How to measure your aquarium
Measure in one unit from start to finish. Length is the longest side along the front, width (or depth) runs front to back, and height is the base to the top rim. Ignore lids, hoods and trims.
Most people measure the outside of the tank, because that is the size printed on the box. The water sits inside the glass, so set "Measured: outside" and enter the glass thickness. The calculator then removes two panels from the length and width and one from the height. Glass is often 4–6 mm on small tanks and 10 mm or more on large ones; the manufacturer’s specification will tell you exactly.
- Bow-front: measure the straight back length, the depth at the ends, and the extra depth at the middle of the bow.
- Hexagonal: measure one glass panel edge. On a regular hexagon the flat-to-flat width is the edge × 1.732.
- Corner, quarter-circle: measure one straight back wall (the radius of the curved front).
- Corner, flat-front (pentagon): measure one back wall and one of the short side panels.
Aquarium volume formulas for every tank shape
Every tank is a floor shape times a height. Work out the floor area, multiply by the height, then convert. 1 litre = 1,000 cm³, 1 US gallon = 231 in³ = 3.785 L and 1 UK gallon = 4.546 L, all exact definitions (NIST).
| Shape | What to measure | Volume |
|---|---|---|
| Rectangular | Length L, width W, height H | L × W × H |
| Cube | Edge s | s × s × s |
| Cylinder | Diameter D, height H | π × (D ÷ 2)² × H |
| Bow-front | Back length L, end depth W, bow depth b, height H | (L × W + circular segment with chord L and depth b) × H |
| Hexagonal | Edge a, height H | 2.598 × a² × H |
| Corner, quarter-circle | Back wall R, height H | π × R² ÷ 4 × H |
| Corner, flat front | Back wall S, side panel F, height H | (S² − (S − F)² ÷ 2) × H |
Gross volume vs real water volume
The gross volume is what the tank would hold filled to the brim with nothing in it. Your fish live in less. Four things take space away:
- Glass. Outside measurements include the glass. On a 60 cm tank, 6 mm panels alone remove about 7.5% of the volume.
- The water line. Tanks are filled a few centimetres below the rim. A 2.5 cm (1 in) gap on a 36 cm tall tank is about 7% of the height.
- Substrate. Gravel and sand are not solid: water fills the gaps between grains. We count the solid part as about 60% of the substrate’s volume (about 50% for aquasoil), so 5 cm of sand removes less water than its full depth suggests.
- Rocks, wood and decor. Enter a share of the remaining water. 5–10% suits a lightly scaped tank; heavy rockwork can reach 20% or more.
If you run a sump or refugium, add its water volume. Medication and salt doses must cover the whole system, while the display volume is what the fish swim in.
Worked example: a 60 cm tank
A common "60 litre" tank measures 60 × 30 × 36 cm on the outside, with 6 mm glass, 5 cm of sand and a little rock and wood.
| Step | Calculation | Result |
|---|---|---|
| Box volume (outside) | 60 × 30 × 36 ÷ 1,000 | 64.8 L (17.1 US gal) |
| Inside the glass | 58.8 × 28.8 × 35.4 ÷ 1,000 | 59.9 L |
| Water 2.5 cm below the rim | 58.8 × 28.8 × 32.9 ÷ 1,000 | 55.7 L |
| 5 cm sand (60% solid) | 8.5 L of sand × 0.6 = 5.1 L removed | 50.6 L |
| 10% rock and wood | 50.6 × 0.9 | 45.6 L (12.0 US gal) |
Common aquarium sizes and their volumes
Box sizes are gross volumes from outside dimensions, so real water volume is always lower. Brands differ by a centimetre or so; measure your own tank for an exact figure.
| Tank | Dimensions | Litres | US gallons | UK gallons |
|---|---|---|---|---|
| 10 gallon | 20 × 10 × 12 in | 39.3 | 10.4 | 8.6 |
| 20 gallon long | 30 × 12 × 12 in | 70.8 | 18.7 | 15.6 |
| 29 gallon | 30 × 12 × 18 in | 106.2 | 28.1 | 23.4 |
| 40 gallon breeder | 36 × 18 × 16 in | 169.9 | 44.9 | 37.4 |
| 55 gallon | 48 × 13 × 21 in | 214.7 | 56.7 | 47.2 |
| 75 gallon | 48 × 18 × 21 in | 297.3 | 78.5 | 65.4 |
| 60 cm | 60 × 30 × 36 cm | 64.8 | 17.1 | 14.3 |
| 80 cm | 80 × 35 × 45 cm | 126.0 | 33.3 | 27.7 |
| 100 cm | 100 × 40 × 50 cm | 200.0 | 52.8 | 44.0 |
| 120 cm | 120 × 40 × 50 cm | 240.0 | 63.4 | 52.8 |
| 150 cm | 150 × 50 × 60 cm | 450.0 | 118.9 | 99.0 |
How much does a full aquarium weigh?
Fresh water at aquarium temperature weighs about 0.997 kg per litre (8.32 lb per US gallon), and seawater about 1.023 kg per litre (USGS). Water is only part of the load. Glass weighs about 2.5 kg per litre of glass, and dry sand or gravel about 1.5–1.6 kg per litre.
In the worked example, 45.6 L of water (45.4 kg), 12.4 kg of glass and 13.5 kg of sand add up to about 71 kg (157 lb) before rocks, the lid and equipment. That is roughly 9 lb per nominal gallon, which is why "about 10 lb per gallon" is a sensible planning figure.
Check the stand and floor
