Aquarium Volume in Gallons: Nominal vs True, US vs UK
An aquarium volume calculator in gallons is only as honest as its unit discipline. A US gallon is not an imperial gallon, a labeled size is not a true water volume, and a formula is only useful if you measure inside the glass. Here is the complete picture.
Published by Nan Liang
One gallon, two countries: US vs UK
The US gallon is defined as exactly 231 cubic inches — about 3.785 liters. The imperial (UK) gallon, still used in some Commonwealth fishkeeping literature, is about 4.546 liters — roughly 20% larger. A tank holding 100 US gallons holds only 83.3 imperial gallons. Dosing and heater sizing go wrong in exactly the proportion you mix these up, so every figure on a tank must say which gallon it means.
The conversion constants worth pinning: 1 US gal = 231 in³ exactly; 1 US gal = 3.785 L; 1 imperial gal = 4.546 L = 1.201 US gal; 1 L = 61.02 in³.
The full conversion table for standard sizes
Converting the eight common US standard tanks (using each maker’s nominal geometry, height measured inside to the waterline) across the three units keepers actually use:
| Nominal size | US gallons | UK gallons | Liters |
|---|---|---|---|
| 5 gal | 5.8 | 4.8 | 22.0 |
| 10 gal | 11.3 | 9.4 | 42.8 |
| 20 high | 20.9 | 17.4 | 79.1 |
| 20 long | 19.9 | 16.6 | 75.3 |
| 29 gal | 29.1 | 24.2 | 110.0 |
| 40 breeder | 42.6 | 35.5 | 161.2 |
| 55 gal | 56.7 | 47.2 | 214.5 |
| 75 gal | 79.0 | 65.8 | 299.0 |
These are geometric volumes from nominal inside dimensions. For dosing, drop to the true water volume in the next section.
Nominal vs true: why the label overshoots the water
Three things take volume back between the label and the waterline: the glass itself (a 3/8 in wall on a 55 displaces over 1,200 in³), the substrate bed (a 2 in bed on the same footprint displaces about 1,250 in³), and the air gap you leave at the rim. Community experience puts the stocked, filled figure around 10–20% under the nominal name — the ~20%-under caliber, estimate-grade.
Worked: a 55 labeled tank with 3/8 in glass, a 2 in substrate bed and a 1 in air gap holds about 43 US gallons of actual water — not 55. That is the number to use when mixing medication, calculating salt, or sizing a heater.
Measuring odd shapes without guessing
Rectangular tanks are trivial: length × width × height ÷ 231. The curved classics each have a usable formula. Cylinder: π × (diameter ÷ 2)² × height. Hexagon measured flat-to-flat: 0.866 × flat-to-flat² × height — note this is not the same as a cylinder approximation, which overcounts a hexagon. Bow-front: the flat back box plus the bow’s half-cylinder bulge; for a 36 in wide tank with a 4 in bow depth that adds roughly 5 US gallons over the flat box alone.
For any shape, measure inside the glass, from the inner bottom to your usual waterline. Outside dims systematically overcount; the error is small on a 10 gallon and material on a 75.
Reverse-checking volume with salt (marine tanks)
If a tank is already running and you genuinely cannot measure it, salt mix is a surprisingly good gauge. Marine salt mixes to a known ratio: Instant Ocean reaches SG 1.022 at 77 °F with about 1.5 lb per 5 US gallons. Divide the pounds of dry salt you added by the mixing ratio and you get the true water volume: 15 lb at a 0.300 lb-per-gallon mix works out to about 50 US gallons.
Treat the result as estimate-grade, good to about ±10–15%: it assumes all the salt went into this tank, the hydrometer reading is accurate, and nothing diluted the mix since. For freshwater dosing, measuring inside the glass beats chemistry every time.
Frequently asked questions
How do I calculate aquarium volume in gallons?
Measure inside the glass: length × width × height in inches, divided by 231 for US gallons (or multiply liters by 0.2642). Measure to your usual waterline, not the rim. For curved shapes use the matching formula — cylinder, hexagon flat-to-flat, or bow-front box-plus-bulge.
How many liters is a 55 gallon tank?
A US 55 holds about 56.7 US gallons by nominal geometry, which is about 214 liters. In imperial gallons the same tank is about 47. The labeled 55 is a US gallon figure — always check which gallon a source means.
Why does my 55 gallon tank hold less than 55 gallons of water?
Three displacements: glass walls and base, the substrate bed, and the air gap at the rim. With 3/8 in glass, 2 in of substrate and 1 in of air gap, a labeled 55 actually holds about 43 US gallons — around 20% under the name, which matches the long-standing community caliber.
What is the difference between US and UK gallons?
A US gallon is 231 cubic inches (3.785 L); a UK imperial gallon is 4.546 L, about 20% larger. A tank of 100 US gallons holds only ~83 imperial gallons. Equipment sizing and medication doses both go wrong if the two are mixed.
How do I measure a hexagon fish tank volume?
Measure the flat-to-flat width (not corner to corner) and the inside height. Volume in cubic inches is 0.866 × flat-to-flat² × height; divide by 231 for US gallons. Approximating a hexagon as a cylinder overcounts the volume.
How do I measure a bow-front aquarium?
Split it: the flat back box (back width × flat depth × height) plus the bow bulge, which is a half-cylinder: π × (width ÷ 2) × bow depth × height ÷ 2. Add the two in cubic inches and divide by 231. Example: 36 × 12 × 18 with a 4-inch bow adds about 5 gallons over the flat box.
Can I check my tank volume with salt?
Yes, for marine tanks: divide the pounds of dry salt added by the mixing ratio for your target specific gravity. Instant Ocean at SG 1.022 is about 0.300 lb per US gallon, so 15 lb implies roughly 50 gallons. Expect ±10–15% accuracy.
Should I dose medicine for the labeled or true volume?
Always the true water volume. Dosing against a labeled 55 when the tank actually holds 43 gallons overdoses by 28% — for medications with narrow safety margins that difference matters. Measure inside the glass to the waterline, subtract substrate height.
Sources
Want the numbers for your own tank? Try the free aquarium volume calculator — no signup, runs in your browser.