To raise cyanuric acid, add granular stabilizer (pure cyanuric acid) — about 13 oz per 10,000 gallons raises CYA roughly 10 ppm. Put it in a sock hung in front of a return jet rather than pouring it in, run the pump, and wait 48–72 hours before retesting. Aim for 30–50 ppm, or 60–80 ppm on a salt pool.
Why Low CYA Quietly Wastes Your Chlorine
Cyanuric acid is chlorine's sunscreen. It bonds loosely to free chlorine and shields it from UV, releasing it back as the chlorine gets used up. Take that shield away and sunlight destroys most of your free chlorine within a few hours — you dose in the evening, and by mid-afternoon the next day the test kit reads near zero.
That is why a pool with no stabilizer feels like it eats chlorine. It isn't demand from swimmers or algae; it's the sun. The symptoms are recognisable:
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Free chlorine that reads near zero by the next afternoon, however much you dosed the evening before.
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A chlorine bill that keeps climbing while nothing about the pool or its use has changed.
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Green tinges returning in warm weather even though you "just shocked it".
Indoor pools are the exception
No sun, no need for sunscreen. Indoor pools are normally run with little or no CYA, because the stabilizer would only weaken the chlorine without protecting it from anything. If your pool is indoors and CYA reads zero, that is not a fault to fix.
The target for an outdoor chlorine pool is 30–50 ppm. Salt pools run higher, 60–80 ppm, because a salt cell produces chlorine continuously in small amounts and benefits from the extra UV protection. Going above that range is its own problem — see how to lower CYA if you overshoot.
How Much Stabilizer to Add
The working figure is 13 oz of granular cyanuric acid per 10,000 gallons for every 10 ppm you want to gain. Test first — you are dosing the gap between where you are and where you want to be, not the target itself.
| Pool volume | +10 ppm | +20 ppm | +30 ppm |
|---|---|---|---|
| 10,000 gal | 13 oz (0.8 lb) | 26 oz (1.6 lb) | 39 oz (2.4 lb) |
| 15,000 gal | 20 oz (1.2 lb) | 39 oz (2.4 lb) | 59 oz (3.7 lb) |
| 20,000 gal | 26 oz (1.6 lb) | 52 oz (3.3 lb) | 78 oz (4.9 lb) |
| 25,000 gal | 33 oz (2.0 lb) | 65 oz (4.1 lb) | 98 oz (6.1 lb) |
A worked example: a 20,000-gallon pool testing 15 ppm, targeting 40 ppm, needs 25 ppm — roughly 65 oz, about four pounds. That is a lot of granular product going into one pool, which is exactly why it needs to dissolve slowly rather than sit on the floor.
If your volume isn't a round number, the CYA stabilizer calculator does the arithmetic from your current and target readings. Not sure of the volume itself? The pool volume calculator comes first — every chemical dose on the site depends on getting that number right.
How to Add It Without Wrecking Your Filter
Granular cyanuric acid is stubborn. It dissolves far more slowly than any other pool chemical, and undissolved granules are mildly acidic — left sitting on a vinyl liner or plaster floor they can bleach or etch a patch. The method matters more here than with most products.
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The sock method — the default
Put the measured amount into an old sock or a nylon stocking, tie it off, and hang it in front of a return jet with the pump running. The flow does the dissolving. Squeeze the sock every few hours if you're passing by. Nothing lands on the floor and nothing goes through the filter.
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Pre-dissolving in a bucket — faster, more effort
Stir the granules into a bucket of warm water until they're fully dissolved, then pour the solution slowly around the pool with the pump running. Warm water helps; cold water barely touches it. Add chemical to water, never water to chemical.
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The skimmer — don't
Pouring stabilizer into the skimmer is common advice and a bad idea. Slow-dissolving granules pack into the filter and keep leaching acid through it — hard on a cartridge, worse on a sand bed, and it makes your CYA reading jump around for weeks as the residue releases unpredictably.
Run the pump continuously for the first 24 hours, then wait a full 48–72 hours before retesting. This is the step people skip. Test at 12 hours and you'll read a fraction of what you added, conclude it didn't work, and dose again — which is how pools end up at 90 ppm and needing a partial drain.
The Other Route: Let Stabilized Chlorine Do It
Trichlor tablets and dichlor granules are chlorine with CYA built in, so they raise stabilizer as a side effect of sanitizing. Roughly, dichlor adds about 9 ppm of CYA for every 10 ppm of FC it delivers, and trichlor about 6 ppm per 10 ppm FC.
If your CYA is only slightly low — say 20 ppm and you want 40 — running tabs for a few weeks will drift you up on its own, and there's no reason to buy granular stabilizer as well. Test as you go rather than assuming.
The trap in this route
CYA never leaves the water on its own. Tabs that conveniently raise your stabilizer this month keep raising it every month after, and the pools that end up at 100+ ppm almost all got there this way. Use stabilized chlorine to reach your target, then switch your routine sanitizing to liquid chlorine or cal-hypo, which add none.
Starting from zero is the one case where granular is clearly better: reaching 40 ppm on tabs alone means weeks of over-chlorinating first. Dose it directly, get it done, then manage chlorine separately. Our chlorine type comparison covers what to use day to day.
Where People Go Wrong
Retesting too early, then double-dosing
The single most common way a pool goes from low CYA to unfixably high. Give it the full 48–72 hours. The stabilizer is in the water whether the test strip has noticed yet or not.
Trusting a strip reading near zero
Test strips are poor at CYA and worst at the low end, where the pad colours barely differ. Before dosing several pounds of anything, confirm with a turbidity-style CYA test from a drop kit or a shop test. Strips vs liquid kits covers the difference.
Aiming high "to be safe"
More sunscreen is not more protection — past the target range CYA starts blunting the chlorine it's meant to protect, and the only way back down is replacing water. Target the middle of the range, not the top of it.
Forgetting that CYA changes your chlorine target
Free chlorine is not an absolute number — the right FC depends on your CYA, and raising stabilizer raises the FC you need to hold. After the CYA settles, re-check your chlorine against the new level with the chlorine calculator.