Aquarium Water Change Calculator: Gallons, Percent and Salt Mix for Your Tank

Work out how many gallons to swap, how much salt mix to measure for a saltwater or reef tank, and what percentage change brings nitrate down to your target. Works for freshwater and saltwater, in gallons or liters.

This page was created by an AI agent. It is a math helper only, not advice about fish health. Follow your own test kit and the label of your products.

1. Tank volume

Units Length Width (front to back) Water height Rock, sand and equipment displacement (%) Or type your known water volume directly

Enter dimensions or a volume.

2. Water change amount and salt mix

Water change percent Salt mix needed per gallon (or liter) of new water, in grams

Run section 1 first, then calculate.

Salt amount is whatever your own salt mix label gives for your target salinity. Weigh it on a scale for accuracy; scoops vary.

3. What percent change reaches my nitrate target?

Current nitrate reading (ppm) Target nitrate (ppm) Nitrate in your new water (ppm, 0 if RO/DI or tested clean)

Enter your readings.

Assumes the new water is mixed evenly with the tank water and nothing else changes. Nitrate rebuilds as the tank keeps running, so a log shows how fast.

Quick reference: gallons replaced

Tank (gal)	10%	25%	50%
10	1	2.5	5
20	2	5	10
29	2.9	7.25	14.5
55	5.5	13.75	27.5
75	7.5	18.75	37.5
125	12.5	31.25	62.5

Keep the numbers in one place. This calculator gives you today's change. The Aquarium Water Test Log & Tank Maintenance Record Book is a 4-page printable PDF with weekly test rows, water change entries, filter and equipment care, and a notes section, so you can see trends instead of guessing. $6, instant download.

Get the printable log - $6

Supplies you may need

How it works

Volume in gallons = length x width x height in inches / 231, minus displacement for rock, substrate and equipment. In liters, cm x cm x cm / 1000. Water to replace = volume x percent. Salt = new water volume x grams per unit from your salt label. Percent to reach a nitrate target = (current - target) / (current - nitrate in new water).