Tank volume calculator: litres, US gallons & UK gallons
Every stocking rule, heater wattage chart and water-change schedule is really a statement about water volume — so the volume number is where aquarium math starts. Enter your tank's dimensions below to get litres, US gallons and imperial (UK) gallons side by side, then check the converter if a guide quotes a unit you don't use.
Key takeaways
- 1 US gallon = 3.785411784 L exactly; 1 imperial gallon = 4.54609 L exactly — both fixed by definition, not approximation.
- Glass thickness, water line and substrate typically shave 10–15% off the volume printed on the box.
- Stocking rules built on "gallons" almost always mean US gallons — a UK gallon holds about 20% more.
The calculator
| Volume | — |
| US gallons | — |
| Imperial (UK) gallons | — |
Enter INTERNAL water dimensions, or add glass thickness to convert from external.
Aquarium unit converter
The three units fishkeepers meet constantly, converted exactly. The US gallon is defined as 231 cubic inches; since the inch is exactly 2.54 cm, that is 3.785411784 litres. The imperial gallon has been 4.54609 litres by law in the UK since 1985. Both figures come from NIST's SI guide (Appendix B), so there is no rounding judgment involved.
| Litres | — |
| US gallons | — |
| UK gallons | — |
| Cubic inches | — |
| Cubic centimetres | — |
Exact conversions: 1 US gal = 3.785411784 L · 1 UK gal = 4.54609 L · 1 in³ = 16.387064 cm³ (NIST SP 811).
Methodology
Rectangular tanks use internal length × width × height ÷ 1000 = litres. Cylindrical tanks (bow-front front area varies, cylinders are exact) use π × (diameter ÷ 2)² × height ÷ 1000. If you enter a glass thickness, the calculator treats your dimensions as external and subtracts one bottom panel plus two side walls from each horizontal dimension — a deliberately simple, slightly conservative model that ignores rim bracing and corner silicone.
Worked example: a tank externally 45 × 25 × 30 cm with 4 mm glass has an internal volume of about 31.7 L against 33.75 L external — the glass alone displaces 2.09 L (6%). A standard 60 × 30 × 40 cm tank filled to the brim is exactly 72 L (19.0 US gal, 15.8 UK gal), but with substrate and a sensible water line it usually holds closer to 60 L of actual water.
Sources
- NIST Special Publication 811, Appendix B.8 — exact gallon, litre and inch relations (checked 2026-11-16).
- Geometry: standard volume formulas; no hobby source needed.
FAQ
How many litres are in a 20-gallon tank?
20 US gallons is 75.7 litres; 20 imperial gallons is 90.9 litres. The difference is large enough to change a stocking decision, so check which unit a care guide uses — UK and older Commonwealth sources may mean imperial.
Why does my tank hold less than its rated size?
Manufacturers quote external, brim-full figures. Glass thickness, a water line a few centimetres below the rim, substrate and hardscape easily remove 10–15%. For dosing medication or fertilizer, always calculate from measured internal dimensions at the real water line — overdosing a smaller real volume is a classic way to hurt fish.
US or UK gallon — which do stocking rules mean?
Almost always the US gallon, because most published stocking rules (including the legacy "1 inch per gallon") are American in origin. Our stocking calculator labels every unit it uses.
How do I measure a tank for volume?
Inside glass to inside glass, and only up to the intended water line. If you have just external measurements, type them in and set the glass thickness so the displacement is subtracted for you.
Example: starter tank kits (affiliate concept)
When a calculator shows your real water volume, an upgrade is sometimes the honest answer. This is where we would list vetted tank kits.
Check price →Heuristic, not gospel
Volume is exact arithmetic, but what a volume means for stocking is not: swimming space, filtration and territory matter too. Treat these numbers as planning inputs and research your specific species — start with our guides on tank sizes for common fish and why the inch-per-gallon rule fails.