Aquarium Volume Calculator
Calculate an aquarium's volume in gallons and liters from length, width, and height in inches. For stocking, heater, and filter sizing.
How this calculator works
An aquarium is a rectangular box, so its volume is length times width times height. Because tanks are measured in inches, the calculator divides by 231 cubic inches per gallon.
Formula: Gallons = (length in × width in × height in) ÷ 231. Liters = gallons × 3.785.
Worked example
A 48 in × 18 in × 21 in tank (a standard 75-gallon footprint):
- Cubic inches: 48 × 18 × 21 = 18,144 in³
- Gallons: 18,144 ÷ 231 ≈ 78.55 gallons
- Liters: 78.55 × 3.785 ≈ 297 L
Reference (common tanks)
| Dimensions (L×W×H in) | Gallons |
|---|---|
| 24 × 12 × 16 | 20 |
| 36 × 18 × 16 | 45 |
| 48 × 18 × 21 | 75 |
How to measure
Measure the inside length, width, and height in inches. Subtract about 10% for substrate and decor to estimate actual water volume, then size your heater and filter from that figure.
Frequently asked questions
How do I calculate aquarium gallons?
Multiply length, width, and height in inches, then divide by 231 — the number of cubic inches in a US gallon. The calculator does this and also converts to liters for equipment rated in metric.
Is the calculated volume the same as the water volume?
It's the tank's full internal volume. Real water volume is a bit less: you fill below the rim, and substrate, rocks, and decorations displace water. A planted or aquascaped tank can hold 10–15% less water than the glass volume.
Why does aquarium volume matter for stocking?
Fish stocking, filtration turnover, and heater wattage all scale with water volume. The common 'one inch of fish per gallon' guideline, heater sizing (3–5 watts per gallon), and filter flow (4–6× turnover per hour) all start from an accurate gallon count.
Does this work for a bowfront or hexagon tank?
It's exact for a rectangular tank. For curved or angled shapes, it slightly overestimates — a bowfront holds a bit more than a rectangle of the same footprint, a hexagon less. Use it as a close starting figure and adjust.
How is this different from a pond calculator?
Aquariums are measured in inches and hold gallons; ponds are measured in feet and hold thousands of gallons. Use this for tanks, and the Koi Pond or Pond Volume calculators for outdoor water features.
Estimates only. Verify quantities with your supplier before purchasing.