# How to calculate density at 15 °C

> A step-by-step guide to finding the density of petrol or diesel at 15 °C from a hydrometer reading and temperature with ASTM Table 53B, including interpolation and two worked examples.

Canonical: https://dipdensity.watsinfo.com/guides/how-to-calculate-density-at-15-degree/
Last updated: 2026-11-02

**Short answer:** Read the hydrometer and the thermometer in the same sample at the same time. Look up the pair in ASTM Table 53B, with the reading across and the temperature down; where they meet is the density at 15 °C. Between printed figures, read in a straight line.

## What you need

- A clean, clear glass jar (the hydrometer jar).
- A calibrated hydrometer for the range of your fuel. Petrol and diesel need different ones.
- A calibrated thermometer.
- Your company's copy of Table 53B, the [Table 53B look-up](https://dipdensity.watsinfo.com/table-53b/) or the [calculator](https://dipdensity.watsinfo.com/tools/density-at-15-calculator/).

The MDG makes it mandatory for a dealer to have calibrated hydrometers, thermometers and the ASTM density conversion table at the outlet.

## Step by step

1. **Take the sample.** From the nozzle in the morning, after letting a little fuel through; from each compartment's manifold when a tanker arrives.
2. **Fill the jar** and let air bubbles rise. Keep it out of the wind and direct sun.
3. **Lower the hydrometer** gently. Let it float free, away from the jar's wall. Give it a slight spin to break any surface film.
4. **Read the hydrometer** at the level of the fuel's flat surface, with your eye level with it. This is the observed density.
5. **Read the thermometer** in the same jar straight away.
6. **Look it up.** Reading across, temperature down.

## Example 1: petrol

Hydrometer **741.0 kg/m³**, temperature **26.5 °C**.

Column 741, row 26.5 °C: **751.2 kg/m³** at 15 °C.

## Example 2: diesel, between rows

Hydrometer **815.0 kg/m³**, temperature **27.3 °C**. The table is printed every 0.5 °C, so 27.3 falls between two rows. Read in a straight line between them, 27.3 being 0.6 of the way from 27.0 to 27.5. The answer is **823.7 kg/m³**. The [calculator](https://dipdensity.watsinfo.com/tools/density-at-15-calculator/) does this step for you.

## What to do with the answer

- **Morning:** write it in the density register before sales start, by 10 AM at the latest.
- **Tanker:** compare it with the invoice density. The difference must stay within ±3.0 kg/m³. See [the density variation limit](https://dipdensity.watsinfo.com/guides/density-variation-limit/).
- **Check the range:** BS-VI petrol is 720 to 775 kg/m³ and BS-VI diesel 810 to 845 kg/m³ at 15 °C. A figure outside is a reason to stop and check.

## Questions

### Can I just add or take off a fixed figure per degree?

Not safely. The change per degree depends on the density and is not quite constant. For petrol near 745 kg/m³ it is roughly 0.9 kg/m³ per degree; for diesel near 830 it is roughly 0.7. Use the table or the calculator.

### How precise should the reading be?

Read the hydrometer to 0.5 kg/m³ and the thermometer to 0.5 °C. The table is printed to 0.1 kg/m³.

## Sources

- [BPCL, Marketing Discipline Guidelines 2024, clauses 1.2 and 1.4.1](https://www.bharatpetroleum.in/images/files/MDG-2024-Effective-24-10-2024.pdf)