Tank Volume Calculator UK: Litres & Gallons

This tank volume calculator works out the capacity of rectangular, cylindrical, and elliptical tanks in litres, UK gallons, and US gallons. You enter the tank’s dimensions (length, width, height, or diameter) and the calculator returns the total volume and, if you specify a fill height, the volume of liquid currently in the tank. The tool is designed for UK users who need to size water tanks, fuel storage, fish tanks, or chemical containers and want results in imperial or metric units.

Formula

The calculator uses the standard geometric volume formulas for each shape, then converts cubic centimetres to litres and gallons.

• Rectangular tank: Volume = length × width × height (all in cm). Divide by 1000 to get litres.
• Cylindrical tank: Volume = π × (diameter/2)² × height. The radius is half the diameter.
• Elliptical tank: Volume = π × (length/2) × (width/2) × height. This treats the horizontal cross-section as an ellipse.

All dimensions are in centimetres, so the volume comes out in cubic centimetres (cm³). One litre equals 1000 cm³, so dividing by 1000 gives litres. To convert litres to UK gallons, multiply by 0.219969 (since 1 UK gallon = 4.54609 litres). For US gallons, multiply by 0.264172.

Worked Example

Using the default values: a rectangular tank with length 120 cm, width 60 cm, height 45 cm, and fill height 40 cm.

1. Total volume in cm³ = 120 × 60 × 45 = 324,000 cm³
2. Total volume in litres = 324,000 / 1000 = 324.00 litres
3. Liquid volume in cm³ = 120 × 60 × 40 = 288,000 cm³
4. Liquid volume in litres = 288,000 / 1000 = 288.00 litres
5. UK gallons = 324 × 0.219969 = 71.27 gallons (rounded to 2 decimals)

The calculator shows the same: 324.00 litres total, 288.00 litres liquid, and 71.27 UK gallons.

Why Tank Volume Matters

Knowing the exact capacity of a tank is essential for several practical reasons. For a fish tank, the volume determines the number of fish you can safely keep and the dosage of water treatments. For a rainwater harvesting system, the tank size must match your roof area and average rainfall to avoid overflow or underuse. For fuel or chemical storage, volume is critical for safety, transport, and compliance with local regulations. Even for a simple garden water butt, knowing the volume helps you plan watering schedules.

In the UK, tanks are often measured in litres, but some products (e.g., fuel tanks, oil storage) are rated in UK gallons. This calculator gives both, so you don’t have to convert manually.

How to Measure Your Tank Correctly

Accurate dimensions are the key to a correct volume. Use a tape measure and measure the internal dimensions, not the external ones, because the wall thickness reduces the usable capacity. For a cylindrical tank, measure the internal diameter at the widest point and the internal height from the bottom to the top (not including any lid or fittings). For rectangular tanks, measure length, width, and height from the inside surfaces.

If the tank is not perfectly full, you can measure the fill height – the vertical distance from the bottom to the liquid surface. The calculator then computes the liquid volume using the same formula with the fill height instead of the full height. This is useful for checking how much water is left in a tank or how much you have used.

Common Mistakes and How to Avoid Them

• Using external dimensions: Always measure inside. Wall thickness can reduce volume by 5–10%.
• Mixing units: If you measure in inches, convert to cm first (1 inch = 2.54 cm). The calculator expects cm.
• Confusing UK and US gallons: A UK gallon is about 20% larger than a US gallon. This calculator uses UK gallons, but you can easily convert to US gallons by multiplying the litres by 0.264172.
• Forgetting the height of the fill: The liquid volume is not the same as the total volume unless the tank is full. Always enter the actual fill height.
• Elliptical tanks: The formula assumes a perfect ellipse. If your tank has a different shape (e.g., oval with flat sides), the result will be approximate.

