Why do litres, US gallons and UK gallons all differ?

The US gallon (3.785 L) and the UK/imperial gallon (4.546 L) are different units with no connection to each other. The litre is the SI base unit. When you buy fish food, additives or filter media, check whether the label refers to US or UK gallons — the difference is about 20%, which matters for dosing.

Does the calculator account for the glass thickness?

No — it calculates the gross internal volume from the dimensions you enter. If you measure the external dimensions, subtract about 1–1.5 cm per glass side (roughly 0.6 in) to get a more accurate net water volume. For most purposes the gross figure is fine for stocking and equipment sizing.

What formula is used for each shape?

Rectangular: L × W × H ÷ 1000. Cylinder: π × r² × H ÷ 1000. Bow-front: modelled as L × average(rear W, front W) × H ÷ 1000. Hexagonal: (3√3/2) × s² × H ÷ 1000 where s is side length. L-shaped: (L1×W1 + L2×W2) × H ÷ 1000. All inputs are in cm, result in litres.

How accurate is the bow-front calculation?

The bow-front formula averages the rear and front widths, which slightly underestimates the true volume because the curved front panel adds a lens-shaped pocket. The real volume is typically 5–10% higher than the displayed figure. For an exact result you would need to integrate the curved profile, but the average-width approximation is accurate enough for stocking and equipment decisions.

What is the 1 cm per 2 litres stocking rule?

This is a popular rule of thumb for small tropical community fish: add up the total adult length of all fish in centimetres; that number should not exceed half the tank volume in litres. For example, a 100 L tank could hold around 50 cm worth of fish — say ten 5 cm neon tetras. The rule breaks down for large, messy, or territorial species, and experienced fishkeepers prefer surface-area or bioload calculations instead.

How many gallons does a standard 120 cm aquarium hold?

A typical starter tank measuring 120 cm × 45 cm × 50 cm (approximately 48 in × 18 in × 20 in) holds 270 litres, which is about 71 US gallons or 59 UK gallons. These are gross figures; net water volume after substrate and decor is usually 10–15% less.

Aquarium Volume Calculator

An aquarium volume calculator that works for five common tank shapes — rectangular, cylinder, bow-front, hexagonal and L-shaped — and outputs the result in litres, US gallons and UK gallons simultaneously. It also shows the worked arithmetic so you can check the numbers at a glance.

How it works

Every volume formula reduces to one idea: find the horizontal cross-sectional area of the tank and multiply it by the water height. The only difference between shapes is how that cross-section is calculated.

Shape	Formula
Rectangular / Cube	L × W × H
Cylinder	π × r² × H
Bow-front	L × avg(rear W, front W) × H
Hexagonal	(3√3 ⁄ 2) × s² × H
L-shaped	(L₁ × W₁ + L₂ × W₂) × H

All dimensions are entered in the same unit (cm or inches) and the result is divided by 1 000 (for cm inputs) to convert cubic centimetres to litres. For inch inputs the tool first converts each measurement to centimetres (1 in = 2.54 cm) before applying the formula. Litres are then converted to US gallons (÷ 3.785) and UK gallons (÷ 4.546).

The bow-front is approximated as a rectangle whose width is the average of the rear and front widths. This slightly underestimates the true volume (typically by 5–10%) because the convex front panel adds extra water — but it is accurate enough for equipment sizing and fish stocking.

The hexagonal formula comes from the regular-hexagon area: a hexagon with side length s has area (3√3 / 2) × s², which equals roughly 2.598 × s². For a 25 cm side hexagon that gives an area of about 1,623 cm², and a 50 cm tall tank would therefore hold roughly 81 litres.

Worked example

A standard community tank measuring 120 cm × 45 cm × 50 cm:

120 × 45 × 50 = 270,000 cm³ → 270 litres 270 ÷ 3.785 ≈ 71.3 US gallons 270 ÷ 4.546 ≈ 59.4 UK gallons

Subtract roughly 10% for substrate, rocks and plants — actual water volume is around 243 litres (64 US gal / 53 UK gal). At one centimetre of fish per two litres of gross volume, this tank supports approximately 135 cm worth of small tropical fish — for example twenty-seven 5 cm neon tetras, or a mixed community of similar sized species.

Unit note

The US gallon and the UK (imperial) gallon are not the same unit and cannot be used interchangeably. The UK gallon is approximately 20% larger than the US gallon. Fish-keeping products sold in North America use US gallons; those sold in the UK and Australia typically quote litres or imperial gallons. When following dosing instructions, always confirm which gallon the label means — the calculator shows both so you can match either.

Accuracy and limitations

The calculator computes gross interior volume from the dimensions you enter. It does not account for:

Glass thickness — measure the inside dimensions for the most accurate result.
Substrate and decor — gravel, rocks, wood and filter equipment typically displace 10–20% of the gross volume.
Partial fills — the height field should reflect the intended water level, not the full tank height, if you do not fill to the brim.

All arithmetic runs entirely in your browser. Nothing is uploaded or stored.