What this calculator does
Aquarium volume is simply length × width × height, the same box-volume formula as any rectangular tank, converted into the litres or gallons that fish-keeping guides, filter ratings and dosing instructions are usually written in. Enter the three inside dimensions of the tank and the volume comes out in litres, US gallons and UK gallons at once.
This is a genuinely different question from converting a tank size you already know between gallons and litres: it starts from the tank's physical dimensions and works out the volume calculator aquarium keepers actually need before they know a gallon or litre figure at all, whether for a new build, a custom tank, or checking a manufacturer's stated capacity.
The formula
Multiply length, width and height together to get the volume in cubic centimetres, then divide by 1,000 to convert to litres. Gallons follow from the standard conversion factors: 1 US gallon is 3.78541 litres, and 1 UK (Imperial) gallon is 4.54609 litres.
| Term | Meaning |
|---|---|
| Litres | Volume in cubic centimetres divided by 1,000. |
| US gallon | 3.78541 litres, the gallon used in the United States. |
| UK (Imperial) gallon | 4.54609 litres, the gallon used in the UK, Ireland, Canada and Australia in older references. |
The inputs explained
| Field | What to enter |
|---|---|
| Length (cm) | The inside length of the tank. |
| Width (cm) | The inside width (front to back) of the tank. |
| Height (cm) | The inside height of the tank, from base to water line if you are not filling to the very top. |
When to use it
Sizing a new or custom-built tank
Before buying a filter, heater or lighting rated for a specific volume, working out the actual volume from the planned dimensions avoids under- or over-sizing that equipment.
Checking a manufacturer's stated capacity
Advertised tank volumes sometimes round generously; recalculating from the actual measured dimensions gives the real figure to plan stocking and water changes against.
Working out how much water a partial water change moves
Multiplying the tank's footprint (length × width) by the drop in water height during a water change gives the volume removed, using the same formula as the full tank calculation.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How volume changes with tank height
A fixed 60 by 30 cm footprint, with the height varying.
| Height | Volume | In US gallons |
|---|---|---|
| 20 cm | 36.0 L | 9.51 US gal |
| 30 cm | 54.0 L | 14.27 US gal |
| 36 cm | 64.8 L | 17.12 US gal |
| 45 cm | 81.0 L | 21.40 US gal |
| 60 cm | 108.0 L | 28.53 US gal |
Volume for common tank lengths
A fixed 30 cm width and 36 cm height, across a range of common tank lengths.
| Length | Volume | In US gallons | In UK/Imperial gallons |
|---|---|---|---|
| 30 cm | 32.4 L | 8.56 US gal | 7.13 imp gal |
| 45 cm | 48.6 L | 12.84 US gal | 10.69 imp gal |
| 60 cm | 64.8 L | 17.12 US gal | 14.25 imp gal |
| 90 cm | 97.2 L | 25.68 US gal | 21.38 imp gal |
| 120 cm | 129.6 L | 34.24 US gal | 28.51 imp gal |
| 150 cm | 162.0 L | 42.80 US gal | 35.64 imp gal |
Questions
Should I use the tank's outside or inside dimensions?
Use the inside dimensions. Glass or acrylic walls take up space that the outside measurements include but the water itself does not occupy, so outside dimensions overstate the true volume, more noticeably on thicker-walled tanks.
Will the tank actually hold this much water?
Not quite. This is the volume up to the height you entered; most tanks are not filled completely to the rim, and substrate, decorations and any internal filter housing all displace some water, so the true water volume is somewhat less than the brim-full figure.
Why does aquarium volume matter for stocking?
Stocking guidelines, filtration turnover rates and medication dosing are almost all expressed per litre or per gallon of tank volume, so an accurate volume figure is the starting point for applying any of them correctly.
How is this different from converting gallons to litres?
This calculator starts from physical dimensions and works out the volume from scratch. If you already know a tank's size in gallons or litres and just need the other unit, the dedicated gallons-to-litres conversion is the quicker route.
If you already know the tank's size in gallons or litres and just need to convert between the two, use the fish tank gallons to litres calculator. For the volume of a plain rectangular box, see the rectangle calculator.