Drain and refill calculator: how much water to replace
Stabilizer, salt and calcium don’t burn off — the only way down is to swap pool water for fresh water. Enter your test result, your target and what’s in your fill water.
Enter your measurements
More options: Max per drain
- Gallons to drain and refill
- 5,000 gal
- Drop in water level (one drain)
- 27 in
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% × 10,000 gal = 5,000 gal; × 4.5 ft average depth = 27 in drop
- 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.
Quick answers for common sizes
| Example | Answer |
|---|---|
| How much water to drain to lower CYA | 50% |
| How much water to drain to lower salt | 20% |
Share of the water to replace, by target (from the level above, fill water as above)
| Target | Replace this share of the water | Gallons to drain and refill | Drop in water level (one drain) |
|---|---|---|---|
| 30 ppm | 70% | 7,000 gal | 38 in |
| 40 ppm | 60% | 6,000 gal | 32 in |
| 50 ppm | 50% | 5,000 gal | 27 in |
| 60 ppm | 40% | 4,000 gal | 22 in |
| 70 ppm | 30% | 3,000 gal | 16 in |
| 80 ppm | 20% | 2,000 gal | 11 in |
Chlorine burns off and pH can be pushed either way, but stabilizer (cyanuric acid), salt, calcium and total dissolved solids only leave the pool with water. When they build up — usually CYA from tablets or dichlor, salt from overdosing, or calcium in hard-water areas — the fix is dilution: drain some water and refill with fresher water.
How the dilution math works
If you replace a fraction of the water with fill water, the new level is a weighted average of the old water and the fill water. Solve for the fraction:
fraction to replace = (now − target) ÷ (now − fill water) gallons = fraction × pool gallons water-level drop ≈ fraction × average depth staged drains: rounds = log((target − fill) ÷ (now − fill)) ÷ log(1 − share per drain)
With fill water at zero (normal for CYA and salt), halving the level means replacing half the water. Fill water that already contains some calcium makes calcium much harder to lower — you can never get below the tap’s own level.
One big drain or several small ones?
Draining in stages uses more water: each refill dilutes water that has already been partly diluted. Going from 100 to 50 ppm CYA in one go replaces 50% of the water; doing it in 25% steps takes three drains and 75% of the pool’s volume. Stages are still sensible when the pool can’t safely be drained that far — a vinyl liner left exposed in the sun can shrink, and an inground pool in high groundwater can float.
| From → to (fill water 0) | One drain | In 30% drains | Water used in 30% drains |
|---|---|---|---|
| 100 → 50 ppm | 50% | 2 | 60% |
| 120 → 60 ppm | 50% | 2 | 60% |
| 90 → 40 ppm | 56% | 3 | 90% |
| 4,000 → 3,200 ppm | 20% | 1 | 30% |
| 5,000 → 3,200 ppm | 36% | 2 | 60% |
Worked examples: how much water to drain to lower CYA and how much water to drain to lower salt.
Before you drain
- Confirm the test. CYA strips often read high; use a turbidity drop test. For salt, cross-check strips against the cell.
- Check the structure. Ask your builder or liner maker how low you can safely go. Don’t leave a liner exposed for long.
- Mind where it goes. Local rules often control discharge of pool water; one water authority recommends no more than 12 gallons a minute to the sewer.
- Refill, then rebalance. Fresh water lowers everything — retest chlorine, pH, alkalinity and calcium after it has mixed.
- For fecal-incident response, the CDC asks pools to lower CYA to 15 ppm or less before hyperchlorinating — this calculator works out that drain too.
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.
See options on Amazon (opens in a new tab) - 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.
See options on Amazon (opens in a new tab) - Salt test stripsBudget price
Check salt independently of the salt cell’s own reading, which drifts as the cell ages.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How much water do I drain to lower CYA by half?
Half the pool, if your fill water has no stabilizer. From 100 to 50 ppm in a 15,000 gallon pool that is 7,500 gallons — see how much water to drain to lower CYA.
Is there a chemical that lowers cyanuric acid?
Some products are sold to remove CYA, but results vary from pool to pool. Replacing water is the method that works every time.
How do I lower salt in my pool?
Replace water. From 4,000 to 3,200 ppm with salt-free fill water, replace 20% of the pool. See how much water to drain to lower salt.
Why does draining in stages use more water?
Because each refill mixes fresh water with water that has already been partly diluted, so each later drain removes less of the chemical. One big drain is the most efficient — if the pool can safely take it.
Related guides
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.