Type the hydrometer reading in kg/m³ and the temperature of the sample. The density at 15 °C comes from ASTM D1250-80 Table 53B, read in a straight line between the printed figures. A reading of 741 at 26.5 °C is 751.2 kg/m³ at 15 °C.
ASTM D1250-80 Table 53B, as printed in the oil companies' workbook. Readings 685 to 929 kg/m³, 0 to 50 °C.
How to use it
- Take the sample in a clean glass jar and let it settle.
- Lower the hydrometer gently and let it float free of the sides. Read the scale where the fuel’s surface cuts it.
- Read the thermometer in the same jar at the same time.
- Type both figures. The answer is the density at 15 °C.
The calculator also shows the volume correction factor for that temperature (Table 54B), which turns litres at the tank’s temperature into litres at 15 °C.
Where the figure comes from
The answer is read from the printed ASTM D1250-80 Table 53B, the same table the oil companies issue to their dealers. Between two printed figures it is read in a straight line, as you would by hand. We check every printed cell against the 1980 formula: they agree within 0.1 kg/m³ for petrol and diesel from 10 to 50 °C.
When to take the reading
The Marketing Discipline Guidelines ask a dealer to record density at 15 °C three times:
- Every morning before sales start, from a nozzle sample, by 10 AM at the latest.
- When a tanker arrives, from each compartment, before you decant.
- After decanting, from the nozzle once at least 50 litres have gone through it.
See the daily density check for the full routine.
Figures here are for reference. Your oil company’s certified calibration chart, approved instruments and the Marketing Discipline Guidelines always prevail.
Questions
Why is density corrected to 15 °C?
Fuel expands as it warms, so the same fuel reads lighter on a hot afternoon than on a cold morning. Correcting every reading to one temperature, 15 °C, lets you compare a reading with the invoice and with yesterday's reading.
My hydrometer reads in g/ml. Can I type 0.741?
Yes. A figure below 2 is read as g/ml and multiplied by 1,000, so 0.741 is taken as 741 kg/m³.
What density should petrol and diesel have?
BS-VI petrol (IS 2796) must be 720 to 775 kg/m³ at 15 °C. BS-VI diesel (IS 1460) must be 810 to 845 kg/m³ at 15 °C. The calculator warns you if your answer falls outside.