Quick Answer
Aquarium volume depends on the tank shape, and the water you actually have depends on substrate and how far below the rim you fill. A "20 gallon long" measures 30 × 12 × 12 in, which is 70.8 L or 18.7 US gallons gross — and only 53.4 L (14.1 US gallons) once a 5 cm substrate bed and a 2.5 cm rim gap are subtracted, 24.6% less. Dosing that tank against its label would be 41.8% too much.
How to Use the Aquarium Volume Calculator
Pick the shape first — the input fields change to match it — then enter the measurements. Everything recalculates as you type.
- Choose the tank shape. Rectangular, cube, cylinder, bowfront, hexagonal or corner. Each one asks for different dimensions, so a cylinder wants a diameter and a height while a hexagon wants a side length.
- Enter the dimensions. Every field carries its own unit picker — mm, cm, m, inches or feet — and switching a picker converts the number already typed instead of clearing it.
- Or tap a standard size. With the rectangular shape active you get quick-fill chips for the common US glass tanks, from 5 gal up to 75 gal, in inches.
- Add substrate depth and how far below the rim you fill. These two fields are what separate the water you will actually have from the box’s raw capacity, and they default to zero.
- Read the result. The headline is net water volume, with US and imperial gallons underneath, plus gross volume, water weight, footprint area and the volume your substrate occupies.
The Six Shapes and Their Formulas
| Shape | Inputs | Volume formula |
|---|---|---|
| Rectangular | Length, height, width | V = L × W × H |
| Cube | Side length | V = side³ |
| Cylinder | Diameter, height | V = π × (d ÷ 2)² × H |
| Hexagonal | Side length, height | V = (3√3 ÷ 2) × side² × H |
| Corner | Radius, height | V = (π × r² ÷ 4) × H |
| Bowfront | Length, height, side width, bow depth | V = (L × W × H) + (⅔ × L × bow × H) |
Worked Example: a “20 Gallon Long”
A 20 long measures 30 × 12 × 12 inches. Here is what the calculator does with it, and where the label starts to mislead.
- Gross volume. 30 × 12 × 12 in works out to 70.8 L, which is 18.7 US gallons — already 6.5% under the name on the box.
- Subtract the substrate. The footprint is 2,323 cm². A 5 cm sand bed displaces 11.61 L.
- Subtract the rim gap. Nobody fills to the very top; 2.5 cm below the rim removes another 5.81 L.
- Net water. 53.4 L, or 14.1 US gallons — 24.6% less than the gross figure and nearly six gallons short of the label.
- Weight. That water alone is 53.3 kg (117.4 lb), before glass, stand, rock or substrate.
That gap matters for anything dosed by volume. Medication, fertiliser and water conditioner measured against “20 gallons” would be 41.8% overdosed in a tank actually holding 14.1.
Standard Tank Sizes: Nominal vs Actual
Tank names are marketing labels, not measurements, and they miss in both directions. These are the calculator’s own quick-fill sizes, computed as gross volume before any substrate:
| Sold as | L × W × H (in) | Litres | US gal | Imp gal | vs the label |
|---|---|---|---|---|---|
| 5 gal | 16 × 8 × 10 | 21 L | 5.5 | 4.6 | +10.8% |
| 10 gal | 20 × 10 × 12 | 39.3 L | 10.4 | 8.7 | +3.9% |
| 20 long | 30 × 12 × 12 | 70.8 L | 18.7 | 15.6 | −6.5% |
| 29 gal | 30 × 12 × 18 | 106.2 L | 28.1 | 23.4 | −3.3% |
| 40 breeder | 36 × 18 × 16 | 169.9 L | 44.9 | 37.4 | +12.2% |
| 55 gal | 48 × 13 × 21 | 214.7 L | 56.7 | 47.2 | +3.1% |
| 75 gal | 48 × 18 × 21 | 297.3 L | 78.5 | 65.4 | +4.7% |
A “40 breeder” is the outlier in the generous direction at 44.9 US gallons, while the 20 long is the one most people over-estimate. Measure your own tank rather than trusting the name — and remember every figure in this table is still gross, before a single gram of substrate goes in.
Why Substrate and Rim Offset Change the Answer So Much
Both losses are footprint × depth, which is why they bite hardest in shallow, wide tanks: the same 5 cm bed costs proportionally far more water in a 30 cm deep tank than in a 60 cm one. On the 20 long above, 7.5 cm of combined substrate and air removed almost a quarter of the volume.
Holding the same 20 long and the same 2.5 cm rim gap, and changing only the substrate depth:
| Substrate depth | Substrate volume | Net water | US gal | Lost vs gross |
|---|---|---|---|---|
| 0 cm (bare bottom) | 0 L | 65 L | 17.2 | 8.2% |
| 2 cm | 4.65 L | 60.3 L | 15.9 | 14.8% |
| 5 cm | 11.61 L | 53.4 L | 14.1 | 24.6% |
| 8 cm | 18.58 L | 46.4 L | 12.3 | 34.4% |
| 10 cm | 23.23 L | 41.8 L | 11 | 41% |
Even a bare-bottom tank loses 8.2% to the rim gap alone. A deep aquascaping bed of 10 cm takes the same 70.8 L box down to 41.8 L — the tank now holds 11 US gallons and is still sold as a 20. Above 10 cm the calculator flags the depth as unusual, since that territory belongs to planted aquascapes rather than general fishkeeping.
That works in reverse when you are buying rather than dosing. The same 5 cm bed is 11.61 L of material, and because substrate is sold by weight that comes to 18.60 kg of gravel — or 14.88 kg if you use a planted aquasoil, which is lighter per litre. The aquarium substrate calculator works that side of the problem, turning a footprint and a bed depth into kilograms and whole bags.
What the Water Alone Weighs
The calculator uses 0.998 kg per litre, water’s density at normal room temperature. It is the number that decides whether your floor and cabinet are adequate:
| Water volume | US gallons | Weight |
|---|---|---|
| 20 L | 5.3 | 20 kg (44 lb) |
| 50 L | 13.2 | 49.9 kg (110 lb) |
| 100 L | 26.4 | 99.8 kg (220 lb) |
| 200 L | 52.8 | 199.6 kg (440 lb) |
| 300 L | 79.3 | 299.4 kg (660 lb) |
| 450 L | 118.9 | 449.1 kg (990 lb) |
Glass, stand, substrate and rock typically add another 20–30% on top. Above 1,000 litres of net water the calculator raises a warning with the weight spelled out, because at that point the cabinet and the floor matter more than the fish do. If you are building the tank rather than buying it, the aquarium glass thickness calculator sizes the panes for the same dimensions, and the general volume calculator handles shapes outside the six here.
US Gallons, Imperial Gallons and Litres
The two gallons are genuinely different units, and aquarium listings rarely say which one they mean. The imperial gallon is defined in UK law as exactly 4.54609 cubic decimetres — see Schedule 1 of the Weights and Measures Act 1985 — while the US liquid gallon is 231 cubic inches, or 3.785411784 litres, listed among the NIST conversion factors.
That makes an imperial gallon 20.1% larger. A tank advertised as 55 gallons holds 214.7 L, which is 56.7 US gallons but only 47.2 imperial — so a UK buyer reading an American listing as imperial would over-estimate the tank by a fifth. The calculator shows both figures side by side underneath the main result for exactly this reason. Water temperature also shifts density slightly; if you are converting heater settings while you are at it, the centigrade to fahrenheit converter covers that.
