How much water to drain to lower CYA?

Short answer

To lower cyanuric acid from 100 to 50 ppm with stabilizer-free fill water, replace 50% of the water: 7,500 gallons from a 15,000 gallon pool, a drop of about 27 in at 4.5 ft average depth.

How to work it out

In short: replacing part of the water with fresh water dilutes what’s in it, so the share to replace is how far you need to come down.
(100 − 50) ÷ (100 − 0) = 0.5 → 50%
50% × 15,000 gal = 7,500 gal; × 4.5 ft average depth = 27 in drop

A pool that has run on trichlor tablets or dichlor all summer often ends the season at 100 ppm of stabilizer or more. At that level chlorine has to be kept high just to keep algae away, and the cure is fresh water.

Adjust for your project

More options: Max per drain

Most you’re willing to drain at once

Set lower (e.g. 30%) to drain in stages — it takes more water in total.

Replace this share of the water50%

Gallons to drain and refill: 7,500 gal
Drop in water level (one drain): 27 in

The Model Aquatic Health Code caps CYA at 90 ppm in public pools. After refilling, chlorine needs drop too — recheck the target with the chlorine and stabilizer calculator.
Fresh water dilutes everything — chlorine, alkalinity, calcium and stabilizer — so retest and rebalance after refilling.
One water authority recommends draining no faster than 12 gallons a minute; check your local rules on where pool water may go.

Before you drainDraining can damage a pool: an inground shell can lift in high groundwater, and vinyl liners can shrink or shift if left exposed. Check your builder’s or liner maker’s guidance before lowering the water a lot, follow local rules for where pool water may be discharged, and dechlorinate if required.

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FAS-DPD drop test kit (chlorine, pH, TA, CH, CYA)Confirm CYA with a turbidity test before you drain — strips often read stabilizer high, and a drain is expensive to get wrong.

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15,000 gallons from 100 ppm CYA, by target

15,000 gallons from 100 ppm CYA, by target
Target CYA	Replace this share of the water	Gallons to drain and refill	Drop in water level (one drain)
80 ppm	20%	3,000 gal	11 in
70 ppm	30%	4,500 gal	16 in
60 ppm	40%	6,000 gal	22 in
50 ppm	50%	7,500 gal	27 in
40 ppm	60%	9,000 gal	32 in
30 ppm	70%	10,500 gal	38 in

Halving the stabilizer

With fill water at zero CYA, the math is simple: to halve the level, replace half the water. In 15,000 gallons that is 7,500 gallons — about 27 in off a pool that averages 4.5 ft deep, which in many pools means the shallow end is nearly empty.

In stages. If you’d rather not go that low, two drains of 30% bring 100 ppm down to 49 ppm, using 9,000 gallons in all — 1,500 more than one big drain.
How far to go. The Model Aquatic Health Code caps CYA at 90 ppm for public pools. For a pool chlorinated by hand, 30–50 ppm keeps the chlorine you need reasonable; salt pools often run higher. The chlorine and stabilizer calculator shows the trade-off.
Stop the cause. If tablets put it there, switching to liquid chlorine after the refill stops CYA from creeping back up — see how many chlorine tablets for a 10,000 gallon pool for the weekly build-up.
Fecal-incident response needs CYA at 15 ppm or less (CDC); from 100 ppm that is an 85% water change.

Once refilled, raise chlorine to the right level for the new CYA, and if algae took hold while CYA was high, follow how much liquid chlorine to SLAM a pool. Salt instead of stabilizer? See how much water to drain to lower salt.

Different size? Use the full Drain & Refill Calculator (Lower CYA, Salt, Calcium).

What you’ll need

FAS-DPD drop test kit (chlorine, pH, TA, CH, CYA)Mid-range price
A drop-count kit reads free chlorine to 0.5 ppm (or 0.2 ppm), plus pH, alkalinity, calcium and stabilizer. Every dose on this site starts from a test, so this is the one thing worth buying first.
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Liquid pool chlorine (sodium hypochlorite)Budget price
Adds chlorine without stabilizer or calcium. Strength is printed on the label — commonly 10% or 12.5% — and it weakens with age and heat, so buy it fresh.
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Hose water flow meterBudget price
Screws onto the hose and counts gallons, so you know exactly how much went in — and your real fill rate.
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Frequently asked questions

How much water to drain to lower CYA from 80 to 50 ppm?

Replace 38% of the water: 5,625 gallons of a 15,000 gallon pool.

How much water to drain to lower CYA from 150 to 50 ppm in 30% drains?

4 drains of 30% (18,000 gallons in all); one big drain would replace 67%.

Other common sizes

How much water to drain to lower salt — 20%

Sources

CDC — Model Aquatic Health Code, 5th edition (2024), code (opens in a new tab) — Free chlorine minimum 1.0 ppm (2.0 with CYA, 3.0 in spas); pH 7.0–7.8; CYA at or below 90 ppm; CYA:FC above 45:1 is an imminent hazard; TA 60–180 ppm; turnover tables.
CDC — Fecal incident response recommendations for aquatic staff (opens in a new tab) — Formed stool: 2 ppm for 25–30 min at pH ≤ 7.5. Diarrhea: CT 15,300 (20 ppm for 12.75 h); with CYA lowered to 15 ppm or less, 20 ppm for 28 h, 30 ppm for 18 h or 40 ppm for 8.5 h.
Southern Nevada Water Authority — Pools and spas (opens in a new tab) — A recommended maximum pool drain discharge rate of 12 gallons per minute; local drain rules apply.
CDC Healthy Swimming — Home pool and hot tub water treatment and testing (opens in a new tab) — pH 7.0–7.8; free chlorine at least 1 ppm in pools (2 ppm with cyanuric acid) and 3 ppm in hot tubs; no CYA in hot tubs; test twice a day.