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Aquarium volume calculator
Measure your fish tank and get its volume in US gallons, liters and imperial gallons, plus how much the water weighs.
Your tank's volume in gallons and liters, and the weight of the water, will show here.
Measure inside the glass if you can; outside measurements slightly overstate the volume. Last reviewed October 7, 2026.
How to calculate aquarium volume
For a rectangular tank, multiply length × width × height to get the volume in cubic inches or cubic centimeters, then convert:
| Convert | Divide by |
|---|---|
| Cubic inches to US gallons | 231 |
| Cubic inches to imperial (UK) gallons | 277.42 |
| Cubic centimeters to liters | 1,000 |
Example: a 24 × 12 × 16 inch tank holds 4,608 cubic inches, or about 19.9 US gallons (75.5 liters).
For a round (cylinder) tank, the base area is π × radius², and for a corner tank shaped like a quarter circle it is a quarter of that, with the radius equal to the length of one straight side.
Common tank sizes
| Outside size (L × W × H) | Volume by these dimensions |
|---|---|
| 16 × 8 × 10 in | 5.5 US gal · 21 L |
| 20 × 10 × 12 in | 10.4 US gal · 39 L |
| 24 × 12 × 16 in | 19.9 US gal · 75 L |
| 30 × 12 × 18 in | 28.1 US gal · 106 L |
| 48 × 13 × 21 in | 56.7 US gal · 215 L |
| 48 × 18 × 21 in | 78.5 US gal · 297 L |
A tank sold as "20 gallons" usually holds a little less water than its outside dimensions suggest, because of the glass thickness, the gap below the rim, and the gravel and decor inside.
How heavy is a full aquarium?
Fresh water weighs about 1 kg per liter, or 8.34 lb per US gallon. A 20-gallon tank holds about 167 lb of water, before adding the glass, stand, gravel and rocks. Make sure the stand and floor can carry the full weight.
Questions people ask
How do I calculate how many gallons my fish tank holds?
Measure the length, width and height in inches, multiply them, and divide by 231 for US gallons. For example, 24 × 12 × 16 inches = 4,608 cubic inches ÷ 231 = about 19.9 gallons.
How do I convert aquarium liters to gallons?
Divide liters by 3.785 for US gallons, or by 4.546 for imperial gallons. A 100-liter tank is about 26.4 US gallons.
Should I measure inside or outside the glass?
Inside measurements give the true volume. Outside measurements are easier but include the glass, so they slightly overstate it.
How much does a 55-gallon aquarium weigh with water?
The water alone weighs about 460 lb (55 × 8.34). The glass, stand, gravel and rocks add more on top.