Aquarium Volume Calculator
Find how many gallons or liters your fish tank holds. Choose a rectangle, cylinder, bow-front or hexagon tank, enter its size, and get the full volume, the real water volume after substrate and fill gap, and how much the water weighs.
0 if you measured the inside. Otherwise the calculator subtracts it.
0 if you measured the inside. Otherwise the calculator subtracts it.
Most tanks are filled to about 1 in (2-3 cm) below the rim.
Most tanks are filled to about 1 in (2-3 cm) below the rim.
Gravel or sand on the bottom.
Gravel or sand on the bottom.
How to use the Aquarium Volume Calculator
- Choose units and the tank shape.
- Enter the dimensions. For a bow-front, enter the straight back length, the depth at the ends and the depth at the middle of the bow.
- Enter the glass thickness if you measured the outside, or 0 if you measured inside.
- Set the fill gap and substrate depth to see how much water the tank really holds.
- Read the volume and weight, and check that your stand and floor can carry it.
Aquarium volume formulas
- Rectangle: V = length × width × height
- Cylinder: V = π × diameter² ÷ 4 × height
- Hexagon (regular): V = 3√3 ÷ 2 × side² × height ≈ 2.598 × side² × height
- Bow-front: V = (length × side depth + segment area) × height, where the segment has chord c = length and bulge s = center depth − side depth: R = (c²/4 + s²) ÷ 2s, θ = 2·acos((R − s) ÷ R), segment = R²/2 × (θ − sin θ)
Convert the result: US gallons = cubic inches ÷ 231, liters = cm³ ÷ 1000, 1 US gallon = 3.785 L. Water weighs about 8.33 lb per gallon (0.998 kg per liter at room temperature).
Common rectangular tank sizes
| Nominal size | Typical outside dimensions (L × W × H) | Calculated volume | Water weight (approx.) |
|---|---|---|---|
| 10 gal | 20 × 10 × 12 in | 10.4 gal / 39 L | 87 lb |
| 20 gal long | 30 × 12 × 12 in | 18.7 gal / 71 L | 156 lb |
| 29 gal | 30 × 12 × 18 in | 28.1 gal / 106 L | 234 lb |
| 40 gal breeder | 36 × 18 × 16 in | 44.9 gal / 170 L | 374 lb |
| 55 gal | 48 × 13 × 21 in | 56.7 gal / 215 L | 472 lb |
| 75 gal | 48 × 18 × 21 in | 78.5 gal / 297 L | 654 lb |
| 125 gal | 72 × 18 × 21 in | 117.8 gal / 446 L | 981 lb |
Volumes are from outside dimensions filled to the rim, so the real water volume is lower. Exact sizes vary by manufacturer.
Frequently asked questions
How do I calculate how many gallons my fish tank holds?
For a rectangular tank, multiply length × width × height in inches and divide by 231 (cubic inches in a US gallon). A 48 × 13 × 21 in tank holds 13,104 ÷ 231 = 56.7 gallons. In centimeters, multiply the three sides and divide by 1000 to get liters.
How do I calculate the volume of a bow-front aquarium?
Treat it as a rectangle plus a circular segment. The rectangle is the length × the depth at the sides; the segment has a chord equal to the length and a bulge equal to the center depth minus the side depth. Multiply the total base area by the height.
How do I calculate the volume of a hexagon tank?
The base of a regular hexagon is 3√3 ÷ 2 × side² (about 2.598 × side²). Multiply by the height, then divide cubic inches by 231 for gallons.
Why is the real water volume less than the tank size?
Tanks are sold by their nominal outside size, but glass thickness, the gap below the rim and gravel or sand on the bottom all take up space. A '55 gallon' tank usually holds about 45-50 gallons of water once set up.
How much does a full aquarium weigh?
Freshwater weighs about 8.3 lb per US gallon (1 kg per liter) and saltwater about 8.5-8.6 lb. With glass, substrate, rock and the stand, a common rule of thumb is about 10 lb per gallon in total, so a 55 gallon tank weighs roughly 550-600 lb.
How many liters is a gallon?
One US gallon is 3.785 liters and one UK (imperial) gallon is 4.546 liters. A tank labelled in UK gallons holds about 20% more than the same number of US gallons.