A rectangular (box) aquarium is the most common tank shape, and its volume comes from three measurements: length, width (depth) and height. The volume on a retailer's label is usually the gross volume calculated from the outside dimensions; this calculator also subtracts the glass, the substrate and the headroom to show how much water the tank really holds.
Along with the result you get approximate figures for water weight, filter flow rate and heater power, so you can plan the stand and equipment before you buy.
How is the volume of a rectangular aquarium calculated?
Gross volume (litres) = length × width × height ÷ 1000, with all sizes in centimetres, because 1 litre is 1000 cm³. In inches, divide by 231 to get US gallons. For the net water volume, two glass thicknesses are subtracted from the length and the width, and the bottom glass, substrate and headroom are subtracted from the height.
How to take the measurements
- Measure the length along the front glass, from outer edge to outer edge.
- Measure the width (front-to-back depth) across the outside of the side panels.
- Measure the height from the bottom to the top rim along the glass.
- Measure the glass thickness at an edge. Thickness ranges from a few millimetres to over 1 cm depending on tank size; if unsure, keep the default 0.8 cm.
Worked example
For a rectangular aquarium with length 100 cm, width 40 cm, height 50 cm, the gross volume is 200 litres. With 0.8 cm glass, no substrate and 2 cm of headroom, the net water volume is 178.3 litres.
Example sizes and their volumes
| Length (cm) | Width (cm) | Height (cm) | Gross (L) | Net water (L) | Net water (US gal) |
|---|---|---|---|---|---|
| 60 | 30 | 30 | 54 | 45.1 | 11.9 |
| 80 | 35 | 40 | 112 | 97.4 | 25.7 |
| 100 | 40 | 50 | 200 | 178.3 | 47.1 |
| 120 | 50 | 50 | 300 | 270.5 | 71.5 |
Net water volume assumes 0.8 cm glass, 0 cm substrate and 2 cm headroom; results are approximate.
Frequently asked questions
How do I calculate how many litres or gallons my aquarium holds?
Enter the outside dimensions. Length × width × height ÷ 1000 gives litres when measured in centimetres, so a 100 × 40 × 50 cm tank is 200 litres gross. In inches, length × width × height ÷ 231 gives US gallons, so a 48 × 18 × 24 in tank is about 89.8 gallons gross.
Why is the net water volume lower than the size on the box?
Retailers usually state the gross volume from the outside dimensions. The real water volume is lower because of the glass thickness, the substrate and the water level (headroom). The calculator shows this as the net water volume.
How much headroom should I leave?
The water level is usually kept a few centimetres below the top rim. The default here is 2 cm; you can change it in the advanced settings to match your tank.