Cyanuric acid (stabilizer): how much to add and how to lower it
Cyanuric acid shields chlorine from the sun. Too little and chlorine burns off fast; too much and it stops working. Here is the balance.
Safety: dosing figures are estimates. Follow the product label, add one chemical at a time, never mix pool chemicals, add chemical to water (never water to chemical), keep chemicals away from children, and re-test before swimming. Add about ¾ of a dose, re-test, then top up. Not a substitute for professional pool service.
Cyanuric acid (CYA), also called stabilizer or conditioner, acts like sunscreen for chlorine. Ultraviolet light destroys unprotected free chlorine within hours, so an outdoor pool with no CYA loses its sanitizer almost as fast as you add it. A modest amount of stabilizer dramatically slows that loss — but too much CYA locks chlorine up and weakens it, so this is a balance, not a “more is better” additive.
The formula
The standard increment is about 1 lb of stabilizer per +13 ppm per 10,000 gallons:
pounds of stabilizer = (Δppm ÷ 13) × (gallons ÷ 10,000)
Δppm is the CYA rise you want. Stabilizer dissolves slowly, so give it time — and never rush it through the skimmer as a shortcut without checking your filter manufacturer’s guidance.
Worked example
Raise CYA by 30 ppm in a 10,000-gallon pool:
(30 ÷ 13) × (10,000 ÷ 10,000) = 2.31 lb of cyanuric acid.
How much CYA should you have?
Outdoor chlorine pools are commonly kept around 30–50 ppm CYA; salt pools often run a little higher because the generator produces chlorine continuously and benefits from extra sun protection. The catch is that higher CYA raises the free chlorine you must maintain to keep the water sanitary — there is a rough proportionality between CYA and the FC target. Let CYA climb too high and you end up chasing ever-larger chlorine doses to compensate. Indoor pools need little or no CYA, since there is no sunlight to protect against.
Lowering CYA — only dilution works
There is no practical chemical that removes cyanuric acid; the reliable fix is to dilute by draining part of the pool and refilling with fresh water. The fraction to drain is:
drain fraction = 1 − (target ÷ current)
Example: to go from 60 ppm to 30 ppm, drain fraction = 1 − (30 ÷ 60) = 0.5 — replace 50% of the water. This same dilution math lowers salinity and calcium hardness too, since none of them can be chemically removed.
Why CYA sneaks up
If you sanitize with stabilized chlorine — dichlor or trichlor — you add CYA with every dose, whether you meant to or not. Over a season that can push CYA well past your target, at which point chlorine seems weak no matter how much you add. If your pool is hard to keep sanitary despite plenty of chlorine, test CYA: over-stabilization is a common, easily missed cause. Switching some dosing to unstabilized liquid chlorine or cal-hypo keeps CYA from creeping up.
How to test CYA
Cyanuric acid is measured with a turbidity test, not a color match. You mix water with a reagent that clouds in proportion to the CYA, then slowly fill a small tube until a black dot at the bottom just disappears; where the water line sits reads your ppm. It is inherently approximate — reading it in shade, at a consistent eye level, improves repeatability — so treat single-digit differences as noise. Because CYA changes slowly, testing it monthly in season is usually enough.
How over-stabilization sneaks up
The reason CYA creeps upward is arithmetic. Stabilized chlorine — dichlor and trichlor — carries cyanuric acid, so every ppm of free chlorine you add from those products also adds close to a ppm of CYA. Use them all season and CYA can climb well past your target, at which point chlorine seems weak no matter how much you add. This is the grain of truth behind the “chlorine lock” idea: chlorine is not truly locked, but at very high CYA you need a much higher free-chlorine level to get the same sanitizing power. CYA does fall slowly on its own — dilution from rain and refills, and some seasonal loss — but the dependable cure for a high reading is to drain and replace part of the water. Indoor pools need little or no CYA at all, since there is no sunlight to guard against, so adding stabilizer there simply weakens your chlorine for no benefit.
The bottom line
Cyanuric acid is chlorine’s sunscreen, and the goal is enough to protect it without so much that it smothers it. Raise it at about 1 pound of stabilizer per 13 ppm per 10,000 gallons, and aim for roughly 30 to 50 ppm outdoors, a touch higher for many salt pools. Keep free chlorine proportional to whatever CYA you run, because higher stabilizer means you need more chlorine to stay sanitary. The trap is over-stabilization: dichlor and trichlor add CYA with every dose, so if your chlorine seems weak despite plenty going in, test CYA before adding more chlorine. There is no chemical remover — the only cure for a high reading is to drain and dilute, with the drain fraction equal to 1 minus target over current. Indoor pools need little or none.
Size a stabilizer addition on the cyanuric acid calculator, and match your chlorine to your CYA using the chlorine calculator and shock calculator.