Room · The instrument
The Aquarium Calculator
Type your tank size or drag the tape measures. Gallons, litres, water weight, water changes and substrate update as you go. Free, no sign-up.
Substrate depth capped at tank height.
Scale drawing, filled to the rim. Substrate layers are schematic, not to scale.
Standard sizes
SLESS kits are made for these four. See the kits
All the numbers
Water weight is water only. Glass, substrate and hardscape add more.
How to measure
- Length along the front glass, width along the side, height from the bottom glass to the top rim.
- Outside measurements include the glass, so the real water volume is a little lower than the result.
- For a bowfront, measure the width at the sides, then the extra depth at the middle of the bow. For a bowl, measure the diameter and how high the water sits.
Now plan the whole tank
Substrate, light, water, plants and animals, checked against a sourced dataset. Ends with an assessment you can save. Free, no sign-up.
Aquarium calculator, explained
How do I calculate aquarium volume in gallons?
Multiply length × width × height to get cubic inches, then divide by 231 for US gallons. A 30 × 12 × 12 in tank is 4,320 cubic inches, about 18.7 US gallons. The calculator above does this instantly and also gives litres.
How much does a full aquarium weigh?
Water weighs about 8.34 lb per US gallon, or roughly 1 kg per litre. A 20 gallon tank holds around 166 lb of water on its own. Glass, substrate and hardscape add more, so plan your stand for well above the water figure.
How much substrate do I need?
Substrate volume is the footprint (length × width) × the depth you want. The calculator returns it in litres and quarts. Weight is not estimated, because soil, sand and gravel differ in density.
US gallons or UK gallons?
A US gallon is about 3.785 L. A UK (imperial) gallon is about 4.546 L, so the same tank holds fewer of them. Use the US/UK switch above. Most of the hobby, and most tank labels in the US, use US gallons.
What is a water change volume?
A 25% water change on a 20 gallon tank is about 5 gallons; a 50% change is about 10. The calculator shows both for your exact tank, so you know how much to prepare and dechlorinate.
How far does the water level drop?
In a straight-sided tank the level drops by the same fraction as the volume. A 25% change on a 12 in tall tank lowers the water about 3 in. Pick a percent above and the drawing shows the drop for your tank.
Every figure is calculated directly from your dimensions with standard conversions: 231 in³ per US gallon, 3.785 L per US gallon, 4.546 L per UK gallon, 0.998 kg per litre of fresh water.