Cetane number: diesel's answer to octane rating
Cetane number measures how readily diesel ignites under compression, the opposite of what octane rating measures for petrol. A higher cetane number generally means smoother running and easier cold starts, not more power or better fuel economy.
In this entry
What cetane number measures — cetane rating, cetane value, and what it means for diesel
Cetane number — also called the cetane rating or cetane value — measures how readily diesel fuel ignites once it is compressed in an engine cylinder, with no spark plug needed. A higher cetane number means a shorter ignition delay between injection and combustion, which is what a diesel engine needs to run smoothly.
What is cetane? Cetane (hexadecane, C₁₆H₃₄) is the reference hydrocarbon on which the scale is based — it is assigned a cetane number of 100 because it ignites very readily. Standard diesel fuel, by contrast, is a complex mixture of hydrocarbons, and its cetane rating of diesel fuel depends on which molecules are present and in what proportions. Minimum cetane requirements are 51 in the EU (EN 590) and 40 in the US (ASTM D975), though most retail diesel exceeds those minimums, particularly premium grades.
What is cetane number in practice? It is the single most important ignition-quality metric for diesel. A fuel with a low cetane value takes longer to ignite after it is injected, which can cause rough running, harder cold starts, and more white smoke on start-up. A fuel with a high cetane rating ignites almost immediately, giving smoother, quieter combustion. That is why premium diesel is often marketed with a cetane figure as one of its headline claims.
Cetane vs octane: opposite properties
This is the point most worth getting right, because the names invite confusion.
Opposite properties, similar-sounding names
A higher octane rating means petrol resists igniting too early. A higher cetane number means diesel ignites quickly and reliably when it's compressed. Both "higher is generally better," but they describe opposite chemical behaviours, so the numbers aren't comparable to each other at all.
How it's measured
Cetane number can be found two ways: by running the fuel in a dedicated test engine, or by estimating it from other measurable properties.
Typical values
Minimum cetane requirements are set by national or regional fuel standards, and premium diesel is often marketed with a higher figure as part of its appeal.
Illustrative scale. The EU's EN 590 standard sets a minimum cetane number of 51; premium diesel is often marketed somewhat higher.
Why it matters in practice
A higher cetane number shows up in a few practical ways for a driver, even if it doesn't change fuel economy on its own.
- Easier, quicker cold starts, since the fuel ignites more reliably in a cold engine.
- Smoother running and less of the knocking diesel clatter, since combustion starts more evenly.
- Often marketed with premium diesel, alongside other additives, as a higher-performance option.
How this shows up on FuelTheGuide
We show diesel and premium diesel as separate grades where a country publishes both, since a numeric cetane figure isn't part of the underlying price data.
We show diesel and premium diesel as separate grades where a country publishes both, but we don't link a numeric cetane figure since it isn't part of the price data.
Questions and answers
What is cetane number and what does it measure?
Cetane number (also called cetane rating or cetane value) measures how readily diesel fuel ignites under compression. A higher cetane number means a shorter ignition delay between injection and the start of combustion. The scale is based on pure cetane (hexadecane, C₁₆H₃₄), which is assigned a cetane number of 100. Standard diesel must meet a minimum cetane rating of 51 in the EU (EN 590) and 40 in the US (ASTM D975). Premium diesel typically exceeds those minimums.
What is the typical cetane rating of diesel fuel?
The typical cetane rating of diesel fuel at retail pumps in the EU is 51–55, meeting the EN 590 minimum of 51. US diesel typically falls in the 40–55 range, exceeding the ASTM D975 minimum of 40. Premium diesel is often marketed with a cetane value of 55 or above. Exact values vary by refinery, feedstock, and season — winter diesel may use different additives that can affect the cetane number slightly. The US DOE AFDC covers diesel fuel quality in detail.
Does a higher cetane number mean more fuel economy or power?
Not directly. It mainly affects how readily the fuel ignites, which shows up as easier starting and smoother running, not as a meaningful change in fuel economy or engine power on its own.
Is cetane number the same everywhere in the world?
No. Minimum requirements vary by country. The EU mandates a minimum of 51 for standard diesel under EN 590. The US sets a minimum of 40 under ASTM D975, though most fuel exceeds that. Premium diesel in most markets is formulated to a higher cetane figure than the legal minimum.
Can I raise the cetane number of diesel I already have?
Yes, with cetane improver additives, sometimes sold as diesel treatment products. They can raise the effective cetane number a few points and may help with cold-start behaviour, but they don't change the underlying fuel chemistry as much as using a higher-grade diesel from the pump. The US DOE Alternative Fuels Data Center covers diesel fuel properties and additives.
Does biodiesel affect cetane number?
Yes, usually upward. Most biodiesel feedstocks produce a fuel with a higher cetane number than petroleum diesel, so blending biodiesel into diesel (B5, B7, B20) tends to raise the blended fuel's cetane number slightly.
Diesel cetane rating standards around the world
The diesel cetane rating required at the pump varies by region. In the European Union, the European Commission's fuel quality framework underpins EN 590, which mandates a minimum cetane number of 51 for standard road diesel. Premium diesel marketed in Europe typically reaches 55 or higher. In the United States, ASTM D975 sets a lower floor of 40, though most retail diesel exceeds that. Understanding cetane vs octane is important when crossing between fuel types: octane measures a petrol's resistance to premature ignition, while cetane measures how readily diesel self-ignites under compression. The two scales are not comparable, and a fuel that scores well on one would perform poorly on the other.
For fleet operators and logistics companies, knowing the local diesel cetane rating matters because it affects cold-start reliability, engine noise, and emissions. The U.S. EIA diesel fuel explainer covers how cetane requirements interact with seasonal blending and sulphur limits. The IEA's oil and fuel technology overview tracks how diesel quality standards are evolving globally as emissions regulations tighten.
More questions
What is the difference between cetane vs octane?
Cetane vs octane is a comparison of two opposite fuel-quality scales. The cetane number measures how easily diesel ignites under compression — higher is better because diesel engines need rapid self-ignition. The octane rating measures how well petrol resists premature ignition — higher is better because petrol engines use a spark and need the fuel to wait for it. A high-cetane fuel would knock badly in a petrol engine, and a high-octane fuel would resist the compression ignition a diesel engine depends on. The two numbers cannot be converted into each other.
Sources and further reading
- Diesel fuel quality and cetane number US Department of Energy, Alternative Fuels Data Center, afdc.energy.gov
- EN 590 automotive diesel fuel standard European Committee for Standardisation, cen.eu
- Fuel prices tool and grade lists OpenVan.camp, openvan.camp/compare/fuels/fuel-prices