1 US gallon = 231 cubic inches exactly (NIST HB 44). Enter your tank dimensions and the calculator gives you gross volume in gallons and liters. Actual water volume will be 10–15% lower after substrate, decorations, and equipment are in place.
The formula
Rectangular tank:
Volume (in³) = Length × Width × Height
Volume (US gallons) = Volume (in³) ÷ 231
Volume (liters) = Volume (gallons) × 3.785411784
Cylindrical tank:
Volume (in³) = π × (diameter ÷ 2)² × Height
Volume (US gallons) = Volume (in³) ÷ 231
The factor 231 cubic inches per US gallon is exact by US legal definition (NIST HB 44). The gallon-to-liter factor (3.785411784 L/gal) is exact per NIST SP 811.
Common standard tank sizes
Nominal sizes are marketing labels — actual internal dimensions vary by manufacturer and glass thickness.
| Nominal size | Typical dimensions (L × W × H, in) | Gross volume (gal) | Gross volume (L) |
|---|---|---|---|
| 5 gallon | 16 × 8 × 10 | 5.5 | 20.8 |
| 10 gallon | 20 × 10 × 12 | 10.4 | 39.3 |
| 20 gallon (high) | 24 × 12 × 16 | 19.9 | 75.3 |
| 20 gallon (long) | 30 × 12 × 12 | 18.7 | 70.8 |
| 29 gallon | 30 × 12 × 18 | 28.1 | 106.3 |
| 40 gallon (breeder) | 36 × 18 × 16 | 44.8 | 169.5 |
| 55 gallon | 48 × 12.75 × 21 | 55.7 | 210.8 |
| 75 gallon | 48 × 18 × 21 | 83.5 | 315.9 |
| 125 gallon | 72 × 18 × 22 | 129.1 | 488.7 |
Measure your specific tank's internal dimensions rather than relying on the nominal label for precision calculations.
Gross volume vs. water volume
The calculator produces gross volume — the total cubic capacity of the tank if filled to the brim. In practice:
- Substrate (gravel, sand, or planted medium) displaces 10–20% of the bottom volume
- Decorations and hardscape (rocks, driftwood) add additional displacement
- Equipment (filter intakes, heaters, powerheads) occupy some space
- Fill level is typically 1–2 inches below the rim
For practical purposes, estimate actual water volume at roughly 85% of the calculator's gross result for a moderately decorated freshwater setup. Heavily aquascaped or reef tanks may be lower.
Stocking and water chemistry
Volume is one input into stocking decisions, but not the only one. Filtration capacity, surface area for gas exchange, water flow rate, and species compatibility all affect how many fish can live comfortably. For stocking guidance specific to your fish species, consult a veterinarian or aquatics specialist.
For calculating how much substrate you need for your tank, see the Aquarium Substrate Calculator →.