Aquarium Volume Calculator
The Number on the Tank Label Is Rarely the Real Water Volume
Aquarium tanks are sold under nominal size labels — "20 gallon," "55 gallon" — but the actual water volume depends entirely on the tank's real internal dimensions, and stocking, filtration sizing, and medication dosing all need the real figure, not the marketing one. Measuring length, width, and height and running them through simple geometry produces an exact volume you can trust for equipment sizing.
The Formula
Centimeters: Liters = (Length × Width × Height) ÷ 1000
231 cubic inches is the volume of exactly one US gallon, and 1,000 cubic centimeters is one liter — both are the exact conversion constants used regardless of tank shape, as long as it's a rectangular prism.
Why Exact Volume Matters
- Stocking limits — fish stocking guidelines (like the fish-per-gallon rule) are only as accurate as the gallon figure feeding into them.
- Filter and heater sizing — both are rated for a gallon range; undersizing a filter for the true volume leaves the tank under-filtered even if it matches the "labeled" size.
- Medication and treatment dosing — aquarium medications and water conditioners are dosed per gallon or per liter of actual water, and overdosing from an inflated volume estimate can harm fish.
- Water changes — knowing exact volume lets you calculate precisely how much replacement water and dechlorinator a partial water change requires.
| Length × Width × Height (in) | Volume (cu in) | US Gallons | Liters |
|---|---|---|---|
| 20 × 10 × 12 | 2,400 | 10.39 | 39.33 |
| 30 × 12 × 12 | 4,320 | 18.7 | 70.79 |
| 48 × 13 × 20 | 12,480 | 54.03 | 204.51 |
| 36 × 18 × 21 | 13,608 | 58.91 | 223.0 |
Computed directly from the length × width × height ÷ 231 formula above; these are worked examples, not universal standard tank sizes.
How to Use This Calculator
- Enter the tank's Length.
- Enter the tank's Width.
- Enter the tank's Height.
- Select the unit — Inches or Centimeters.
- Select Calculate to get the volume in both liters and US gallons.
Related Calculations
Once you know the true volume, use the Fish Stocking Calculator to estimate a safe stocking level for that tank.