Limitations of the Formula

The formulas assume a simple geometric shape with straight sides and flat ends. Real tanks may have dished ends, conical bottoms, or internal baffles that change the volume. For a cylindrical tank with domed ends, the formula gives only the cylindrical part; you would need to add the volume of the domes separately. Similarly, a tank with a sloped bottom (like some IBC containers) will have a different volume than a straight-sided one. The calculator is a good estimate for standard tanks, but for precise measurements (e.g., for chemical dosing), you should calibrate with a known quantity of water.

Also, the fill height method assumes the tank is upright and the liquid surface is level. If the tank is tilted or the liquid is sloshing, the reading will be inaccurate.

Choosing the Right Tank Size

When you need to buy a new tank, use this calculator to check the volume of a candidate model. For example, a rectangular tank that is 100 cm × 50 cm × 50 cm holds 250 litres. If you need 300 litres, you might choose a tank that is 120 cm × 60 cm × 45 cm (324 litres). Always allow a small margin – about 10% – for expansion, air space, or fittings. For fish tanks, the general rule is 1 cm of fish per litre of water, but this depends on the species and filtration.

For rainwater harvesting, a typical 4-person household in the UK might use 200–300 litres per day for gardening. A 1000-litre tank could cover a few days of dry weather. The calculator helps you match the tank to your needs and available space.

Safety and Accuracy Notes

When measuring a tank that contains liquid, be careful: if it is a fuel or chemical tank, follow the site safety rules. Do not open hatches or enter confined spaces without proper training and equipment. For water tanks, ensure the water is safe to handle. Always double-check your measurements and calculations, especially if the tank is for a critical application like drinking water storage or chemical containment. If in doubt, consult a professional.

Frequently Asked Questions

How many litres are in a UK gallon?

One UK gallon is exactly 4.54609 litres. To convert litres to UK gallons, divide by 4.54609 (or multiply by 0.219969). This is different from the US gallon, which is 3.78541 litres.

What is the difference between a cylinder and an elliptical tank?

A cylinder has a circular cross-section, so its volume is π × r² × h. An elliptical tank has an elliptical cross-section, which is like a squashed circle. Its volume is π × (length/2) × (width/2) × h, where length and width are the two diameters of the ellipse. If the ellipse is a circle (length = width), the formula gives the same as a cylinder.

Can I use this calculator for a fish tank?

Yes. Enter the internal dimensions of your aquarium. The calculator gives the total volume and the liquid volume if you enter the fill height. Remember that the actual water volume will be slightly less due to gravel, decorations, and the thickness of the glass, so subtract about 10% for a more realistic water volume.

How do I measure a round tank’s diameter?

Measure the widest distance across the opening, from one inner edge to the opposite inner edge, passing through the centre. If the tank is closed, measure the outside and subtract twice the wall thickness. For a horizontal cylindrical tank, the diameter is the vertical height of the cylinder when lying on its side.

Why does the calculator ask for fill height?

The fill height lets you calculate the volume of liquid currently in the tank, not just the total capacity. This is useful for monitoring usage or determining how much is left. If you want the total volume, set the fill height equal to the tank height.

Is the result in UK or US gallons?

The calculator outputs UK gallons by default. If you need US gallons, multiply the litre result by 0.264172. The difference is significant: 100 litres is about 22 UK gallons but 26.4 US gallons.

Related

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in shape | Tank shape | Rectangular=1, Cylindrical=2, Elliptical=3 | 1 | 0
in length | Length (cm) | cm | 120 | 0
in width | Width (cm) | cm | 60 | 0
in height | Height (cm) | cm | 45 | 0
in fill | Fill height (cm) | cm | 40 | 0
out volume | Total volume | litres | (shape==1 ? length*width*height/1000 : shape==2 ? pi*(width/2)^2*height/1000 : pi*(length/2)*(width/2)*height/1000) | 2
out fill_vol | Liquid volume | litres | (shape==1 ? length*width*fill/1000 : shape==2 ? pi*(width/2)^2*fill/1000 : pi*(length/2)*(width/2)*fill/1000) | 2
out gallons_uk | UK gallons | gal | volume*0.219969 | 2