Aquarium volume calculator
Measure the inside of the tank. For the height, use the water level. The substrate and decor take up room, so the displacement field takes a share off the total.
Use 164 L in the water change calculator
How much does the substrate take?
A 5 cm layer of gravel in a 45 cm deep tank is about 11% of the height. Gravel is about two-thirds solid (pea gravel has around 32% pore space and larger gravel about 30%, per the Kansas Geological Survey), so by my own estimate a shallow bed like this displaces about 7–8% of the volume. Rock and driftwood add more. My guess is about 10% for a lightly decorated tank and 15–20% for a heavy aquascape.
The label size on a tank is a nominal figure for the glass box. A set-up tank holds less water than that, so measure yours.
Common tank sizes
Outer dimensions from Aqueon's critical dimensions chart (PDF, rounded to the nearest 1/8 inch where the chart gives fractions). The last two columns run the calculator's rectangular math on those outer measurements with no substrate and the water at the very top of the box. Real water volume is lower: the glass is thick, the water sits below the rim and the bottom pane takes some height.
| Label | L x W x H (in) | Box liters | Box US gal |
|---|---|---|---|
| 10 gal | 20.25 x 10.5 x 12.56 | 43.8 | 11.6 |
| 20 long | 30.25 x 12.5 x 12.75 | 79.0 | 20.9 |
| 29 gal | 30.25 x 12.5 x 18.75 | 116.2 | 30.7 |
| 40 breeder | 36.19 x 18.25 x 16.94 | 183.3 | 48.4 |
| 55 gal | 48.25 x 12.75 x 21 | 211.7 | 55.9 |
| 75 gal | 48.5 x 18.5 x 21.13 | 310.6 | 82.1 |
| 125 gal | 72.5 x 18.5 x 23.13 | 508.3 | 134.3 |
The 40 breeder and the 75 come out well above their labels: a 40 breeder box holds 48 gallons and a 75 holds 82. The label is the nominal figure, and other brands differ by an inch or so in each direction. Measure inside your own glass.
Worked example: a round tank
A cylinder 40 cm across, filled to 50 cm, has a radius of 20 cm. The volume is 3.1416 x 20 x 20 x 50 / 1000 = 62.8 L. With 10% taken by sand and a couple of rocks, that leaves 62.8 x 0.9 = 56.5 L, or 14.9 US gallons. Pick Round (cylinder) above, enter 40, 50 and 10 to see the same numbers.