Aquarium Calculator

Work out your tank's water volume in gallons or liters from its length, width, and height, plus the water weight at the fill level you actually use.

Quick Facts

Formula
Volume = Length × Width × Height ÷ 231 (gallons) or ÷ 1000 (liters)
Standard rectangular-tank formula; measure the inside of the glass.
Water weight
≈ 8.345 lb per US gallon, ≈ 1 kg per liter
Freshwater at room temperature; saltwater runs a few percent heavier.

Your Results

Calculated
Water volume
-
At your fill level
Full tank volume
-
Brim-full geometric volume
Alternate unit
-
Water volume converted
Water weight
-
Approximate, freshwater

Ready

Enter your tank's dimensions and fill level, then press Calculate.

About the Aquarium Volume Calculator

This calculator finds how much water a rectangular aquarium holds. It multiplies length by width by height to get the geometric volume, then converts that figure to US gallons (dividing by 231) or liters (dividing by 1000), depending on the unit system you pick. Because most tanks are not filled all the way to the rim, it also applies a fill-level percentage so the "water volume" result reflects what you actually pour in, not the tank's maximum brim-full capacity.

The formula

  • Full tank volume (US): Length × Width × Height (inches) ÷ 231 = gallons
  • Full tank volume (metric): Length × Width × Height (centimeters) ÷ 1000 = liters
  • Water volume: Full tank volume × (Fill level ÷ 100)
  • Water weight: Water volume × 8.345 lb/gal (US) or × 1 kg/L (metric), for freshwater

How to measure your tank

  • Measure the inside dimensions of the tank, not the outer glass or frame — outer measurements overstate the volume.
  • For "Water height," use where the waterline actually sits, not the full rim height — most tanks run 80-90% full to allow for a lid, filter flow, and evaporation.
  • Substrate, rock, driftwood, and equipment displace additional water once the tank is set up, so the calculated figure is a starting estimate, not the exact volume after decorating.

Practical context

Use the water volume result for tasks like dosing water conditioner or medication (both are usually rated per gallon or per liter), sizing a heater or filter (rated in watts or gallons-per-hour relative to tank volume), or estimating stocking capacity. Use the water weight result to confirm a stand, shelf, or floor can support the tank once filled — water alone in a 55-gallon tank weighs well over 400 pounds, before adding the glass, substrate, and rock.

Frequently Asked Questions

What is the formula for aquarium volume?
For a standard rectangular tank, volume equals length times width times height. In inches, dividing that product by 231 converts cubic inches to US gallons. In centimeters, dividing by 1000 converts cubic centimeters to liters. Measure the inside of the glass, not the outside.
Should I use the full tank height or the water height?
Use the water height, not the rim height. Most aquariums are filled to roughly 80-90% of their full height to leave room for a hood, filter return flow, and evaporation, so the usable water volume is smaller than the tank's brim-full geometric volume.
Why does my calculated volume not match the tank's labeled size?
Manufacturer size names like "20 gallon long" are nominal, rounded figures. The actual interior volume is usually a bit less once glass thickness is accounted for, and substrate, rock, and decorations displace additional water once the tank is set up.
How much does the water in my tank weigh?
Freshwater weighs about 8.345 pounds per US gallon, or about 1 kilogram per liter. Multiply your water volume by that figure for an estimate. This matters for checking that a stand, floor, or shelf can support a filled tank, since a 55-gallon tank holds well over 400 pounds of water alone.

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