Aquarium Volume Calculator
Find out how many gallons or liters your fish tank really holds. Works for rectangle, cylinder, bow-front and hexagon tanks, in inches or centimeters.
How to measure your tank
- Measure inside the glass if you can. Outside measurements include the glass, which can add several percent on larger tanks.
- Use the water depth, not the tank height, if you want the volume you actually fill. Most tanks are filled 1 to 2 inches (3 to 5 cm) below the rim.
- For medicine and fertilizer doses, use the "likely real water" figure. Substrate, rock and decorations push water out, so the real volume is usually 10 to 20% lower than the box size.
Formulas used
All shapes work the same way: find the area of the tank's footprint, then multiply by the water depth.
For a bow-front, the curved front is treated as an arc of a circle. The bow's bulge is the center width minus the end width, and the area of that circular segment is added to the rectangle behind it. This is more accurate than the common "average the two widths" shortcut, which underestimates bow-front tanks.
Converting the result:
Liters = cubic centimeters ÷ 1,000
1 US gallon = 3.785 liters · 1 UK gallon = 4.546 liters
Worked example: a standard 55 gallon tank
A standard 55 gallon tank measures about 48 × 13 × 21 inches on the outside.
48 × 13 × 21 = 13,104 cubic inches. 13,104 ÷ 231 = 56.7 US gallons (214.7 liters).
That is the outside volume. Take away the glass, a 1 inch gap at the top and 2 inches of gravel, and the real water volume is closer to 46 to 48 gallons. That is why your dosing should use the lower number.
Standard aquarium sizes
Common US glass tank sizes and their approximate outside dimensions. Exact sizes vary by brand.
| Tank | Dimensions (L × W × H) | Calculated volume |
|---|---|---|
| 10 gallon | 20 × 10 × 12 in | 10.4 gal / 39 L |
| 20 gallon high | 24 × 12 × 16 in | 19.9 gal / 76 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 |
Frequently asked questions
How do I calculate how many gallons my fish tank holds?
Measure the inside length, width and water depth in inches, multiply them together to get cubic inches, then divide by 231. One US gallon is exactly 231 cubic inches. For centimeters, multiply length × width × depth and divide by 1,000 to get liters.
Why does my 55 gallon tank calculate as more than 55 gallons?
Tank sizes are nominal and usually based on outside dimensions. Glass thickness, the gap between the water line and the rim, substrate and decorations all reduce the real water volume, often by 10 to 20 percent.
How much does water in an aquarium weigh?
Fresh water weighs about 8.34 pounds per US gallon (1 kg per liter). Seawater at normal reef salinity weighs about 8.55 pounds per gallon (1.025 kg per liter). The tank, substrate and rock add more on top. Use the aquarium weight calculator for the full total.
Should I use the US or UK gallon?
Use US gallons if you are in the United States, where tank sizes and product doses are labelled in US gallons. The UK (imperial) gallon is about 20 percent larger. Most of Europe uses liters.
Related calculators
- Substrate calculator: how much gravel or sand to buy
- Heater size calculator: wattage for your tank volume
- Aquarium weight calculator: can your stand or floor take it?
Sources: Gallon and liter definitions from NIST, US customary to metric conversions. Seawater density from NOAA (about 1.025 kg/L at 35 ppt).
Last updated: October 2026