How the calculation works

When you drain part of the pool and top it back up, the new water dilutes what is left. If you replace a share of the water with fill water at F ppm, the new level is F + (current − F) × (1 − share). Turning that around:

Share to replace = (current − target) ÷ (current − fill water level).
Water to drain = share × pool volume.
Water-level drop = share × average depth. This is only right for straight, vertical walls, where the surface area stays the same as the level falls, and only while the water stays above the shallow-end floor.
Rounds: if each drain can only lower the level so far, each round replaces at most (drop limit ÷ average depth) of the water. The calculator finds the fewest full rounds that reach the target and makes the last round just big enough.
Drain first, then refill. The formulas assume the water is well mixed and that you finish draining before you refill. If you run the hose while you drain, some of the new water goes straight back out, so it takes more.
Impossible targets: replacing water can only bring the pool closer to the fill water's level, never past it. If your target is at or below the level in your fill water, no amount of draining will reach it.

Worked example

A 15,000 gallon pool has stabilizer (CYA) at 100 ppm and you want 50 ppm. Your tap water has none, and the pool's average depth is 5 ft (60 inches).

Share to replace: (100 − 50) ÷ (100 − 0) = 0.5, so 7,500 gallons in one go.
With vertical walls that lowers the water by 0.5 × 60 = 30 inches (2.5 ft).
If you only want to lower it 12 inches at a time, each round replaces 12 ÷ 60 = 20% of the water (3,000 gallons). After three rounds the level is 80, then 64, then 51.2 ppm. A fourth, small round of 352 gallons (1.4 inches) brings it to 50 ppm. That is 4 rounds and 9,352 gallons in total, 62.3% of the pool.

In metric, a 60,000 litre pool with an average depth of 1.5 m needs 30,000 litres replaced in one go, a 75 cm drop. In rounds of up to 30 cm it is three rounds of 12,000 litres and a final 1,406 litres (3.5 cm): 37,406 litres in total.

Target levels

Stabilizer (CYA): the Hayward and Pentair salt chlorinator manuals ask for 30–50 ppm in outdoor pools and none indoors; Jandy's AquaPure manual asks for 50–75 ppm. The CDC's Model Aquatic Health Code, written for public pools, caps it at 90 ppm and requires public pools to close at 300 ppm or more until it is fixed. The CDC says disinfection takes longer when stabilizer is present, recommends at least 2 ppm free chlorine (instead of 1 ppm) in home pools that use it, and advises against using it in hot tubs.
Calcium hardness: Hayward and Pentair, quoting pool industry (APSP) levels, give 200–400 ppm for pools; Jandy gives 150–400 ppm.
Salt: use the level in your chlorinator's manual. Hayward's optimum is 3,200 ppm (range 2,700–3,400), Pentair's current guide asks for 3,600 ppm (range 3,600–4,500) and Jandy's AquaPure 3,000–3,500 ppm. Hayward says partial draining and refilling is the only way to lower salt. The pool salt calculator works out how much to add if you overshoot the other way.
Total dissolved solids (TDS): Pentair's guide says TDS in a salt pool includes the salt and gives 3,000 ppm minimum to 5,700–6,000 ppm maximum. We didn't find a manufacturer's figure for non-salt pools, so ask your pool professional.

Before you drain

Don't empty the pool without professional advice. Latham, a pool manufacturer, warns that groundwater under an emptied pool can push up and crack or float the floor, a risk that grows if the drained water soaks into the ground around the pool. It says in-ground pools typically don't need draining, and recommends a pool professional for the job. The Model Aquatic Health Code requires new public pools to be built with a hydrostatic relief valve or underdrain where groundwater could lift a drained pool, so ask your builder whether yours has one.
Fiberglass pools: Latham says never to drain one without first consulting your dealer, because the walls can bulge when the water's weight is gone, and that most manufacturers forbid draining a fiberglass pool, which may void the warranty.
Vinyl liner pools: Latham says a liner can shrink and crack if the pool is completely emptied, and not to remove more than half of the total volume. The Coated Fabrics and Film Association says not to drain a liner pool completely for any reason without consulting a pool professional, and that older liners are more likely to shrink and not stretch back.
Drain a little at a time. For high salt, an earlier Pentair IntelliChlor guide says to drain and refill 1 ft (30 cm) at a time. Use the drop limit box to plan rounds like this, and retest after each refill.

