What is CYA (pool stabilizer)? Levels, sources, and what happens when it's off
It protects your chlorine from UV breakdown — but it also raises the free chlorine level you need to maintain effective sanitization. Ideal range: 30–50 ppm for regular chlorine pools, 60–80 ppm for saltwater (SWG) pools.
CYA is one of those pool parameters that gets added once and then forgotten — until it causes a problem. Too little, and chlorine disappears from the pool within hours on a sunny day. Too much, and your required chlorine level climbs to a point where maintaining it becomes expensive and difficult, leaving the pool vulnerable to algae.
This guide covers what CYA does, what the right level is for your pool type, how it builds up, and what to do when it's outside range.
What is CYA and why do pools need it?
Cyanuric acid (CYA) is a stabilizing compound that forms a reversible bond with hypochlorous acid (HOCl) — the active sanitizing form of chlorine. That bond shields the HOCl molecule from UV light, which would otherwise split it into components that leave the pool as gas.
Without stabilizer, chlorine in an outdoor pool can lose 50–90% of its strength within a few hours of direct sunlight. A pool that tested fine in the morning can be dangerously low by midday. CYA prevents this by holding chlorine in a protected, UV-stable reservoir.
How the reservoir works
CYA doesn't destroy chlorine — it holds it. When active HOCl is consumed by sanitizing bacteria or reacting with contaminants, more is released from the CYA-chlorine bond. Think of it as a buffer: UV attacks the reservoir instead of depleting your free chlorine directly. The tradeoff is that the reservoir-bound chlorine is less active than free HOCl, which is why higher CYA requires higher total FC.
Indoor pools need little or no CYA because they have no UV exposure. Outdoor pools without a cover need CYA to function at all during daylight hours. This is why stabilizer is standard in virtually every outdoor residential pool.
CYA and free chlorine: the tradeoff you can't ignore
CYA's protective effect comes with a cost: as CYA increases, you must maintain a higher free chlorine level to keep your pool sanitized.
The reason is the same chemistry that makes CYA useful. CYA-bound chlorine is much less active as a sanitizer than free HOCl. Higher CYA means more of your total FC is sitting in that bound, less-active form. To maintain adequate active sanitizer in solution, you need more total FC as CYA rises.
Minimum FC ≈ CYA × 5% (saltwater / SWG pool)
At CYA 50, the minimum FC is 3.75 ppm. At CYA 80, it's 6 ppm. The old advice of "keep FC between 1–3 ppm" was written for pools with no or very low CYA. At a typical outdoor pool's stabilizer level, that range leaves you well below the sanitization threshold. See the FC/CYA chart for exact minimums, target ranges, and shock levels at every CYA value.
Ideal CYA levels by pool type
| Pool Type | Recommended CYA | Why |
|---|---|---|
| Regular chlorine pool (liquid or tablets) | 30–50 ppm | Good UV protection; FC minimums stay practical |
| Saltwater / SWG pool | 60–80 ppm | Higher CYA extends how long the cell's output lasts; SWG's lower FC minimum (CYA × 5%) compensates |
| Indoor pool | 0–20 ppm | No UV exposure; CYA's FC-raising effect isn't worth the tradeoff |
| CYA above 90 ppm (any pool) | Too high | FC minimums become impractical; shock levels extreme; partial drain required |
Within the 30–50 range, lower is generally better for regular pools. At CYA 30, the FC minimum is about 2.25 ppm and the shock level is 12 ppm — both easy to manage. At CYA 50, the minimum is 3.75 ppm and shock requires reaching 20 ppm. Both are workable, but CYA 30–40 leaves the most margin.
How to test CYA
CYA is measured with a turbidity test: a reagent is added to a water sample that causes dissolved CYA to form a white precipitate. The sample is read by looking for the disappearance of a black reference dot as the solution becomes opaque. The reading corresponds to the CYA level at which the dot disappears.
This test has lower precision than most pool tests — typical accuracy is ±10 ppm. A result of 45 ppm might actually be anywhere from 35 to 55 ppm. This is normal and expected. Use the midpoint of the plausible range when making decisions.
Test CYA at the start of each season, after significant water additions (rain, top-offs, or partial drains), and anytime you switch chlorine products. In pools that use trichlor tablets, test monthly — CYA accumulates steadily and the only signal is the test result.
How CYA builds up in your pool
CYA doesn't evaporate, break down in sunlight, or get filtered out. It only leaves the pool through water removal: splash-out, backwash waste, or intentional draining. The result is that any CYA added to the pool stays there until water is removed.
