FinFettle tools
Aquarium volume calculator (gallons & litres)
Find out how many litres and gallons your tank holds — rectangular, bowfront, cylinder, hexagon or corner. Need extra precision for a sensitive treatment? The optional Advanced section estimates the actual water after glass, gravel and decor.
Measure the outside of the tank.
Measure the outside of the glass for each labelled edge.
Advanced: estimate the actual water (optional)
Glass, the gap below the rim, gravel and decor take up some space. For most medicines and fertilisers the tank size is close enough. This is worth doing for shrimp or snail tanks, copper or formalin-based treatments, or very rock-heavy tanks. All figures here are rough estimates.
Count plastic/silk plants and ornaments — live plants displace almost no water.
Stored on this device only.
Once you have the number, use it everywhere dosing matters: a confirmed ich treatment needs the actual water volume rather than the box rating, and a new quarantine tank should be sized and dosed from this figure too.
Add this calculator to your website
Run an aquarium club, shop or fishkeeping blog? You're welcome to embed this calculator for free. Paste the code below into any page that accepts HTML — it stays up to date automatically.
<iframe src="https://finfettle.com/embed/aquarium-volume" title="Aquarium volume calculator" width="100%" height="1100" style="border:0;max-width:760px" loading="lazy"></iframe> <p style="font-size:14px">Aquarium volume calculator by <a href="https://finfettle.com/tools/aquarium-volume">FinFettle</a></p>
The auto-height version adds a small script so the calculator grows to fit instead of scrolling. Some site builders strip scripts — the plain version works everywhere iframes do.
Common questions
- How much water does my tank actually hold?
- A bit less than its full size, because glass, the gap below the rim, gravel and decor take up space. For most medicines and fertilisers the full tank size is close enough. If you keep shrimp or snails, use a copper or formalin-based treatment, or have a very rock-heavy tank, open the calculator's Advanced section for a rough estimate of the actual water.
- How do I work out how many gallons my tank is without a calculator?
- Measure the inside length, width and height in inches, multiply them together, then divide by 231 to get US gallons (measure in centimetres and divide by 1,000 for litres instead). That gives you the tank's volume. This calculator does the same maths for awkward shapes too — bowfront, cylinder, hexagon and corner tanks.
- What units does this use?
- Results are shown in litres, US gallons and UK (imperial) gallons. A UK gallon (4.546 litres) is about 20% bigger than a US gallon (3.785 litres), so check which one a product label means — mixing them up is an easy mistake. Inputs inside the dose panel let you pick the label's unit.
- Do I need to know my glass thickness?
- No. Glass thickness is only used in the optional Advanced section. Leave it blank and we assume a typical thickness for the tank's size.
- Are the results exact?
- The tank volume is straight geometry from your measurements, so it's as accurate as your tape measure. The Advanced 'actual water' figure is a rough estimate — the space gravel, rock and decor take up can't be measured precisely from a few inputs.
Husbandry-first guidance — educational, and never a substitute for a vet. The actual-water figure is a rough estimate; displacement can't be measured perfectly.