How to use the aquarium volume calculator
- Choose your tank shape.
- Enter the dimensions. For the most accurate figure measure the inside; if you measure the outside, enter the glass thickness too.
- Set the fill level to allow for the gap at the top and the gravel, rock and décor.
- Choose freshwater or saltwater to get the right water weight and salt mix.
Volume formulas by tank shape
Rectangle V = L × W × H Cylinder V = π × (D ÷ 2)² × H Half cylinder V = ½ × π × (D ÷ 2)² × H Hexagon V = 2.598 × side² × H Corner V = ¼ × π × r² × H Bow front V = (L × side depth + circular segment) × H US gallons = cubic inches ÷ 231 Litres = cubic cm ÷ 1,000
Worked example
A standard “55 gallon” tank measures 48 × 13 × 21 in: 48 × 13 × 21 = 13,104 in³, and 13,104 ÷ 231 = 56.7 US gallons (215 L). At a 90% fill, about 51 gallons of water weigh roughly 426 lb before you add gravel, rock and the tank itself.
Common tank sizes
| Tank | Dimensions (L × W × H) | Volume |
|---|---|---|
| 10 gallon | 20 × 10 × 12 in | 10.4 gal / 39 L |
| 20 gallon long | 30 × 12 × 12 in | 18.7 gal / 71 L |
| 29 gallon | 30 × 12 × 18 in | 28.1 gal / 106 L |
| 40 gallon breeder | 36 × 18 × 16 in | 44.9 gal / 170 L |
| 55 gallon | 48 × 13 × 21 in | 56.7 gal / 215 L |
| 75 gallon | 48 × 18 × 21 in | 78.5 gal / 297 L |
| 125 gallon | 72 × 18 × 21 in | 117.8 gal / 446 L |
Figures are from outside dimensions; real water volume is lower.
Why the exact volume matters
- Medication and water conditioner are dosed per gallon or litre — dose for the water actually in the tank, not the label.
- Salt for reef tanks: about 36 g of salt mix per litre (roughly ½ cup per gallon) gives a specific gravity near 1.025. Always confirm with a refractometer.
- Heater: 3–5 W per gallon keeps tropical fish at 76–80 °F in a normal room.
- Filter: aim to move 4–6 times the tank volume per hour; planted and reef tanks often more.
- Stocking: rules of thumb such as “1 inch of fish per gallon” are rough at best; think about adult size, activity and waste.
Is my floor strong enough?
A full 75 gallon tank with stand, rock and gravel can weigh 850 lb or more on a footprint of just 4 × 1.5 ft. Place large tanks against a load-bearing wall or across floor joists rather than in the middle of a span, and on upper floors consult a builder for anything over about 100 gallons.