Calculate aquarium tank gallons or find the missing length, width, height, or volume from dimensions in inches, feet, cm, meters, or liters.
Aquarium Gallons Formula
The aquarium gallons calculator uses the rectangular tank volume formula. All length, width, and height values are first converted to inches, then the volume is converted to US gallons or liters.
- G_us = tank volume in US gallons
- L = aquarium length in inches
- W = aquarium width in inches
- H = aquarium height in inches
- 231 = cubic inches per US liquid gallon, exactly
- Liters = volume in liters
Choose the quantity in Solve for, then enter the other three values. The answer appears below the form. Volume is shown in US gallons and liters; dimensions are shown in inches, feet, centimeters and meters. Exact unit definitions are used before rounding the display.
Common Aquarium Size Reference
Standard aquarium sizes can vary slightly by manufacturer. Use inside measurements when you need a closer water-volume estimate.
| Common Tank Name | Typical Dimensions | Approx. Capacity |
|---|---|---|
| 10 gallon | 20 in × 10 in × 12 in | 10.4 US gal |
| 20 gallon high | 24 in × 12 in × 16 in | 19.9 US gal |
| 20 gallon long | 30 in × 12 in × 12 in | 18.7 US gal |
| 40 gallon breeder | 36 in × 18 in × 16 in | 44.9 US gal |
| 55 gallon | 48 in × 13 in × 21 in | 56.7 US gal |
| Conversion | Factor |
|---|---|
| 1 cubic inch | 1/231 US gallon (approximately 0.004329) |
| 1 US gallon | 231 cubic inches |
| 1 US gallon | 3.785411784 liters |
| 1 liter | Approximately 0.264172 US gallons |
Example Calculations
Example 1: Find gallons from tank dimensions
You have an aquarium that is 24 inches long, 12 inches wide, and 16 inches high.
The tank holds about 19.95 US gallons before subtracting space for substrate, decorations, and the fill line.
Example 2: Find the missing height
You want a 20 gallon tank with a length of 24 inches and a width of 12 inches.
The needed height is about 16.04 inches.
FAQ
Should you measure the inside or outside of the aquarium?
Use inside dimensions if you want the closest estimate of actual water volume. Outside dimensions include glass or acrylic thickness, so they usually give a slightly larger result than the amount of water the tank can hold.
Why is the actual water volume lower than the calculated volume?
The formula calculates the full rectangular volume. In real use, the water level is usually below the rim, and gravel, sand, rocks, driftwood, filters, and decorations take up space. For a more realistic water amount, measure the height only up to the planned water line and remember that objects inside the tank reduce the final water volume.
Does this calculator work for bow-front, hexagon, or cylinder tanks?
This calculator is for rectangular aquariums. Bow-front, hexagon, cylinder, and corner tanks need different volume formulas because their shape is not a simple rectangular box.