The most common source of unintentional CYA buildup is trichlor tablets. Every 3-inch tablet added to the pool contains both chlorine and a significant amount of cyanuric acid. Over the course of a season, CYA can climb from 40 ppm to 80 ppm or higher from tablets alone.
| Chlorine Source | Adds CYA? | Notes |
|---|---|---|
| Trichlor tablets (3-inch) | Yes — significant | Most common source of CYA accumulation; ~6 ppm CYA per lb in 10K gal |
| Dichlor granules | Yes — moderate | Adds less per dose than trichlor but accumulates with regular use |
| Liquid chlorine (bleach / sodium hypochlorite) | No | Preferred for pools that already have CYA at target |
| Cal-hypo granular shock | No | Adds calcium instead — raises CH, not CYA |
| Saltwater generator (SWG) | No | Produces chlorine from salt; CYA must be added separately and stays stable |
Many pool owners start the season with trichlor tablets to raise CYA to target, then switch to liquid chlorine for daily maintenance once CYA reaches 40–50 ppm. This keeps CYA stable while avoiding the expense and handling of tablets in the heat of summer. For more detail, see does liquid chlorine raise CYA.
What happens when CYA is too low
Low CYA means your chlorine is exposed to direct UV without protection. In full summer sun, an unstabilized pool can lose the majority of its free chlorine within a few hours. The practical consequences:
- Rapid chlorine loss. Even with normal dosing, FC can drop to zero between additions on sunny days. You are essentially dosing to replace what UV destroys, not to build a sanitizing reserve.
- Inconsistent protection. A pool tested at 3 ppm FC in the morning may be at 0.5 ppm by afternoon. Swimmers are exposed to under-sanitized water during peak use hours.
- Higher chemical costs. You're burning through more chlorine than a stabilized pool would need for the same protection level.
The fix is straightforward: add granular cyanuric acid (sold as "stabilizer" or "conditioner") until CYA reaches the target range. It dissolves slowly — add it to a skimmer or in a sock near a return, and allow a full pump cycle before retesting. Do not add more than about 10 ppm at a time without retesting; overshooting is easier than correcting.
What happens when CYA is too high
High CYA is a slow-developing problem that catches many pool owners off guard. Because CYA accumulates gradually over a season, the shift from "fine" to "too high" can happen without any dramatic event. The pool just becomes harder and harder to keep sanitized.
At CYA 80, the FC minimum is 6 ppm. At CYA 100, it's 7.5 ppm — and shock requires holding FC at 40 ppm, roughly a gallon of liquid chlorine per 10,000 gallons, held for hours. Most pool test kits lose accuracy above 10 ppm, so even verifying that you've hit the shock target is difficult. Meanwhile, the margin between "sanitized" and "algae bloom" is razor-thin.
The algae connection
Most summer algae blooms in pools that "have chlorine" trace back to CYA that has crept too high. The owner adds chlorine faithfully, but at CYA 90 with FC maintained at 3–4 ppm, they are well below the required minimum of 6.75 ppm. The chlorine reads positive on the test strip, but it isn't doing its job. The only fix is bringing CYA back into range.
Above 90 ppm, a partial drain and refill is the standard solution. There is no reliable chemical that removes CYA. For a full walkthrough of calculating how much to drain, see how to lower CYA in a pool. After refilling, switch to liquid chlorine or cal-hypo to prevent CYA from climbing again.
How to raise CYA
Add granular cyanuric acid (sold as stabilizer or conditioner at pool supply stores) until CYA reaches your target range. The standard dose is approximately 13 oz per 10,000 gallons to raise CYA by 10 ppm, though this varies by product concentration.
- Pre-dissolve in a bucket of warm water before adding, or place in a sock near a skimmer or return jet
- Run the pump for at least 24–48 hours before retesting — CYA dissolves slowly
- Add in increments of 10 ppm and retest; don't overshoot the 30–50 target
- Do not add directly to the skimmer basket — undissolved granules can damage the pump
Alternatively, using trichlor tablets exclusively for a week or two will raise CYA while also chlorinating the pool. This approach is more common at season start when CYA needs to go from 0 to target. Once at 30–40 ppm, switch to liquid chlorine to prevent further CYA accumulation. For a step-by-step guide, see how to raise CYA.
How to lower CYA
The only reliable method is dilution. There is no pool chemical that dependably removes CYA — several products are marketed as CYA reducers but the evidence for consistent effectiveness is poor.
