Aquarium Volume Calculator
How many gallons is your aquarium?
Measure your fish tank, pick a shape and get precise gross and net volume in gallons and litres — with a live 3D preview, water weight, glass-thickness guidance and surface area for every rectangular, cylinder, hexagon, bow-front, cube and corner tank.
Updated July 2026 · Reviewed by the AquariumStocking team
Tank volume calculator
Tank Dimensions
Measure the interior of the glass, not the outside frame. Length is the front face, width is front-to-back, height is top-to-bottom.
Leave blank to use the standard 90% net-volume estimate.
Net water volume accounts for substrate, hardscape and the 1–2 inch freeboard below the rim. When you enter a substrate depth we subtract the exact displaced volume instead of using the flat 10% estimate.
Live 3D preview
Drag to rotate. Blue fill shows the real water level.
Volume & weight
Rectangular — interior capacity.
Glass thickness
Rough hobbyist rule of thumb from tank height and longest span. It is not a structural spec — always defer to the manufacturer or a qualified builder for custom tanks.
How to calculate aquarium volume
For a standard rectangular fish tank, aquarium volume is length × width × height. Measure the inside of the glass in inches, multiply the three numbers, then divide by 231 to convert cubic inches to US gallons. For example, a 20 × 10 × 12 in tank = 2,400 in³ ÷ 231 ≈ 10.4 gallons. In metric, multiply the same three numbers in centimetres and divide by 1,000 to get litres (a 50 × 25 × 30 cm tank is 37,500 cm³ = 37.5 litres).
Cylinders use π × radius² × height. Hexagons use (3√3 / 2) × side² × height, where the side length is the flat-to-flat width divided by √3. Bow-front tanks add a half-ellipse of area (π / 8) × length × bow-depth to the rectangular footprint before multiplying by height. Corner (quarter-round) tanks are a quarter of a full cylinder — (π × R²) / 4 — times height.
Why real water volume is less than the rated size
The gallon number stamped on a tank is the empty interior. The moment you add substrate, rocks, driftwood and leave the standard 1–2 inches of freeboard under the rim, real water volume drops. A typical planted or aquascaped 40-gallon tank actually holds closer to 34–36 gallons. That matters for dosing fertilisers, calculating water changes and stocking correctly — always use the net figure, not the label on the box.
Common aquarium sizes
Standard glass fish tank dimensions from most US manufacturers. Click any row to load its size into the calculator, or follow the link to a full stocking guide.
| Nominal | Dimensions | Litres | Water weight | Load |
|---|---|---|---|---|
| 5 gal | 16 × 8 × 10 | 19 L | 42 lb | |
| 10 gal | 20 × 10 × 12 | 38 L | 83 lb | |
| 20 gal | 24 × 12 × 16 | 76 L | 167 lb | |
| 29 gal | 30 × 12 × 18 | 110 L | 242 lb | |
| 40 gal | 36 × 18 × 16 | 151 L | 334 lb | |
| 55 gal | 48 × 13 × 21 | 208 L | 459 lb | |
| 75 gal | 48 × 18 × 21 | 284 L | 626 lb | |
| 90 gal | 48 × 18 × 24 | 341 L | 751 lb | |
| 125 gal | 72 × 18 × 21 | 473 L | 1043 lb | |
| 180 gal | 72 × 24 × 24 | 681 L | 1502 lb |
Before you fill it
- Confirm your stand is rated for the filled weight above.
- Level the tank — a 1° tilt puts uneven stress on silicone seams.
- Floors above ground level: check joist rating for tanks over 55 gal.
- Leave 1–2 inches of freeboard for surface agitation and lids.
The maths behind every shape
- Rectangular: L × W × H
- Cube: S³
- Bow-front: (L × W + π/8 × L × bow) × H
- Cylinder: π × r² × H
- Half-cylinder: ½ × π × r² × H
- Quarter-cylinder: ¼ × π × r² × H
- Hexagon: (3√3 / 2) × s² × H
- Corner pentagon: (π × R²) / 4 × H
- L-shape: (L × W − cut²) × H
1 US gallon = 231 in³ = 3.78541 L. 1 UK gallon = 4.54609 L. Water = 8.34 lb/gal (1 kg/L).