Checked against the Marketing Discipline Guidelines 2024 (Ver. 1, effective 24.10.2024) on 26 October 2026. Published by MDG Services on 26 October 2026.
What happens when fuel warms up
Warm fuel takes more room than cool fuel. A cubic metre of it therefore weighs a little less, and the hydrometer sinks deeper and shows a smaller number. The fuel is the same; only the reading moved.
For example (made-up samples, worked out with the 15°C calculator): each fuel below has one fixed density at 15°C, but the hydrometer shows a different figure as the sample warms.
| Fuel | Sample temperature | Hydrometer reads (kg/m³) | Density at 15°C (kg/m³) |
|---|---|---|---|
| Petrol | 20°C, cool morning | 748.7 | 753.1 |
| Petrol | 27°C | 742.5 | 753.1 |
| Petrol | 35°C, hot afternoon | 735.3 | 753.1 |
| Diesel | 20°C | 828.4 | 831.9 |
| Diesel | 32°C | 820.0 | 831.9 |
| Diesel | 38°C | 815.8 | 831.9 |
The printed ASTM table decides if it differs from the calculator, and it may differ in the last digit MDG 1.5(vi). Do not turn these rows into a rule of thumb. Use the table.
Why MDG fixes 15°C
One fixed temperature lets any two readings be compared, whenever and by whoever they were taken. MDG states density at 15°C wherever it matters: the invoice MDG 2.2.1(iv), the tanker check MDG 1.2(iv), the density after decanting MDG 1.2(xiv), the morning reading MDG 1.4.1(c) and the ranges for each fuel MDG 7.6.
A raw hydrometer reading can sit outside a range that is stated at 15°C. For example, a petrol at the bottom of the petrol range, 720 at 15°C, reads about 697 at 40°C. Convert first, then compare.
At the pump
- Put the hydrometer and the thermometer in the same sample and read both MDG 1.2(viii).
- Convert to 15°C with ASTM Table 53B or the 15°C calculator MDG 1.2(iv) MDG 1.5(vi).
- Compare or record: a tanker against its invoice, within ±3.0 kg/m³ (MDG 1.2(iv), see the invoice check); the morning reading in the register, by 10 AM (MDG 5.1.9, see the 10 AM rule).
Note: a wrong thermometer spoils the answer too. In the calculator, a petrol read at 742.5 gives 753.1 at 27°C but 755.8 at 30°C, a 2.7 gap from 3°C of error alone, most of the ±3.0 allowance.
For what a hydrometer would read at other temperatures, see the temperature chart.
Questions dealers ask
01Why does density change with temperature?
Fuel expands as it warms and shrinks as it cools. The same weight of fuel takes more space when warm, so a cubic metre of warm fuel weighs less and the hydrometer reads lower. That is a general explanation, not MDG's words.
02Petrol density at night: is it higher or lower?
If the sample is cooler than it was in the afternoon, the hydrometer reads higher. The density at 15°C does not change. Read the temperature of the same sample you take the density from MDG 1.2(viii).
03What does density at 15 degree celsius mean?
It is the density the fuel would show at 15°C, worked out from your hydrometer reading and the sample's temperature. MDG uses it for the invoice, the tanker check, the morning reading and the ranges MDG 2.2.1(iv) MDG 1.2(iv) MDG 1.4.1(c) MDG 7.6.
04What is the diesel density at 15°C?
MDG gives a range: 810–845 kg/m³ for BS VI diesel up to B7, and 820–860 for B8–B20 blends MDG 7.6. Each tanker's own 15°C figure is printed on its invoice MDG 2.2.1(iv).
Read the official text: Marketing Discipline Guidelines 2024 (PDF on Bharat Petroleum's website) (also published on iocl.com).
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