Estimate partial drain-and-refill water for CYA, calcium hardness, salt, borates or another dissolved reading.
How the estimate works
Replacement fraction = (current − target) ÷ (current − fill). For n equal stages, each stage replaces 1 − ((target − fill) ÷ (current − fill))^(1/n). Mix and retest between stages. The result assumes uniform mixing and no other additions or losses. Test replacement water: hard fill water can prevent the desired hardness target. A target equal to fill-water concentration requires a complete theoretical replacement, not a recommended drain.
Do not drain a pool solely because of an estimate. Groundwater, shell and liner conditions can make draining unsafe. Arrange a qualified local assessment before a large drain.
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Units and assumptions
Volume uses US gallons; dimensions use feet and inches. All budget inputs use the same currency chosen by you. Replace demonstration values with measurements and quotes. Calculations are planning estimates, not equipment specifications or contractor offers.
Unit reference: NIST SI conversion guidance.
