Fish Tank Water Volume Calculator UK

This calculator works out the water volume of a fish tank in litres and UK gallons. You enter the tank’s length, width and height in centimetres (or inches), and it returns the total volume, the water volume when filled to a typical level, and the weight of that water. It is designed for UK aquarists who need to dose treatments, choose a heater or filter, or simply know how much water their tank holds.

Formula

The volume of a rectangular tank is length × width × height. For a tank measured in centimetres, the volume in litres is:

Volume (litres) = (L × W × H) / 1000

where L is the internal length in cm, W is the internal width in cm, and H is the internal height in cm. Dividing by 1000 converts cubic centimetres to litres (1 litre = 1000 cm³).

To convert litres to UK gallons, multiply by 0.219969 (1 UK gallon = 4.54609 litres). To convert to US gallons, you would multiply by 0.264172, but this calculator uses UK gallons as the default.

Because tanks are rarely filled to the brim, the calculator also lets you enter a fill level percentage (default 95%). The actual water volume is then the full volume multiplied by that percentage.

The weight of the water is simply the volume in litres multiplied by 1 kg (freshwater at room temperature weighs approximately 1 kg per litre). For saltwater, the density is slightly higher, about 1.025 kg per litre, so the weight would be about 2.5% more.

Worked Example

Using the default values: length = 60 cm, width = 30 cm, height = 36 cm, fill level = 95%.

1. Full volume in cm³ = 60 × 30 × 36 = 64,800 cm³
2. Full volume in litres = 64,800 / 1000 = 64.8 litres
3. Water volume at 95% fill = 64.8 × 0.95 = 61.56 litres
4. Water volume in UK gallons = 61.56 × 0.219969 = 13.54 UK gallons
5. Water weight = 61.56 kg (freshwater)

The calculator rounds these to 61.6 litres, 13.5 UK gallons, and 61.6 kg.

Why You Need the Water Volume

Knowing your tank’s water volume is not just a curiosity – it is essential for safe fishkeeping. Most treatments, medications, and water conditioners are dosed per litre or per gallon. An overestimate means you add too much chemical, which can harm fish; an underestimate means the treatment is too weak to work. The volume also determines the correct size of heater (a rough guide is 1 watt per litre for tropical tanks) and the filter flow rate (usually 4-5 times the tank volume per hour for most freshwater setups).

When you buy a tank, the advertised size (e.g., “60 litre�) is often the nominal or exterior volume, not the actual water volume. Substrate, decorations, and the fact that you don’t fill to the top all reduce the real water volume. This calculator gives you a practical figure for daily use.

How to Measure Your Tank Correctly

To get an accurate result, measure the inside dimensions of the tank, not the outside. The glass thickness (typically 4-10 mm depending on tank size) can make a noticeable difference. Use a tape measure or ruler and measure from the inside glass surface to the opposite inside surface.

• Measure length, width, and height in centimetres – do not mix units.
• For the height, measure from the bottom glass to the top edge, but remember you will not fill to the very top.
• If your tank is not a simple rectangle (e.g., a bow-front or hexagonal tank), this calculator is not accurate – you would need a more specific formula or a physical measurement (e.g., using a bucket to add water and counting litres).

If you already know the volume in litres, you can work backwards: the calculator’s formula is straightforward, but you can also just use the litre value and multiply by 0.219969 for UK gallons.

Common Mistakes and Misconceptions

One of the most common mistakes is mixing up UK and US gallons. A UK gallon is about 4.55 litres, while a US gallon is about 3.79 litres – a significant difference. If you are following advice from an American source, make sure you know which gallon they mean. This calculator uses UK gallons, which is standard in the UK.

Another mistake is forgetting to account for the fill level. Most tanks are filled to about 2-3 cm below the top rim to leave space for the lid and to prevent fish jumping out. That can reduce the volume by 5-10%. Also, if you have a thick substrate (gravel or sand) or large decorations, they displace water, so the actual water volume is even lower. For dosing medications, you should ideally measure the real water volume, not the tank’s nominal size.

Finally, some people confuse the volume of the tank with the volume of water needed to fill it. The tank’s total volume includes the air space at the top, which is not water.

Limits of the Formula

This formula is only accurate for rectangular tanks with straight sides. For bow-front, corner, cylindrical, or other irregular shapes, you cannot use a simple length × width × height calculation. In those cases, the best approach is to fill the tank with a known volume of water (e.g., using a 5-litre bucket) and count how many buckets it takes. That gives you the true water volume, including the effect of substrate and decorations, though you would need to do it when the tank is empty.

The calculator also assumes freshwater (density 1 kg/L). For saltwater, the weight is slightly higher, but the volume is the same. If you need the weight for floor loading (e.g., checking if a stand can support the tank), remember to add the weight of the glass, substrate, and equipment – the water is only part of the total.

Frequently Asked Questions

How many litres is a 4ft by 2ft by 2ft tank?
Assuming the dimensions are in feet, convert to cm first: 4 ft = 122 cm, 2 ft = 61 cm, 2 ft = 61 cm. Volume = 122 × 61 × 61 / 1000 = 454 litres. That is the full volume; actual water volume at 95% fill is about 431 litres.

What is the difference between UK and US gallons?
A UK gallon is 4.54609 litres, while a US gallon is 3.78541 litres. So a UK gallon is about 20% larger. When you see “gallons� in a UK context, it is almost always UK gallons, but always check if the source is American.

Do I need to measure the inside or outside of the tank?
Always measure the inside. The glass thickness can add several centimetres to the outside dimensions, which would overestimate the volume. For example, a tank with 10 mm glass and outside dimensions of 60×30×36 cm has inside dimensions of about 58×28×34 cm, giving a volume of about 55 litres instead of 64.8 litres – a significant difference.

How much does a full 60-litre tank weigh?
Water weighs about 1 kg per litre, so 60 litres of water weighs about 60 kg. Add the weight of the glass (a 60-litre tank might weigh 10-15 kg), substrate (gravel can add 10-20 kg), and equipment, and the total can easily exceed 90 kg. Always check your floor can support this.

Can I use this calculator for a saltwater tank?
Yes, the volume is the same. The only difference is the weight: saltwater is denser, about 1.025 kg per litre, so 60 litres of saltwater weighs about 61.5 kg. The volume in litres or gallons is unaffected.

Why does my tank’s advertised size not match the calculated volume?
Manufacturers often list the nominal size based on exterior dimensions or include the air gap. Also, some tanks are described by the volume of the glass box, not the water it holds. Always use the actual water volume for dosing and equipment sizing.

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in l | Length | cm | 60 | 0
in w | Width | cm | 30 | 0
in h | Height | cm | 36 | 0
in level | Fill level | % | 95 | 0
out vol_l | Water volume | litres | l * w * h / 1000 * (level / 100) | 1
out vol_ukg | Water volume | UK gallons | l * w * h / 1000 * (level / 100) * 0.219969 | 1
out weight_kg | Water weight | kg | l * w * h / 1000 * (level / 100) | 1