The gallon number on the box is a rounded, outside-the-glass figure. To know what your tank really holds (for medication doses, heater size or filter flow) you need three measurements and two small corrections. This guide walks through them; the volume calculator does the arithmetic.
What you need
- A tape measure or ruler (a cloth tape helps on curved glass).
- Something to read the glass edge: on rimless tanks you can see it; on framed tanks, check the manufacturer's spec or estimate.
- For a final check, a bucket of known volume.
Step by step (rectangular tanks)
- Length: measure along the front, end to end. Outside is fine; you will take the glass off later.
- Width: measure front to back along one end.
- Height: measure from the bottom of the base glass to the top of the rim (exclude the stand and any trim below the base).
- Glass thickness: on a rimless tank, measure the edge of a pane. Most hobby tanks use roughly 4–10 mm (about 3/16–3/8 in) glass, thicker on taller tanks.
- Water level: measure from the top of the rim down to the water surface. Most tanks are filled to an inch or so below the rim.
The formula
For a rectangular tank, multiply the three dimensions. In inches, divide by 231 to get US gallons (a US gallon is defined as exactly 231 cubic inches). In centimeters, divide by 1,000 to get liters.
To get the inside volume, take two glass thicknesses off the length and the width (one pane at each end) and one off the height (the base pane), then use the water height instead of the full height.
Worked example: a 20 gallon high
Outside, a 20 gallon high is 24 × 12 × 16 inches: 24 × 12 × 16 = 4,608 in³ ÷ 231 = 19.9 gallons. With 5 mm glass and the water 1 inch below the rim it holds about 17.6 gallons (66 liters). That is 12% less than the outside figure, before any gravel or rock goes in.
Gravel, rock and wood
Everything in the tank takes the place of water. A 2 inch gravel bed, a few rocks and a piece of driftwood commonly take 5–15% of the volume; a heavy rockscape can take 25% or more. The calculator has a slider for this. If you dose medication, dose for the water that is actually there.
Odd shapes
- Cylinder: π × radius² × height. The calculator's cylinder mode takes the diameter and does this for you.
- Hexagon (regular, equal sides): 2.598 × side² × height, with the side measured along one pane.
- Bowfront: measure the depth at the center and at the ends, use the average as the width in the rectangular formula, and expect the result to be within a few percent.
- Bowls and spheres: use the calculator's sphere mode; the water level matters a lot because the bowl narrows at the top.
Checking with a bucket
The most accurate method is the simplest: on the next water change, count the buckets it takes to refill a known drop, or fill a new tank bucket by bucket. Five-gallon buckets are usually a little over five gallons to the brim, so mark a line at exactly 5 first.
Not sure of your tank's dimensions? The aquarium sizes chart lists every standard size.