Where the water can go

Rules are set locally and differ a lot. Two examples show what to expect:

Washington, DC prohibits discharging pool water into public space. It may go to the sanitary or combined sewer under the plumbing code, or to the storm drain only after dechlorinating to undetectable levels (less than 0.1 mg/L), and never to a septic system. The DC Department of Energy & Environment says chlorine left in sunlight dissipates within 10 days if you stop adding it, or you can use a dechlorinating product as its label directs. Chloramines from city water don't dissipate readily and need a product made to remove them. A first offense can be fined up to $1,000.
Portland, Oregon asks for home pool water to go to the sanitary sewer or soak into the ground on your property, and never onto the sidewalk, street or a neighbor's land. The storm system is a last resort: the water must be dechlorinated to 0.1 mg/L or less, left to sit for at least two weeks, cooled, pH-neutral and released at less than 50 gallons per minute. Salt-treated pool water, and water containing bromine or copper, may never go to the storm system.

Wherever you live, check with your city, county or water utility (or your local council or water company outside the US) before draining, and mention if it is a salt pool.

After refilling

New water changes the rest of your water balance, so test before anyone swims. The CDC advises testing free chlorine and pH at least twice a day, keeping pH at 7.0–7.8 and free chlorine at 1 ppm or more in home pools (2 ppm or more with stabilizer, 3 ppm or more in hot tubs). Those are minimums for health; the manufacturers' ranges for your equipment may be higher.

Pool chemical safety

Rebalancing after a refill usually means adding chemicals. The CDC advises:

Read the whole product label before each use and follow it.
Wear safety equipment such as goggles and gloves.
Never mix chlorine products with acid, which can create toxic gases, and never mix different pool chemicals with each other or with anything else. Open one container at a time and close it before opening another.
Only pre-dissolve a product if the label tells you to. Then add the chemical to water, never water to the chemical, which can cause a violent reaction.
Store each chemical separately in its original container, dry, below 95°F (35°C) and away from heat. Keep them secure and away from children and pets.

Add large corrections in stages and retest between them. Keep swimmers out while you add anything by hand: the Model Aquatic Health Code only allows public pools to add chemicals by hand when no one is in the water, and it is a sensible rule at home too.

Common questions

How much water do I need to replace to halve my stabilizer?: Half of it, if your fill water has no stabilizer in it. Lowering CYA from 100 to 50 ppm in a 15,000 gallon pool means replacing 7,500 gallons. With an average depth of 5 ft and straight walls, that is a 30 inch drop, so it is usually better done in several smaller rounds.
Why do several small drains use more water than one big one?: Each round drains some of the fresh water you added in the round before. In the example above, four rounds of up to 12 inches use 9,352 gallons (62.3% of the pool) instead of 7,500 gallons (50%) in one go. The benefit is that the water level never drops more than 12 inches.
Does the water I refill with matter?: Yes, especially for calcium. To bring calcium hardness down from 600 to 400 ppm you replace 33.3% of the water (5,000 of 15,000 gallons) if the fill water has no calcium, but 40% (6,000 gallons) if it has 100 ppm. If your fill water is at 400 ppm or more, replacing water can never get you down to 400.
Can I just empty the pool and start again?: Not without professional advice. Latham, a pool manufacturer, warns that groundwater can crack or lift the floor of an emptied pool and that fiberglass walls can bulge, and it says not to remove more than half the water from a vinyl liner pool. The Coated Fabrics and Film Association says never to drain a vinyl liner pool completely without consulting a pool professional.
Can I let the water run into the street or a storm drain?: Often not. Washington, DC and Portland, Oregon both restrict pool water in storm drains, require it to be dechlorinated first if it goes there at all, and Portland never allows salt pool water in its storm system. Ask your city, county or water utility before you drain.

Sources

Last checked and updated: October 31, 2026.