Enter the inside dimensions of the glass, choose inches or centimetres, and the calculator returns US gallons, litres and cubic volume. Turn on the fill height and substrate options to see how much water you actually keep in the tank after leaving a gap at the top and burying part of the floor under gravel or sand.
How to measure a rectangular tank
- Measure between the inside faces of the glass, not across the outer frame or trim.
- Take the height from the inside floor (the bottom pane) up to the rim, then use the fill-height option for the real water line.
- If the tank has a centre brace or built-in overflow, measure the open interior and treat the overflow box as displacement.
Always measure inside the glass. Exterior sizes include the panel thickness and any trim, which on larger tanks can remove 5 to 10 percent of the apparent volume. See the measuring guide for the full method.
Rectangular volume formula
V = L × W × H
Multiply the three interior dimensions. In inches, divide the result by 231 for US gallons. In centimetres, divide by 1,000 for litres.
With inches, divide the cubic result by 231 to get US gallons and multiply gallons by 3.785 for litres. With centimetres, divide by 1,000 for litres directly.
Worked example
Inputs: Length 36 inWidth 18 inHeight 16 in
A 36 × 18 × 16 inch interior gives 10,368 cubic inches. Dividing by 231 gives 44.9 US gallons, or about 169.9 litres.
Why the box tank is the volume baseline
Because the floor area of a rectangle is uniform from top to bottom, water volume scales linearly with height. Every inch of water in a 36 × 18 tank is worth 2.8 gallons, which makes fill-height and substrate corrections simple to reason about. Curved and polygon tanks need their own floor-area formulas, but once that area is known, the same height scaling applies.
Getting to real water volume
The formula gives the geometric volume of the tank to the rim. Three things make the real water volume smaller, and the calculator above lets you account for two of them directly:
- Fill height. Most tanks run an inch or two below the rim. Enter the actual water line and the result scales to match.
- Substrate. Gravel and sand sit in the water column. The substrate option removes the solid fraction of the bed (62 percent of its depth for typical gravel) from the total.
- Rock, wood and equipment. Use the displacement calculator to subtract hardscape by weight and equipment by type.
Water weight
Fresh water weighs about 8.34 pounds per US gallon (1 kg per litre). Reef water at a specific gravity of 1.025 is about 8.55 pounds per gallon. The calculator shows both so you can check what your stand and floor will carry before adding glass, substrate and rock on top.