How much water to drain to lower salt in a pool?

Short answer

To lower salt from 4,000 to 3,200 ppm with fresh fill water, replace 20% of the pool: 4,000 gallons from a 20,000 gallon pool, a drop of about 11 in in water level.

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.
(4,000 − 3,200) ÷ (4,000 − 0) = 0.2 → 20%
20% × 20,000 gal = 4,000 gal; × 4.5 ft average depth = 11 in drop

Salt overshoots happen easily: a guessed pool volume, a cell reading low, or an extra bag “to be safe”. Salt never evaporates, so a high reading stays until water leaves — and the most common target, 3,200 ppm, is only about 20% below a 4,000 ppm overshoot.

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 water20%

Gallons to drain and refill: 4,000 gal
Drop in water level (one drain): 11 in

Turn the salt cell off while draining and refilling, then let the water mix and retest before turning it back on — follow your cell’s manual.
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.

For this job · affiliate link

Hose water flow meterA meter on the fill hose tells you exactly how much fresh water went back in.

View on Amazon

20,000 gallons to 3,200 ppm, by starting salt

20,000 gallons to 3,200 ppm, by starting salt
Salt now	Replace this share of the water	Gallons to drain and refill	Drop in water level (one drain)
3,600 ppm	11%	2,222 gal	6 in
3,800 ppm	16%	3,158 gal	9 in
4,000 ppm	20%	4,000 gal	11 in
4,500 ppm	29%	5,778 gal	16 in
5,000 ppm	36%	7,200 gal	19 in
6,000 ppm	47%	9,333 gal	25 in

A 20% water change

Taking 4,000 ppm down to 3,200 ppm replaces a fifth of the pool. In 20,000 gallons that is 4,000 gallons — around 11 in off a pool that averages 4.5 ft deep, usually without uncovering much of the shallow end.

Is it worth it? Many cells tolerate more than their ideal level — Pentair lists 2,600–4,500 ppm for IntelliChlor Plus/LT, Hayward an ideal of 2,700–3,400 ppm for AquaRite. Check your manual before draining for a few hundred ppm.
Cell off. Turn the cell off while draining and refilling, let the new water mix, and retest salt before turning it back on.
Rain and backwash help a little. Overflow after heavy rain and routine backwashing both lower salt slowly.
Next time, add about three-quarters of the calculated salt, wait for it to dissolve and mix, and test before adding the rest — see how much salt for a 20,000 gallon pool.

Stabilizer works the same way; see how much water to drain to lower CYA. For what salt pools need besides salt, read salt water pool basics.

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

What you’ll need

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)
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)
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)

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 to drain to lower salt from 5,000 to 3,200 ppm?

Replace 36% of the water: 7,200 gallons of a 20,000 gallon pool.

How much water to drain to lower salt from 3,800 to 3,400 ppm?

Replace about 11%: 2,105 gallons.

Other common sizes

How much water to drain to lower CYA — 50%

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.