Pool drain and refill calculator

Some things can't be removed from pool water with chemicals: stabilizer (CYA), calcium hardness, salt and dissolved solids. Replacing part of the water with fresh water dilutes them. Enter the current and target levels to see how much to replace.

Current level (ppm)

Target level (ppm)

Level in your fill water (ppm)

Pool volume

Volume unit

Average water depth (inches, optional)

Replace

–

Water to drain

–

Level drop

–

The formula

Fraction to replace = (current − target) ÷ (current − fill water level)

When you drain a fraction of the water, that fraction of the dissolved CYA, calcium or salt leaves with it. The fresh water brings in whatever it contains. For CYA and salt, tap water usually contains little or none, so you can leave the fill water level at zero. Calcium is different: tap and well water often contain a fair amount, so test your fill water and enter it, or the result will be too low.

Worked examples

Stabilizer at 100 ppm, target 60 ppm

(100 − 60) ÷ (100 − 0) = 0.4, so replace 40% of the water: 6,000 gallons of a 15,000-gallon pool. With an average depth of 60 inches, that's a 24-inch drop if done in one go.

Calcium at 600 ppm, target 400 ppm, fill water at 150 ppm

(600 − 400) ÷ (600 − 150) ≈ 0.44, so replace about 44%. Ignoring the fill water's calcium would suggest only 33%, and you'd fall short.

One big drain or several small ones?

If you drain, refill and mix fully each time, two smaller steps need a little more water in total than one big drain. For example, removing 40% at once replaces 6,000 gallons of a 15,000-gallon pool. Doing it in two equal steps takes two drains of about 22.5% each (1 − √0.6), about 6,800 gallons in total. Smaller steps are often the safer choice, because they keep the water level from dropping too far.

Another approach is to drain and refill at the same time, with the hose running in the shallow end and water pumped out of the deep end. Because fresh water mixes with the water being removed, this needs more water than draining first, sometimes much more. Draining, then refilling, uses the least water.

Before you drain. Some pools can be damaged if they're drained too far. In-ground pools can lift out of the ground ("pop") when groundwater is high. Vinyl liners can shrink, wrinkle or pull out of their track when the water comes off them. Check with your pool builder or liner maker how far your pool can safely be drained. Many owners lower the level only a foot or two at a time and refill before continuing. Also check local rules on where pool water may be discharged.

Frequently asked questions

Does rain or evaporation change these numbers?

Evaporation removes only water, leaving CYA, calcium and salt behind, so it slowly concentrates them. That's why levels creep up in pools that are topped up rather than drained. Heavy rain that overflows the pool acts like a small drain and refill.

Can I use this for total dissolved solids (TDS)?

Yes. Enter the TDS of the pool and of your fill water. TDS rarely needs managing in a residential pool, and in saltwater pools most of it is the salt.

How do I measure the water level drop?

Mark the current waterline on the tile or skimmer, then measure down. The level drop shown assumes straight walls, which is close enough for most pools.