How Often to Drain a Pool (and Why TDS Isn't the Reason)

Most pools never need a full drain. A partial drain, where you let out part of the water and refill, fixes the same problems with less water and less risk to the pool.

Calculator: how much water to replace

Choose what is too high (CYA, calcium hardness, or salt), then enter the level now, the level you want, the level in your fill water, your pool volume, and the most you are willing to drain at one time. The calculator gives the share of water to replace, the gallons, and the refill cost at your water price.

With the example values (CYA 90 ppm now, target 50 ppm, fill water 0 ppm, 15,000 gallons), you replace 44% of the water: 6,667 gallons (25,236 L). At an example price of $8 per 1,000 gallons, the refill water costs about $53. Use the price on your water bill.

The formula: share to replace = (level now − target) ÷ (level now − level in fill water). Test your fill water if you can. Tap or well water with some calcium in it means you need to replace more. If the share is bigger than the most you want to drain at once, the calculator splits the job into rounds. Refill, run the pump for a few hours, then retest before the next round.

When replacing water is the only fix

Three things build up in pool water and don't leave by themselves. Evaporation takes only water and leaves them behind, so every top-up makes them a little more concentrated.

Stabilizer (CYA). Chlorine tablets (trichlor) and dichlor shock add CYA every time you use them. As CYA rises, you need more free chlorine to keep the same protection. The pool chlorine levels page shows the chlorine you need for your CYA, and the CYA buildup calculator shows how fast tablets push it up. For the CYA-only details, see how to lower CYA.
Calcium hardness. Hard fill water, calcium-based shock (cal-hypo), and evaporation raise it. High calcium can leave scale on surfaces and equipment. See how to lower calcium hardness, including what to do when your fill water is hard too.
Salt. In a salt pool, salt is lost only with water that leaves, not by evaporation. Hayward's AquaRite manual says the only way to lower salt is to partly drain and refill. See how to lower salt in a pool.

If none of these are high and the water is clear and balanced, you don't need to drain. "Every year" or "every few years" rules are rough guides, not requirements. A pool that gets a lot of rain overflow, splash-out, or backwashing replaces part of its water on its own.

High TDS alone is not a reason to drain

TDS (total dissolved solids) is everything dissolved in the water: calcium, salt, CYA, alkalinity, and leftovers from chemicals, sweat, and sunscreen. A TDS meter gives one number but can't tell you which of these is high.

The CDC's Model Aquatic Health Code (MAHC, 2024 Annex), written for public pools, uses 1,500 ppm above the fill water as a generally accepted limit. The same annex says no known scientific data supports any value as a maximum. It also notes that salt in a salt pool raises TDS a lot, so a salt pool's starting point is its TDS right after the salt goes in.

So test the things TDS is made of instead. If CYA, calcium, and salt are in range and the water is clear, a high TDS reading by itself does not tell you to drain. Our calculator does not estimate TDS, because we found no sourced way to add up the individual readings into a TDS number.

Partial drain, step by step

Test CYA, calcium hardness, and salt (salt pools), and test your fill water if you can. Retest any surprising reading before you drain.
Enter the readings in the calculator above and note the gallons to replace.
Turn off the heater and the salt cell. Drain with a submersible pump or your pump's waste setting, as your equipment manual allows.
Watch the level. Measure the drop in inches and stop at the planned amount, or at the lowest level your pool's manual allows, whichever comes first.
Refill right away. Don't leave a vinyl liner or a partly empty pool exposed to sun for long.
Run the pump for a few hours to mix, then retest everything: CYA, calcium, salt, chlorine, pH, and alkalinity.
If the calculator showed more than one round, repeat steps 3 to 6.

Before you drain: liners, groundwater, and local rules

Vinyl-liner pools. A liner left exposed to air and sun can shrink or pull out of its track. Check the liner maker's guidance before draining low, and prefer several smaller rounds.
Above-ground pools. The water helps the wall keep its shape. Follow the manual's lowest safe level.
Inground pools and groundwater. A nearly empty inground pool can be pushed up by groundwater after heavy rain or where the water table is high. Ask a local pool professional before you drain deeply.
Where the water goes. Cities have different rules on draining pool water to the street, storm drain, or sewer. Check before you drain, and let the chlorine drop first.

Refill water is not free: the pool fill cost calculator prices it with your water and sewer rates.

FAQ

How often should you drain a pool?

Only when a test shows CYA, calcium hardness, or salt too high for your pool, because water replacement is the only way to lower them. There is no fixed number of years. A partial drain is usually enough: bringing CYA from 90 to 50 ppm requires replacing 44% of the water.

Should you drain your pool every year?

No. A yearly drain wastes water if your levels are in range. Test CYA, calcium hardness, and salt at the start of each season, and drain part of the water only if one of them is high.

Is high TDS a reason to drain a pool?

Not by itself. The CDC's public pool code annex uses 1,500 ppm above the fill water as a guide but says no known data supports any maximum. Test CYA, calcium, and salt, and drain only if one of those is high.

How much water should I drain at once?

Only as much as your pool type allows. Many vinyl-liner and above-ground pools should not be drained low, so use several smaller rounds and refill right away. Enter your limit in the calculator, and it splits the job into rounds.

Sources

CDC, Model Aquatic Health Code 2024, Annex (TDS guidance): cdc.gov (PDF)
Hayward, AquaRite owner's manual 092538 Rev D (salt not lost to evaporation; only a partial drain lowers salt): hayward.com (PDF)
Orenda Technologies, Evaporation and accumulation (what concentrates as water evaporates; secondary source): blog.orendatech.com
Dilution formula and chlorine-to-CYA targets: methodology