Rectangular tank formula
If measurements are in centimetres, the result is cubic centimetres. Since 1 litre = 1,000 cm³, divide by 1,000 to get litres.
Worked rectangular tank examples
| Internal dimensions | Calculation | Capacity |
|---|---|---|
| 200 × 100 × 50 cm | 1,000,000 cm³ ÷ 1,000 | 1,000 L |
| 45.7 × 30.5 × 25.4 cm | 35,409.8 cm³ ÷ 1,000 | 35.41 L |
| 346 × 246 × 80 mm | 6,809,280 mm³ ÷ 1,000,000 | 6.81 L |
Use internal dimensions for actual liquid capacity. External dimensions include wall thickness and can overstate usable volume.
Metric shortcuts for litres
When dimensions are in metres, cubic metres convert directly using 1 m³ = 1,000 L. A tank 2 m × 1 m × 0.5 m has a volume of 1 m³ = 1,000 L. When dimensions are in millimetres, divide cubic millimetres by 1,000,000 to get litres.
Cylindrical tank formula
For a vertical cylinder, “length” in the formula is the liquid height. For a horizontal cylinder that is completely full, it is the physical cylinder length. A partially filled horizontal cylinder needs a circular-segment formula, so a dedicated tank calculator is easier and less error-prone.
How to calculate liquid already inside a rectangular tank
Measure the current liquid depth rather than the full tank height. If a 200 × 100 cm tank has 20 cm of liquid depth, the liquid volume is 200 × 100 × 20 ÷ 1,000 = 400 litres.
This method works only when the tank has straight vertical sides and a constant rectangular cross-section.
Allow for unusable space and freeboard
Rated tank capacity is not always the same as usable operating capacity. Pipe connections, internal baffles, sloped bottoms, air space, expansion allowance and safety freeboard can reduce the amount of liquid that should actually be stored. For fuel or chemicals, follow the tank manufacturer and applicable safety requirements.
Gallons versus litres
Do not mix US gallons and imperial gallons. 1 US gallon is about 3.785 litres, while 1 imperial gallon is about 4.546 litres. When converting a tank rating, identify which gallon definition is being used.
Measurement checklist
- Measure internal dimensions when possible.
- Use the same unit for every dimension.
- Identify the tank shape before choosing the formula.
- For partial fill, use the current liquid depth or the correct segment calculation.
- Separate theoretical geometric capacity from safe usable capacity.
Frequently asked questions
How many litres are in a 200 cm × 100 cm × 50 cm tank?
1,000 litres, using internal rectangular dimensions.
How do I convert cubic centimetres to litres?
Divide cubic centimetres by 1,000.
How do I convert cubic millimetres to litres?
Divide cubic millimetres by 1,000,000.
Should I use internal or external tank dimensions?
Use internal dimensions for the best estimate of actual liquid capacity.
Enter your own values, keep the assumptions visible and use this guide to understand the formula behind the result.