Aquarium Volume Calculator
How many gallons is your tank, really? Pick the shape, enter the dimensions in inches (or switch to cm), and get the volume in US gallons and liters — plus the water weight and an honest look at how many fish that volume actually supports.
Tank volume
US gallons
10.4
Liters
39.3
Water weight
87 lb
Water only (8.34 lb per gallon) — add the glass tank itself, stand, substrate and rock for the true load on your floor. Effective water volume after substrate and decor is typically 8.8–9.4 gallons (−10–15%).
Stocking rule of thumb: “1 inch of fish per gallon”
With ~9 effective gallons, the classic rule suggests about 9 inches of small-bodied fish at adult size (for example 4 neon-tetra-sized fish at ~2 in each).
- Only works for small, slim fish (tetras, rasboras, danios) — a 10-inch oscar is not ten neons.
- Count adult size, not the juvenile size at the store.
- Filtration, plants and maintenance schedule matter as much as raw volume.
- Research each species: territory, temperament and swimming level beat any formula.
The formulas, shape by shape
Every shape reduces to cubic inches, then divides by 231 — the number of cubic inches in a US gallon:
Cube: side³ ÷ 231
Cylinder: π × (d/2)² × h ÷ 231
Bowfront: L × (W + ⅔ × (fullW − W)) × H ÷ 231 (approximation)
Worked example: a standard 10-gallon tank measures about 20 × 10 × 12 inches inside. 20 × 10 × 12 = 2,400 cubic inches, and 2,400 ÷ 231 = 10.4 gallons (39.3 liters). The bowfront formula treats the curved front as a parabolic bulge, which lands within a percent or two of the true volume for typical bowfront tanks.
Actual water is always less than the tank's rated size: the glass takes up space, you leave an air gap at the top, and substrate and decor displace another chunk. Budget on 85–90% of nominal volume as real water.
Setting up a new tank? Cycle it first
Volume is step one — a safe tank also needs an established nitrogen cycle. A brand new tank has no beneficial bacteria to convert toxic ammonia from fish waste into nitrite and then relatively harmless nitrate. Run a fishless cycle for 3–6 weeks (dosing ammonia and testing water) or seed the filter with media from an established tank before adding fish. Adding a full stock to an uncycled tank is the single most common cause of early fish deaths — "new tank syndrome".
More volume and unit tools
Need other conversions? The measurement converter switches between gallons, liters, quarts and cups, and the 3D print cost calculator helps if you're printing your own aquarium accessories.