Aquarium Volume Calculator
Estimate tank capacity in US gallons or liters using inches or centimeters.
US liquid gallons (1 US gal = 3.785411784 L). Switching units converts your entries.
Check the entered values and ranges before calculating.
Result
How to calculate aquarium volume
Choose the shape and enter inside dimensions at the waterline. The rectangular and cube options calculate geometric capacity; the cylinder option uses its diameter. All gallons are US liquid gallons, not UK imperial gallons.
Formula and units
Rectangular US gal = length × width × height (in) ÷ 231. Liters = length × width × height (cm) ÷ 1,000. Cylinder volume = π × (diameter ÷ 2)² × height.
Worked example
A rectangular tank measuring 48 × 18 × 20 inches internally holds about 74.81 US gallons (283.17 L) before displacement. Enter the height of the water rather than the full height of the glass.
Assumptions and limits
Bow front uses 85% of a rectangular volume based on maximum width; this is only a rough shape estimate. The optional sump adds geometric volume and does not subtract displacement. The water-weight result excludes the tank, stand, rocks and equipment.
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Frequently Asked Questions
How do I calculate fish tank gallons from inches?
For a rectangular tank, multiply internal length × width × water height in inches and divide by 231 to get US gallons. In centimeters, divide the product by 1,000 to get liters. Measure to the actual waterline.
Does this calculator subtract rock and sand displacement?
No. It calculates geometric volume and optionally adds the sump volume. Rocks, sand, equipment and empty space reduce actual water volume. Use your measured fill volume or a conservative working-volume estimate for dosing.
Are bow front volumes exact?
No. The bow front option multiplies length × maximum width × height by 0.85. Curved tanks vary, so check the result against manufacturer dimensions or a measured fill. All gallons shown here are US gallons.
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