Petrol vs Diesel Density: Which Is Heavier and Why?
Diesel (0.835 kg/L) is 12.8% denser than petrol (0.740 kg/L). Learn why hydrocarbon chain length matters, how weight affects MPG, and fleet fuel math.
You pull up to the pump or review fleet invoices and wonder why diesel fuel feels noticeably heavier and stretches so much further per tank than petrol. If you treat both fuels as interchangeable liquids when calculating freight tare weights or fuel tank loads, you risk payload fines or inaccurate mileage projections. Here is the reality: diesel is substantially heavier than petrol. 1 litre of diesel weighs 0.835 kg, while petrol weighs only 0.740 kg at standard 15°C reference.
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1. Side-by-Side Density Comparison
| Property | Petrol (Gasoline) | Diesel (ULSD) | Difference |
|---|---|---|---|
| Density at 15°C | 0.740 kg/L | 0.835 kg/L | Diesel +12.8% |
| Weight of 1 Litre | 740 g | 835 g | +95 g per litre |
| Weight of 50 Litres | 37.0 kg | 41.75 kg | +4.75 kg |
| Weight of 100 Litres | 74.0 kg | 83.5 kg | +9.5 kg |
| Specific Gravity | 0.740 | 0.835 | Dimensionless ratio |
| API Gravity | 59.7° API | 37.9° API | Inverse scale |
| Carbon Chain Length | C4 to C12 | C12 to C25 | Heavier hydrocarbons |
| Thermal Expansion ($\beta$) | 0.00120 per °C | 0.00095 per °C | Petrol expands faster |
| Volumetric Energy Density | ~32.0 MJ/L | ~35.8 MJ/L | Diesel +11.9% |
Fuel data verified against technical benchmarks from the US Department of Energy (AFDC), standard refining specifications from ASTM D975 Standard Specification for Diesel Fuel, ExxonMobil Marine Fuels Technical Services, and regulatory standards from the NIST Weights and Measures Division.
2. Why Is Diesel Denser Than Petrol?
Both fuels originate from crude oil through fractional distillation, but they are collected at different temperature stages.
Petrol vaporises at low temperatures ($30^\circ\text$ to $200^\circ\text$), yielding short, branched hydrocarbon chains containing 4 to 12 carbon atoms. These short molecules have lower molecular weights and pack loosely.
Diesel condenses at higher temperatures ($200^\circ\text$ to $350^\circ\text$), yielding longer chains of 12 to 25 carbon atoms. These heavier molecules pack together tightly, driving up bulk mass per litre.
We have all noticed how volatile gasoline evaporates almost instantly when spilled, whereas diesel leaves an oily, dense residue. To explore heavier refinery products like aviation fuel, read our comprehensive industrial fuel density guide.
3. Temperature Sensitivity: Summer vs Winter Fuel
Both fuels expand when heated, but petrol expands noticeably faster due to its higher expansion coefficient ($\beta = 0.00120 \text ^\circ\text$).
| Ambient Temp | Petrol Density | Diesel Density |
|---|---|---|
| 0°C (Cold Winter) | 0.749 kg/L | 0.844 kg/L |
| 15°C (Standard Calibration) | 0.740 kg/L | 0.835 kg/L |
| 25°C (Warm Day) | 0.728 kg/L | 0.826 kg/L |
| 40°C (High Summer) | 0.711 kg/L | 0.814 kg/L |
In extreme heat, 1,000 litres of petrol loses nearly 29 kilograms of mass compared to cold winter conditions. To understand the thermodynamics behind volume expansion, visit our guide on why temperature affects liquid density.
4. Frequently Asked Questions About Petrol vs Diesel Density
Is diesel heavier than petrol?
Yes. At the standard 15°C reference temperature, 1 litre of diesel weighs 0.835 kg, while 1 litre of petrol weighs 0.740 kg. Diesel is approximately 12.8% heavier. You can convert diesel volumes directly on our diesel calculator.
Why does diesel provide better fuel economy than petrol?
Diesel contains more mass per litre. Because energy release in hydrocarbons depends on mass, 1 litre of diesel delivers approximately 35.8 megajoules of energy compared to 32.0 megajoules for petrol, providing an immediate 11.9% energy advantage.
How much does a full 60-litre tank of petrol weigh compared to diesel?
A 60-litre tank of petrol weighs 44.4 kg. The same 60-litre tank filled with diesel weighs 50.1 kg, making the diesel fuel load 5.7 kg heavier.
Can you detect fuel contamination with a hydrometer?
Yes. Petrol and diesel mix completely. If petrol is mistakenly pumped into a diesel tank, the fluid density will drop below the 0.820 kg/L minimum threshold, warning mechanics before engine fuel injectors are damaged.
How do aviation fuels compare to petrol and diesel?
Aviation Jet A-1 is a kerosene cut with a density of 0.804 kg/L, placing it between petrol and diesel. Test aviation quantities with our jet A-1 calculator or check kerosene in litres.
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