Draining a portion of the pool and refilling with fresh water proportionally reduces CYA. If CYA is at 80 ppm and you drain 25%, CYA will drop to approximately 60 ppm after refilling. Use the CYA dilution calculator to find the exact gallons to drain for your target reduction. After refilling, switch to a non-CYA chlorine source (liquid chlorine or cal-hypo) to keep CYA from climbing again. For the complete guide including what to do when CYA is extremely high, see how to lower CYA.
Track CYA drift before it causes problems
PoolChem Tracker logs your CYA over time and automatically adjusts your FC minimum based on each reading. You can see CYA creeping up across readings — the signal that action is needed before the pool turns green.
Frequently asked questions
Is pool stabilizer the same as CYA?
Yes. Pool stabilizer, pool conditioner, and CYA all refer to the same compound: cyanuric acid. The labels vary by manufacturer and region, but the chemistry is identical. If a product says "stabilizer" or "conditioner" without specifying the active ingredient, check the label — if it lists cyanuric acid, it is CYA.
What is the ideal CYA level for a pool?
For regular chlorine pools: 30–50 ppm. For saltwater (SWG) pools: 60–80 ppm. For indoor pools: 0–20 ppm. Within the regular-pool range, lower is better — CYA 30–40 keeps the FC minimum low and the shock target manageable. SWG pools run higher because the generator's continuous output benefits from extended UV protection at higher CYA.
Does CYA affect how much chlorine I need?
Yes — this is the most important effect most pool owners miss. The minimum safe FC scales with CYA: approximately CYA × 7.5% for regular pools, CYA × 5% for SWG pools. At CYA 50, you need at least 3.75 ppm FC. At CYA 80, at least 6 ppm. Running FC at 1–3 ppm when CYA is 50 or higher is not adequate sanitization, regardless of what the test strip shows. See the FC/CYA chart for the complete table.
Which chlorine products add CYA?
Trichlor tablets add significant CYA with every use. Dichlor granules add a moderate amount. Liquid chlorine, cal-hypo granular shock, and saltwater generators add no CYA. If you use trichlor tablets as your primary chlorine source, test CYA monthly — it accumulates steadily throughout the season and the only signal is the test result.
What happens if CYA is too high?
Your required FC level rises with it, making effective sanitization more expensive and difficult to maintain. At CYA 80–90+, shock levels become extreme, FC test kits lose accuracy, and the margin between "sanitized" and "algae bloom" shrinks. Most summer algae blooms in pools that "have chlorine" are caused by CYA that has crept above the FC minimum. The only fix for high CYA is a partial drain and refill.
How do I lower CYA in my pool?
Drain a portion of the pool and refill with fresh water. Draining 25% reduces CYA by roughly 25%. No chemical product reliably removes CYA. Use the CYA dilution calculator to find exactly how much to drain. After refilling, switch to liquid chlorine to prevent CYA from climbing again. See how to lower CYA for the full step-by-step guide.
Does CYA affect LSI?
Yes, indirectly. CYA-corrected alkalinity — used in the LSI formula — reduces effective TA by approximately CYA ÷ 3. At CYA 60, about 20 ppm of your alkalinity reading is "tied up" by CYA and contributes less to water balance. Pools with high CYA that look balanced on individual readings may run more corrosive than expected. For a deep dive, see how CYA affects LSI.
CYA / Pool Stabilizer Series
- Pool Stabilizer (CYA) — what it is, ideal levels, and how to manage it (start here)
- Best CYA Level for Liquid Chlorine Pools
- How to Lower CYA in a Pool
- How to Raise CYA in a Pool
- High CYA in a Pool — Causes, Symptoms, and Fixes
- Low CYA in a Pool — Risks and How to Fix It
- How Much Stabilizer to Add to a Pool
- Best CYA Level for Saltwater Pools
Keep reading
- FC/CYA Chart: Safe Chlorine Levels at Every CYA Value — the complete table of minimums, targets, and shock levels
- Does Liquid Chlorine Raise CYA? — which chlorine types affect stabilizer and which don't
- How CYA Affects LSI — CYA-corrected alkalinity and why it matters for water balance
- CYA Dilution Calculator — how many gallons to drain to hit your target CYA
- Chlorine Dose Calculator — dose recommendations that automatically account for your CYA level
- Pool Chlorine Levels Chart — the full chlorine reference for every pool situation
- How to Balance Pool Water in 4 Steps — where CYA fits in the overall balancing sequence
- Pool Turned Green Overnight? — what happens when FC drops below the CYA minimum