Fuel expands as it warms, so a warm litre holds less fuel than a cold one and reads lighter on a hydrometer. Correcting density and volume to one reference temperature, 15 °C, lets every reading be compared with every other, whatever the weather.
Fuel grows when it warms
Every liquid expands with heat, and petrol more than most. Near 745 kg/m³, petrol grows by about 0.12% for each degree. Diesel near 830 kg/m³ grows by about 0.09% per degree.
| 10,000 litres at 15 °C becomes | at 25 °C | at 35 °C |
|---|---|---|
| Petrol, 745 kg/m³ | about 10,123 L | about 10,250 L |
| Diesel, 830 kg/m³ | about 10,087 L | about 10,177 L |
The weight stays the same. Only the volume, and so the density, changes.
Why a reference temperature
Without one, a tanker loaded at a cool depot at dawn and unloaded at a hot pump in the afternoon would show more litres than were loaded, and a lower density. Neither would be real. With every figure corrected to 15 °C, the two ends agree.
15 °C is the reference used across the world’s oil trade, and the one ASTM D1250 Tables 53B and 54B are built on.
In practice
- Density: always compare densities at 15 °C. Use Table 53B.
- Volume: to compare litres at different temperatures, correct them with the volume correction factor.
Figures here are for reference. Your oil company’s certified calibration chart, approved instruments and the Marketing Discipline Guidelines always prevail.
Questions
How much does petrol expand?
About 0.12% per degree near 745 kg/m³. 10,000 litres of petrol at 15 °C becomes about 10,123 litres at 25 °C.
Does that mean customers get less fuel in summer?
Dispensers measure litres at the temperature of the fuel. A warm litre holds slightly less fuel by weight than a cold one. That is why depots and large buyers trade at 15 °C, while retail sells by the litre as dispensed.