Aquarium Tank Volume Calculator

Calculate aquarium volume in gallons and litres from tank dimensions — rectangular, cube, cylinder, hexagonal, or bow-front shapes.

Inputs

Pick the tank geometry; the relevant dimension fields apply.

Units for the length/diameter/height fields below.

Rectangular/bow-front: front-glass length. Cube: side. Ignored for cylinder.

Front-to-back depth (rectangular/bow-front).

Water height (or full tank height).

Cylinder only: inside diameter.

Hexagon only: length of one of the six equal sides.

Bow-front only: how far the curved front bows out beyond the rectangle.

Result

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How to use this calculator

  • Choose the tank shape and whether you are measuring in inches or centimetres.
  • Enter the dimensions the selected shape needs (e.g. length/width/height, or diameter for a cylinder).
  • Read the volume in US gallons and litres, plus imperial gallons.
  • Use the −10% figure as a realistic water volume once substrate and decor are added.

About this calculator

This calculator converts aquarium dimensions into volume in US gallons, imperial gallons, and litres, for the most common tank shapes: rectangular, cube, cylinder, regular hexagon, and bow-front. Knowing the true volume matters for stocking levels (the "one inch of fish per gallon" guideline), dosing water conditioners and medications, and sizing filters and heaters. The math is straightforward geometry — length × width × height for a rectangle, π·r²·h for a cylinder — converted using exact factors: one US gallon is defined as 231 cubic inches, and one litre is 1000 cubic centimetres. Because substrate, rocks, and decorations displace water, the tool also reports an approximate actual water volume at about 90% of the gross figure. For best accuracy, measure the inside dimensions of the glass rather than the outside.

How it works — the formula

Rectangular: V = L × W × H Cube: V = s³ Cylinder: V = π × (d/2)² × H Hexagon: V = (3√3/2) × s² × H Bow-front: V ≈ (L×W + ⅔·L·bow) × H Gallons(US) = V(in³) ÷ 231 | Litres = V(cm³) ÷ 1000

Each shape uses its standard solid-geometry volume formula. The result is converted to gallons and litres with exact unit factors: 231 in³ per US gallon and 1000 cm³ per litre.

Sources: NIST — US customary to metric conversions (gallon = 231 in³) · NIST — Definitions of the SI units (litre)

Worked examples

Example 1
Rectangular 24 × 12 × 16 in
Inputs:
shape=rectangular, units=in, length=24, width=12, height=16
Output:
4608 in³ ÷ 231 = 19.95 gal (75.5 L)
Example 2
Cylinder 20 in diameter × 24 in tall
Inputs:
shape=cylinder, units=in, diameter=20, height=24
Output:
π·10²·24 = 7540 in³ ÷ 231 = 32.64 gal
Example 3
Cube 12 in
Inputs:
shape=cube, units=in, length=12
Output:
1728 in³ ÷ 231 = 7.48 gal

Limitations

  • Uses inside dimensions ideally; outside measurements over-count by the glass thickness.
  • Bow-front and hexagon results are geometric approximations of real tank shapes.
  • The −10% water-volume figure is a typical allowance, not exact for your decor.

Volume estimate for stocking and dosing guidance. Verify critical dosing against measured fill volume.

Frequently asked

How do I convert tank dimensions to gallons?+
Multiply length × width × height to get the volume. If you measured in inches, divide by 231 (the number of cubic inches in a US gallon). If you measured in centimetres, divide by 1000 to get litres, then divide by 3.78541 to get US gallons.
Why is one US gallon 231 cubic inches?+
The US liquid gallon is legally defined as exactly 231 cubic inches. That makes the inch-to-gallon conversion exact, not an approximation. The UK imperial gallon is larger — about 277.4 cubic inches, or 4.546 litres.
Should I use the full height or the water height?+
For the amount of water the tank holds when filled to the trim, use the inside height up to the waterline (usually an inch or two below the top rim). Using the full glass height slightly overestimates the actual water volume.
Why is my real water volume less than the calculated number?+
Substrate (gravel or sand), rocks, driftwood, and equipment all displace water, and tanks are rarely filled to the very top. A common allowance is about 10% less than the gross internal volume, which this tool shows as the approximate water volume.
How is a bow-front tank calculated?+
A bow-front is a rectangle with a curved front that bulges outward. This tool approximates the curved area as a parabolic segment, adding two-thirds of (front length × bow depth) to the rectangular cross-section before multiplying by height — a close estimate for typical gentle bows.
How is a hexagonal tank calculated?+
For a regular hexagon (six equal sides), the base area is (3√3 ⁄ 2) × side², which is then multiplied by the tank height. Measure the length of one of the six glass panels for the side length.

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