Aquarium Water Change Calculator
Take the guesswork out of maintenance day. Set your tank volume and either a change percentage or the amount you want left in the tank, and this tells you exactly how much to drain and refill — plus how much dechlorinator the new water needs.
Use the true filled volume, not the rated tank size.
Products differ widely — match this to your conditioner's label. Set to 0 to skip.
Formula & how it works
By percentage, the amount to drain and refill is volume × percentage, and what stays behind is the rest. Set instead by a target remaining volume and the change becomes volume − remaining, with the percentage worked out for you.
Dechlorinator is dosed on the new water only: (refill ÷ 10) × dose per 10 gallons. Liters use gallons × 3.785.
Worked example
On a 40 gallon tank, a 25% change means draining and refilling 10 gallons (about 38 liters), leaving 30 gallons in the tank. At a dose of 5 mL per 10 gallons, the 10 gallons of new water needs 5 mL of conditioner.
Getting water changes right
Percentage versus target volume
Most people think in percentages — a routine 20% or 25% weekly change. But sometimes you know the level you want to reach instead, such as draining to expose the substrate for cleaning. Switching the calculator to a target remaining volume handles that case and still tells you the effective percentage.
Treat the new water, not the tank
Chlorine and chloramine come in with the fresh tap water, so dechlorinator is dosed for the volume you are adding back, not the whole tank. Because conditioners vary so much in strength, the dose here is a starting default — always cross-check your bottle, especially with concentrated products where a little goes a long way.
Steady beats dramatic
Frequent modest changes keep parameters stable, which fish prefer to big swings. If nitrates or other readings are high, it is usually gentler to do a couple of moderate changes over a few days than one very large one, and matching the refill temperature avoids shocking the tank.