Fish Tank Volume Calculator UK: Litres & Gallons

This fish tank volume calculator works out the water capacity of a rectangular, square, or bow-front aquarium in litres and UK gallons. You enter the tank’s dimensions in centimetres or inches, and the calculator returns the total volume and the actual water volume (allowing for substrate and the glass thickness). The first result shown is the total volume in litres, which is the standard unit for aquarium sizing in the UK.

Formula

The calculator uses the basic geometric volume formula for a rectangular prism:

Volume (cm³) = Length × Width × Height

To convert cubic centimetres to litres, divide by 1,000 (because 1 litre = 1,000 cm³). To convert litres to UK gallons, multiply by 0.219969 (since 1 UK gallon = 4.54609 litres).

For the water volume, the calculator subtracts the glass thickness from the length and width (twice, for both sides) and subtracts the glass thickness plus substrate depth from the height. This gives the internal water-holding dimensions.

Variables and units:

• Length, Width, Height – external tank dimensions in centimetres (cm).
• Glass thickness – thickness of the glass walls in cm (usually 0.4–1.2 cm for home aquariums).
• Substrate depth – average depth of gravel or sand in cm (optional, but reduces water volume).
• Volume – in litres (L) and UK gallons (UK gal).

Worked Example

Using the default values from the calculator: Length = 60 cm, Width = 30 cm, Height = 36 cm, Glass = 0.6 cm, Substrate = 5 cm.

1. Total volume in cm³ = 60 × 30 × 36 = 64,800 cm³.
2. Convert to litres: 64,800 ÷ 1,000 = 64.80 litres.
3. Convert to UK gallons: 64.80 × 0.219969 = 14.25 UK gal.
4. Water volume: internal length = 60 – 2×0.6 = 58.8 cm; internal width = 30 – 2×0.6 = 28.8 cm; internal height = 36 – 0.6 – 5 = 30.4 cm.
5. Water volume in cm³ = 58.8 × 28.8 × 30.4 = 51,455.23 cm³ (approx).
6. Water volume in litres = 51,455.23 ÷ 1,000 = 51.46 litres.
7. Water volume in UK gallons = 51.46 × 0.219969 = 11.32 UK gal.

The calculator shows the same figures (rounded to two decimals).

Why Use Litres Instead of Gallons?

In the UK, aquarium equipment and fish stocking guidelines are almost always given in litres. Most fish tanks sold in the UK are labelled with their litre capacity (e.g., “60L�, “200L�). While US gallons (3.785 L) are common in American guides, UK gallons (4.546 L) are larger, so mixing them up can lead to under- or over-dosing treatments. This calculator uses UK gallons for the gallon output, but the litre figure is the one you should rely on for most practical purposes.

How to Measure Your Tank Correctly

To get an accurate volume, measure the external dimensions of the tank from the outside edges. Use a rigid ruler or tape measure, and measure in centimetres for the most direct conversion to litres. If you measure in inches, convert each dimension to centimetres first (1 inch = 2.54 cm) before entering them.

For the water volume, you need to know the glass thickness. Most standard tanks have glass between 4 mm and 10 mm thick. You can measure it with a caliper or check the manufacturer’s spec. If you don’t have that, you can estimate: for tanks under 60 cm long, 0.4–0.6 cm is typical; for larger tanks, 0.8–1.0 cm or more.

Substrate depth is the average height of the gravel or sand layer. If the substrate is sloped, use the average depth. This matters because substrate displaces water – a 5 cm layer across a 60×30 cm base takes up about 9 litres of space.

Common Mistakes and Misconceptions

• Mixing up UK and US gallons: A UK gallon is about 20% larger than a US gallon. If you use a US-gallon calculator, your 60 cm tank would show about 17.1 US gallons, but only 14.25 UK gallons.
• Forgetting to subtract glass and substrate: The total volume is the outside of the glass box, not the water inside. Water volume is always less – sometimes 10–20% less depending on glass and substrate.
• Assuming 1 litre = 1 kg: For freshwater, that’s true (1 litre of water weighs 1 kg). But the total weight of the tank includes glass, substrate, and equipment – the water weight is only part of the story when checking floor load.
• Rounding too early: If you round each dimension to the nearest cm, you can lose several litres. Measure to the nearest 0.5 cm at least.

