Tank volume calculator
Calculate tank capacity in litres, gallons and weight. Supports rectangular, cylindrical, L-shaped and trapezoidal tanks.
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Worked example
A rectangular diesel tank measuring 600 mm × 400 mm × 300 mm (internal dimensions):
- Volume = 0.6 × 0.4 × 0.3 = 0.072 m³ = 72 litres
- In UK gallons: 72 ÷ 4.546 = 15.8 UK gallons
- Weight when full of diesel (0.845 kg/L): 72 × 0.845 = 60.8 kg
- At 50% fill: 36 litres / 30.4 kg
Use millimetres in the unit selector for easier entry of smaller tanks.
Tank shapes explained
- Rectangular — a simple box, most common for diesel and water tanks
- Cylindrical — round tanks, common for LPG and some water tanks
- L-shaped — two rectangles joined at a corner, sharing the same height
- Trapezoidal — wider at top than bottom, common in hull-fitted tanks
Frequently asked questions
Should I use internal or external dimensions?
Always use internal dimensions — the space the fluid actually occupies. External dimensions include the tank wall thickness, which is typically 3–6 mm for GRP or stainless tanks. For aluminium tanks the walls can be thicker.
How do I calculate how much water my boat tank holds?
Measure the internal length, width and height of the tank in millimetres. Multiply them together and divide by 1,000,000 to get litres. For example: 800 mm × 400 mm × 350 mm = 112,000,000 mm³ ÷ 1,000,000 = 112 litres.
How heavy is a full diesel tank?
Diesel weighs approximately 0.845 kg per litre. A 100-litre tank weighs around 84.5 kg when full — a significant addition to boat trim. Fresh water weighs 1.0 kg/L; a 100-litre water tank adds a full 100 kg.
My tank is an odd shape — which option should I choose?
Most boat tanks approximate a rectangle or trapezoid. If yours is irregular, use the closest shape and accept a small margin of error. For tanks with complex curves, the L-shape option lets you model tanks with a notch cut from one corner.
Fluid densities used
| Fluid | Density (kg/L) |
|---|---|
| Fresh water | 1.000 |
| Sea water | 1.025 |
| Diesel | 0.845 |
| Petrol | 0.745 |
| LPG (liquid) | 0.510 |