigger and the margin for a missed week got thinner. This is why “my chlorine keeps disappearing” is so often a stabilizer problem in disguise: the reading that used to be fine is now below the minimum, and nobody moved the chlorine.

The Puck Trap

Almost nobody adds too much stabilizer on purpose. It arrives with the chlorine. Trichlor is about half cyanuric acid by weight, so a feeder holding free chlorine up by 2 ppm a day adds about 37 ppm of stabilizer over a 30-day month. Dichlor is worse per unit of chlorine — the same run adds around 55 ppm. Two months of pucks on a pool that started the season correct is enough to push it out of the band, which is exactly why a pool that behaved in June fights algae in August on an unchanged routine.

The fix is not to stop using pucks; it is to know what they cost. Test cyanuric acid monthly through the season. Once you are in band, switch the routine chlorine to liquid or cal-hypo, which add none, and keep the stabilized products for the start of the year when the stabilizer is wanted. The pool chemical calculator shows the stabilizer and calcium each product carries on the same screen as its dose, and the pool volume calculator gives the number every one of these doses depends on. Salt-water pools have their own reason to run stabilizer high, and their own arithmetic — the pool salt calculator covers that side.

Getting It Back Down

There is one method and it is water. Draining and refilling dilutes cyanuric acid in direct proportion — half the water out halves the reading — and nothing else does. It is not consumed by chlorine, not broken down by sun, and not removed by any product with published evidence behind it. Do it in stages rather than in one go if the pool is vinyl or fibreglass or the water table is high, because an empty shell can float or crease. And if you live somewhere it rains, the pool has been quietly doing this for you all along.

Free chlorine your pool needs at each stabilizer level

Cyanuric acid	Minimum free chlorine	Everyday target	To clear algae
20 ppm	1.5 ppm	4–6 ppm	10 ppm
30 ppm	2.3 ppm	4–6 ppm	12 ppm
40 ppm	3 ppm	4–6 ppm	16 ppm
50 ppm	3.8 ppm	5–7.5 ppm	20 ppm
60 ppm	4.5 ppm	6–9 ppm	24 ppm
70 ppm	5.3 ppm	7–10.5 ppm	28 ppm
80 ppm	6 ppm	8–12 ppm	32 ppm
90 ppm	6.8 ppm	9–13.5 ppm	36 ppm
100 ppm	7.5 ppm	10–15 ppm	40 ppm

Minimum is 7.5% of the cyanuric acid reading, the everyday band 10–15%, and the algae-clearing level 40% — percentages from the hypochlorous-acid and cyanurate binding equilibrium measured by O'Brien, Morris and Butler (1974), which is what the familiar FC/CYA chart is built on. Two floors come from the chart rather than the percentages, because a lightly stabilized pool still needs a working level and not a fraction of a small number: the everyday band never reads below 4–6 ppm, which is what governs every row under 40 ppm, and the algae level never below 10 ppm, which governs every row under 25. The two floors hand over at different readings, so between 25 and 40 ppm the algae column is already a percentage while the band is still on its floor. The minimum column is the one exception — it is left at an unrounded 7.5%, and published charts round their lowest minimums up from there. The table starts at 20 ppm and runs to 100: below that a pool has a different problem entirely. Computed from the same functions the calculator runs.

Stabilizer to raise cyanuric acid by 10 ppm

Pool size	US units	Metric
5,000 gal (18,927 L)	7 oz	189 g
20,000 L (5,283 gal)	7 oz	200 g
10,000 gal (37,854 L)	13 oz	379 g
40,000 L (10,567 gal)	14 oz	400 g
15,000 gal (56,781 L)	1.3 lb	568 g
60,000 L (15,850 gal)	1.3 lb	600 g
20,000 gal (75,708 L)	1.7 lb	757 g
80,000 L (21,134 gal)	1.8 lb	800 g
25,000 gal (94,635 L)	2.1 lb	946 g
100,000 L (26,417 gal)	2.2 lb	1 kg

Scales linearly: for a 40 ppm lift, multiply by four. Granular stabilizer takes days to dissolve — sock method, pump running, no backwash and no retest for a week.

