How tank volume is calculated
For a rectangular aquarium, US gallons = length × width × height (in inches) ÷ 231, because one US gallon is exactly 231 cubic inches. A 24 × 12 × 16 in tank holds 4608 ÷ 231 ≈ 19.95 gallons. This is the full glass volume.
Working in metric? Multiply litres by 0.264 to get US gallons. A few common sizes for reference: 20 × 10 × 12 in ≈ 10.4 gallons, 36 × 18 × 16 in ≈ 44.9 gallons, and 48 × 18 × 21 in ≈ 78.6 gallons.
Many people follow this up with the gallons to liters calculator.
Actual water vs full volume
Real water volume is a little lower because of substrate, rocks, décor and the gap below the rim. Subtract roughly 10–15% to estimate the true water you'll add. Keeping a backyard pond too? Use the pond volume calculator.
As a worked example, that 19.95-gallon tank with two inches of gravel and a piece of driftwood realistically holds closer to 17 gallons of water. Stocking guidelines and medication doses are based on actual water volume, so using the full glass figure can lead to overdosing. Bow-front and hexagonal tanks are not rectangular, so use an average width or split the tank into simpler shapes.
People working on this usually reach for the liters to gallons calculator too.
How to use this calculator
Measure the inside length, width and height in inches and enter them. The capacity in US gallons appears instantly. For potting mix or substrate by the bag, see the soil calculator.
Measure the inside faces rather than the outside of the glass — thick panels on large tanks can shave a gallon or more off the true figure. Measure height to the waterline you actually keep, usually about an inch below the rim.
Frequently asked questions
- How many gallons is a 24x12x16 inch tank?
- About 19.95 US gallons of total glass volume.
- Why divide by 231?
- One US gallon equals exactly 231 cubic inches, so dividing cubic inches by 231 gives gallons.
- Is the real water volume lower?
- Yes. Substrate, rocks and the empty rim mean you'll add roughly 10–15% less water than the full figure.
- How do I get liters instead?
- Multiply US gallons by 3.785 to convert to liters.
- How do I calculate a bow-front or hexagonal tank?
- Use the average width for a bow-front tank, or split an odd shape into simple rectangles and add the volumes. The answer is an approximation, so round down when deciding stocking levels.