How to Use the Volume Result

The volume tells you several things:

• Stocking fish: A common guideline is 1 cm of fish (adult size) per litre of water, but this is very rough. Always research specific species.
• Medication dosage: Most fish medications are dosed per litre of water. Using the total volume instead of the actual water volume can overdose fish – always use the water volume (with substrate and decor considered).
• Heater and filter sizing: A heater rated for 50–100 L will handle a 60 L tank, but a 100 L tank needs a more powerful heater. Filters are rated by litres per hour – aim for 4–6 times the tank volume per hour.
• Water changes: If you do a 25% water change, you need to know the water volume to mix the right amount of dechlorinator.

Limits of the Formula

The formula assumes a perfect rectangular box. Many tanks are not exactly rectangular – bow-front, corner, hexagonal, or cylindrical tanks have different volume formulas. For those, you need a shape-specific calculator or a more complex formula. Also, the formula doesn’t account for the volume displaced by decorations, plants, or the filter itself. If you need a precise water volume for medication, it’s safer to measure the actual water you add with a bucket or a flow meter.

Another limit: the glass thickness is not always uniform, and the substrate may not be level. The result is an estimate, not an exact measurement. For practical purposes, it’s accurate enough for stocking and equipment sizing.

Frequently Asked Questions

How many litres is a 4ft by 2ft by 2ft tank?

First convert feet to centimetres: 4 ft = 121.92 cm, 2 ft = 60.96 cm, 2 ft = 60.96 cm. Volume = 121.92 × 60.96 × 60.96 = 453,000 cm³ approx, which is 453 litres. In UK gallons, that’s about 99.6 UK gal. This is a common “large� tank size, often sold as a 450–500 litre system.

How do I calculate the volume of a bow-front tank?

For a bow-front (curved front) tank, the formula is more complex. You can approximate it by treating it as a rectangle with an average width – measure the width at the middle and at the ends, take the average, and use that in the rectangular formula. For a more accurate result, use a dedicated bow-front calculator or fill the tank with a known volume of water and measure it.

What is the difference between UK and US gallons?

A UK gallon (imperial gallon) is 4.54609 litres. A US gallon is 3.78541 litres. So a UK gallon is about 20% larger. When a fish tank is labelled “50 gallons� in the UK, it means 50 UK gallons (227 litres), but in the US it would be 50 US gallons (189 litres). Always check which gallon is being used.

Does the volume include the glass?

No, the total volume in this calculator is based on the external dimensions, so it includes the glass. The water volume subtracts the glass thickness, so it represents the actual water capacity. If you want the volume of water the tank holds when filled to the brim, use the water volume with substrate depth set to 0.

How many fish can I put in a 60 litre tank?

There is no single answer – it depends on the species, their adult size, and their behaviour. A very rough rule is 1 cm of adult fish length per litre, but that fails for territorial or high-waste fish. For a 60 L tank, you might keep a small school of tetras (e.g., 6–8 neon tetras) plus a couple of small catfish, but always research each species’ minimum tank size.

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in length | Tank Length | cm | 60 | 0
in width | Tank Width | cm | 30 | 0
in height | Tank Height | cm | 36 | 0
in glass | Glass Thickness | cm | 0.6 | 0
in substrate | Substrate Depth | cm | 5 | 0
out vol_l | Total Volume | litres | (length * width * height) / 1000 | 2
out vol_g | Total Volume (UK gal) | UK gal | ((length * width * height) / 1000) * 0.219969 | 2
out water_l | Water Volume (litres) | litres | ((length - 2*glass) * (width - 2*glass) * (height - glass - substrate)) / 1000 | 2
out water_g | Water Volume (UK gal) | UK gal | (((length - 2*glass) * (width - 2*glass) * (height - glass - substrate)) / 1000) * 0.219969 | 2