Knowing your tank’s real volume
Fish tanks are sold by nominal size, but the water inside is less than the box suggests. The tank is not filled to the brim, and gravel, rocks and wood take up space. The real volume matters for dosing medication and water conditioner, sizing the heater and filter, and choosing how many fish to keep. This calculator works from the inside dimensions.
The formulas
- Tank volume (liters) = length × width × height in cm ÷ 1,000; in inches, divide cubic inches by 231 for US gallons
- Water volume = tank volume × water level × (1 − share taken by substrate and decor)
- Weight ≈ 1 kg per liter of water (8.34 lb per gallon), plus about 1.6 kg per liter of gravel
- Heater ≈ 1 watt per liter (3 to 5 W per gallon) for tropical fish
- Filter flow ≈ at least 4 times the water volume per hour
Worked example: a 30 × 12 × 18 inch tank
This is the classic “29-gallon” footprint.
- Tank volume: 30 × 12 × 18 = 6,480 cu in ÷ 231 = 28.1 gallons (106 L).
- Filled to 95% with 10% taken by gravel and decor: 28.1 × 0.95 × 0.9 = 24.0 gallons of water (91 L).
- Weight of water and gravel: about 238 lb, before the glass and stand.
- Heater: about 91 W, so a 100-W heater.
- Filter: at least 91 × 4 = 363 L/h, about 96 gallons per hour.
- Weekly 25% water change: about 6 gallons.
Common tank sizes
| Nominal size | Dimensions (in) | Gross volume |
|---|---|---|
| 10 gallon | 20 × 10 × 12 | 10.4 gal |
| 20 gallon long | 30 × 12 × 12 | 18.7 gal |
| 29 gallon | 30 × 12 × 18 | 28.1 gal |
| 40 gallon breeder | 36 × 18 × 16 | 44.9 gal |
| 55 gallon | 48 × 13 × 21 | 56.7 gal |
| 75 gallon | 48 × 18 × 21 | 78.5 gal |
Published capacities are rounded and measured on the outside, so your result can differ slightly.
How many fish?
The old rule of one inch of adult fish per gallon is shown as a cautious starting point for small, slim fish such as tetras and rasboras. It does not work for goldfish, cichlids or other large or messy fish, which need far more water per fish. Think of adult size, activity level, swimming space and the filter’s capacity, and add fish gradually so the beneficial bacteria can keep up.
Placement and safety
A filled 55-gallon tank weighs over 600 lb with its stand. Place large tanks on a dedicated aquarium stand, on a level floor, ideally near a load-bearing wall, and never on furniture not designed for the load. Keep electrical equipment on a drip loop and a GFCI outlet.