About this tool
The Water Change Calculator is a free, in-browser tool that works out how a water change moves any measurable parameter — nitrate, TDS, salinity, phosphate — toward a target. Replacing a fraction f of the tank with source water at concentration C_source leaves a final concentration C_final = C_source + (C_current − C_source) × (1 − f). The tool inverts and iterates that dilution equation across three modes and runs entirely in your browser, so nothing you enter is uploaded.
In 'find volume' mode it solves for the single change that hits your target: f = (C_current − C_target) ÷ (C_current − C_source), and change volume = f × tank volume. In 'count' mode it finds how many equal changes of a fixed percentage are needed: n = ln((C_target − C_source) ÷ (C_current − C_source)) ÷ ln(1 − f), rounded up, and reports the concentration actually reached. In 'resulting value' mode it simply applies a percentage change and shows the new concentration. If the target is below the source-water value it is unreachable by dilution, and the tool says so rather than returning a bogus number.
As a worked example, a 200 L tank with nitrate at 40 ppm and RO source water at 0 ppm, targeting 10 ppm: f = (40 − 10) ÷ (40 − 0) = 0.75, so a 75% change (150 L) is required in one go. Because a single 75% change stresses livestock, the count mode shows that four 25% changes drop 40 ppm to about 12.7 ppm and five reach about 9.5 ppm — the safer path. Source water is rarely truly zero, so enter your tap or RO reading for an accurate result.