Aquarium Volume Calculator: Gallons and Liters
How to Measure
- Inside measurements: measure inside the glass, or subtract the glass thickness from the outside measurements.
- Water depth, not tank height: measure from the bottom glass to the waterline, which is usually a little below the rim.
- Odd shapes: bow-front, corner and cylinder tanks need different formulas. The maker's stated volume is the best guide for those.
The Formulas
| You measured in | Do this | Result |
|---|---|---|
| Inches | Length × width × depth ÷ 231 | US gallons |
| Inches | Length × width × depth ÷ 277.4 | UK gallons |
| Centimeters | Length × width × depth ÷ 1,000 | Liters |
Example: a tank 24 × 12 × 16 inches (61 × 30 × 41 cm) holds 4,608 cubic inches, which is about 20 US gallons (75 liters).
US vs UK Gallons
A US gallon is 3.785 liters; a UK (imperial) gallon is 4.546 liters, about 20 percent more. So a "20-gallon" tank holds about 76 liters if the gallons are American, or about 91 liters if they are British. Our fish pages use US gallons with liters alongside. When a product label gives gallons, check which kind it means, or go by the liters.
Measuring and Setup Kit
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A tape measure gets the dimensions right. Once you know the volume, you can size a filter and heater properly and dose water conditioner accurately.
Real Water Volume and Weight
Gravel, sand, rocks, wood and the filter all take up space, so the real amount of water is lower than the calculation. For dosing water treatments, use your best estimate of the actual water, and follow the product's instructions.
Water is heavy: a liter weighs 1 kg (2.2 pounds), and a US gallon about 8.3 pounds (3.8 kg). A 20-gallon (76-liter) tank holds about 167 pounds (76 kg) of water, before glass, gravel and decor. Use a stand made for aquariums and a level floor. See aquariums and stands.
Frequently Asked Questions
Common questions about tank volume.
How do I calculate gallons in a fish tank?
Multiply the inside length, width and water depth in inches, then divide by 231 for US gallons.
How many liters is a 10-gallon tank?
About 38 liters if it's 10 US gallons, or about 45 liters if it's 10 UK gallons.
Why is my tank's real volume less than the label?
Labels usually give the volume of the empty box. Glass, substrate, decor and the gap at the top all reduce the water it actually holds.
How heavy is a full aquarium?
Water alone weighs about 8.3 pounds (3.8 kg) per US gallon. Add glass, substrate and decor on top.
Related Reading
- Heater sizing: matching watts to volume.
- Filters and pumps: choosing by tank size.
- Your first aquarium: setting up step by step.
For help choosing fish for your tank's volume, ask an aquatics store with trained staff.