About this tool
The Pool Drain & Dilution Calculator tells you how much water to drain and replace to lower a chemical that no product can remove — cyanuric acid, salt, calcium hardness, or total dissolved solids (TDS). These all build up over time and the only way down is to swap out old water for fresh. Everything is computed locally in your browser, so nothing you enter is uploaded.
The math is a simple mixing relation. If your fill water already contains some of the chemical, the fraction you must replace is (current − target) ÷ (current − fill-water level), and the volume to drain is pool volume × that fraction. If your top-up water is essentially clean, set the fill level to 0 and the fraction simplifies to 1 − target ÷ current. The resulting concentration after refilling is current × (1 − fraction) + fill × fraction, which lands on your target.
Enter your pool volume, pick the chemical, and give the current level, target level, and (optionally) the level already present in your fill water. The tool returns the gallons or litres to drain, the percentage of the pool that represents, the water remaining, and the level you should read after refilling. Note that you cannot dilute below the concentration of your fill water — if your tap water is 40 ppm CYA you can never get the pool under 40 ppm by dilution. For large replacements, drain in stages and let the pool refill and mix between rounds rather than emptying it in one go.