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Aquarium Calculator

Enter a tank’s length, width and height to get its volume in US gallons and litres — plus the filled weight your stand must hold.

Written by Jude Ramsey · Last updated August 2026 · How these calculators are checked

What this calculator does

Tank volume drives every other aquarium decision — stocking, filtration, heater size, dosing — and the filled weight decides what furniture can safely hold it. Enter the three inside dimensions in inches and this calculator returns US gallons, litres, and the approximate weight of the water alone, with the honest note that substrate and decorations reduce the real water volume by roughly 10–15%.

How to use it

  1. Measure the tank’s length, width and height in inches.
  2. Read the volume in gallons and litres.
  3. Check the filled weight before choosing a stand.

The formula

gallons = L × W × H ÷ 231 · litres = gallons × 3.785 · water weight ≈ gallons × 8.34 lb

A worked example

A 24 × 12 × 16 inch tank works out at 19.9 gallons — the classic “20 gallon high” — about 76 litres, with the water alone weighing roughly 166 lb. Add glass, substrate and rock and the stand is carrying well over 200 lb, which is why aquariums belong on purpose-built stands, not bookshelves.

Common questions

Is the “one inch of fish per gallon” rule true?
Only as a very rough starting point for small fish — it fails for anything large, messy or territorial. Research each species’ adult size and needs; volume is the constraint, not the licence.
Do I measure inside or outside the glass?
Inside gives the true water volume. Outside dimensions include the glass thickness and over-state it slightly — more on thick-glass large tanks.
Why does my “20 gallon” tank hold less than 20?
Nominal sizes are rounded, you never fill to the rim, and substrate and decor displace water — real water volume typically runs 10–15% under nominal.

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