Aquarium Volume Calculator

Calculating aquarium volume and water needed

A rectangular tank is a box, so its volume is length × width × height. What you actually want to know is usually the water it holds, which means converting that box volume into gallons or litres. With the inside dimensions in inches the conversion is a single division, because a US gallon is defined as 231 cubic inches.

gallons = length × width × height (in) ÷ 231

litres = length × width × height (cm) ÷ 1,000

1. Choose US units or Metric units. This decides the units of the answer: cubic yards, gallons and pounds, or cubic metres, litres and kilograms.
2. Enter the length, width and height and pick a unit for each. They can differ, so a tank measured as 4 ft × 18 in × 0.5 yd works as entered.
3. Press Calculate. The results are the aquarium volume, the water needed and the water weight.

Example. A tank that measures 20 in × 10 in × 12 in has 2,400 cubic inches. Divided by 231 that is 10.39 US gallons, or 39.33 litres, and the water in it weighs about 86 lb (39 kg).

How the weight is worked out

Water at tank temperature is a little lighter than the textbook 1 kg per litre. The calculator uses 0.997 kg per litre, so 200 litres weigh about 199 kg, and a US gallon comes to roughly 8.32 lb. Larger totals are shown in a second form: tonnes once the water passes 500 kg, and US short tons (2,000 lb) once it passes 1,000 lb, with the plain weight in brackets.

The water weight is only part of the load. Glass or acrylic, the stand, substrate, rock and equipment all add weight, and a full tank can exceed 500 lb even at 55 gallons. Check the capacity of the floor and the stand before filling a large tank. These figures are estimates; see our disclaimer.

Typical aquarium sizes

Tank makers name sizes by nominal gallons, and the outside shape often gives a slightly different figure when you calculate it from the dimensions. The table uses common footprints and the calculator's own rules for volume and weight.

Real water volume is lower than the figure above for three reasons. The glass has thickness, so you should use the inside dimensions; the water line sits below the rim; and substrate, rocks and decorations displace water. Many keepers plan for around a tenth to a fifth less water than the calculated capacity.

Volume and water calculations for non-rectangular aquariums

The calculator is built for boxes. For other shapes, work out the volume by hand in cubic inches or centimetres and convert it with the same divisors as above.

cylinder: V = π × r² × h

bow-front: V ≈ (rectangle area + area of the curved bulge) × heightApproximate the bulge as a circular segment or half an ellipse and add its area to the rectangle's.

Round tank example. A cylinder 24 in across and 30 in tall has a radius of 12 in. The volume is π × 12² × 30 ≈ 13,572 cubic inches, which is about 58.75 gallons or 222.4 litres. A corner or hexagonal tank can be split into a rectangle and triangles, or into regular prisms. For general solids the volume calculator, the tank volume calculator and the box volume calculator help.

Gallons and litres

The gallon used here is the US liquid gallon, equal to 3.785 litres. The imperial gallon used in the UK is larger, 4.546 litres, so a tank labelled in imperial gallons holds about a fifth more than the same number of US gallons. If a label does not say which one it means, compare it with the dimensions.