Water to replace to bring cyanuric acid down to 40 ppm

Starting level	Share of the pool	In a 10,000 gal pool
60 ppm	33%	3,333 gal (12,618 L)
80 ppm	50%	5,000 gal (18,927 L)
100 ppm	60%	6,000 gal (22,712 L)
120 ppm	67%	6,667 gal (25,236 L)
150 ppm	73%	7,333 gal (27,760 L)

Dilution is proportional and it is the only method. The share does not depend on pool size; the volume column shows what it means for the standard 10,000 gallon pool, and scales with yours.

How to use the cyanuric acid calculator

Four steps, computed in your browser from your own test reading.

Enter your pool volume
Type the volume in gallons or litres — both work natively, and the dose comes back in both. If you already used the pool volume calculator on this site, the number is filled in for you.
Put in your measured stabilizer level
Use a real reading, not a guess. The standard drop-dilution test reads anything under about 20 ppm as zero, so if the answer matters down there, send a sample to a shop with a photometer. Test strips are close to useless for cyanuric acid.
Pick a target and a direction
Raise or lower. Outdoor pools sit at 30–50 ppm, salt-water generators run higher at 60–80 ppm because the cell makes chlorine all day and sunlight would waste it, and an indoor pool needs none at all.
Read the answer, then read the chlorine targets
Raising gives a weight of granular stabilizer; lowering gives the share of the water to replace, because there is no chemical that removes it. Underneath, the panel shows what free chlorine your reading demands: minimum, everyday band, and the level that clears algae.

Frequently Asked Questions

What should my cyanuric acid level be?

30–50 ppm for an outdoor chlorine pool, 60–80 ppm for a salt-water pool with a chlorine generator, and none for an indoor pool, which has no sunlight to protect the chlorine from. The reason the bands differ is not comfort but arithmetic: whatever your stabilizer reads, the free chlorine you must hold is a percentage of it. At 30 ppm the minimum is 2.3 ppm; at 80 ppm it is 6 ppm for the same protection.

How do I lower cyanuric acid in a pool?

By replacing water, and only by replacing water. Nothing consumes cyanuric acid — sunlight does not break it down, chlorine does not use it up, and the enzyme products sold for the job have no published evidence behind them. Going from 100 to 40 ppm means swapping out 60% of the pool, which is 6,000 gal (22,712 L) in a 10,000 gal pool. Drain in stages rather than all at once: an empty fibreglass or vinyl pool can float or crease, especially with a high water table. Rain and routine backwashing dilute it slowly for free.

How much stabilizer do I add per 10,000 gallons?

About 13 oz (379 g) raises cyanuric acid by 10 ppm in 10,000 gal of water — so a fresh fill going to 40 ppm needs roughly 3.3 lb (1.51 kg). It scales straight with volume and with the size of the lift. Add it in a sock hung in front of a return jet rather than through the skimmer, keep the pump running, and do not retest for a week: granular stabilizer takes days to dissolve and an early reading reads low.

Does pool shock raise cyanuric acid?

It depends entirely on which shock. Dichlor is stabilized, so a 10 ppm shock with it adds 9.1 ppm of cyanuric acid — repeat that weekly through a season and the pool is somewhere it should not be. Cal-hypo adds none of it, bringing calcium instead, and liquid chlorine adds none either. If your stabilizer is already at or above target, shock with cal-hypo or liquid chlorine and keep dichlor for the start of the season, when you actually want the stabilizer it carries.

Why is my chlorine gone even though I keep adding it?

Two different failures look identical from the deck. With too little stabilizer, sunlight destroys free chlorine within hours and an unstabilized outdoor pool can lose most of it in an afternoon. With too much, the chlorine is still in the water but bound: at 30 ppm of cyanuric acid a pool needs at least 2.3 ppm of free chlorine to stay ahead of algae, and at 80 ppm it needs 6 ppm to do the same work. Someone reading 2 ppm and calling it fine has a green pool coming. Test cyanuric acid before you decide the chlorine is the problem.

Do indoor pools need stabilizer?

No. Cyanuric acid exists to protect chlorine from ultraviolet light, and an indoor pool has none to protect it from — all the stabilizer would do is bind chlorine that then has to be replaced by holding a higher level. Indoor pools run at zero, which is also why they can be sanitary at free chlorine levels an outdoor pool would be overrun at.