Cetane Index Calculator estimates diesel ignition quality from density at 15°C and 10%, 50%, and 90% D86 recovery temperatures, then checks that result against EN 590 or ASTM D975.
How the Cetane Index Calculator Rates Diesel Fuel
The Cetane Index Calculator estimates the ignition quality of diesel fuel from its density and distillation temperatures. Fuel labs, fleet managers and fuel buyers use it to check a test report against the EN 590 or ASTM D975 minimum.
Enter the density at 15°C as g/mL, kg/m³, °API or specific gravity. Then enter the 10%, 50% and 90% recovery temperatures from an ASTM D86 distillation, in °C or °F, and pick the spec to check against.
The tool returns the calculated cetane index by three methods. It also shows your margin over the minimum, the highest density that would still pass, and your density in the other report units.
Cetane Index vs. Cetane Number
Cetane number is measured by running the fuel in a test engine. The calculated cetane index estimates that number from lab data, without an engine.
ISO 4264, which uses the same four-variable equation as ASTM D4737, states that the index is not another way of expressing cetane number. It is an estimate of the base fuel’s cetane number.
The index cannot see cetane improver additives such as 2-ethylhexyl nitrate. A treated fuel can test higher in an engine than its index suggests.
The Cetane Index Calculator Formula
The main result uses ASTM D4737 Procedure A, the four-variable equation. It works in SI units, with density D in g/mL at 15°C and recovery temperatures in °C.
First, the density and temperatures are shifted around reference points.
$$B = e^{-3.5(D – 0.85)} – 1$$
$$T_{10N} = T_{10} – 215 \qquad T_{50N} = T_{50} – 260 \qquad T_{90N} = T_{90} – 310$$
Those terms then go into the equation.
$$\text{CCI} = 45.2 + 0.0892\,T_{10N} + (0.131 + 0.901B)\,T_{50N} + (0.0523 – 0.420B)\,T_{90N} + 0.00049\,(T_{10N}^2 – T_{90N}^2) + 107B + 60B^2$$
With the defaults, a fuel at 0.835 g/mL with T10 of 205°C, T50 of 268°C and T90 of 335°C has a calculated cetane index of 52.2.
A common mistake is entering the density at the sample temperature instead of the corrected 15°C figure. Lab reports list density at 15°C for this reason, so use that line.
Procedure B for No. 2-D S500 Diesel
ASTM D4737 has a second, linear equation called Procedure B. The current standard assigns it to ASTM D975 Grade No. 2-D S500, and states that Procedure A has been verified for No. 2-D S15.
$$\text{CCI} = -386.26D + 0.1740\,T_{10} + 0.1215\,T_{50} + 0.01850\,T_{90} + 297.42$$
The second card of the Cetane Index Calculator shows the Procedure B result, 49.3 at the defaults. It also shows the gap from Procedure A, 2.8 points lower here, and the margin over your chosen minimum.
The Older Two-Variable Method
ASTM D976 is the original cetane index equation. It uses only density and the 50% recovery temperature.
$$\text{CCI} = 454.74 – 1641.416D + 774.74D^2 – 0.554\,T_{50} + 97.803\,(\log_{10} T_{50})^2$$
It is outdated for estimating cetane number, but the EPA still accepts a D976 index of 40 or more as an alternative to the 35% aromatics limit for highway diesel. The third card shows the D976 result, 52.5 at the defaults, and its margin over 40.
Checking Against EN 590 and ASTM D975
The Check Against menu in the Cetane Index Calculator sets the minimum index. EN 590, the European diesel spec, requires a cetane index of at least 46. The ASTM D975 and EPA option uses 40.
The first card shows how far Procedure A sits above or below that minimum. At the defaults, the fuel clears EN 590 by 6.2 points.
The same card gives a likely cetane number range of plus or minus 2 around the index. That band comes from ASTM’s stated expected error, which it gives for 65% of the fuels it evaluated between 32.5 and 56.5 cetane number.
Why Density Moves the Index So Much
Density has the strongest pull in the equation. With the default distillation, going from 0.825 to 0.845 g/mL drops the index from 56.4 to 48.3.
That is why the first card also shows the highest density that still passes. At the default temperatures, the fuel would meet EN 590 up to 0.8511 g/mL.
Denser diesel tends to contain more aromatics, which ignite less readily. A dense fuel usually needs a higher T50 or a cetane improver to reach the same rating.
Density, API Gravity and Specific Gravity
US reports often list API gravity or specific gravity at 60°F instead of density at 15°C. The Cetane Index Calculator converts either one to density at 15°C before running the equations.
$$\text{API} = \frac{141.5}{\text{SG}} – 131.5$$
The fourth card shows your density in the other units. At the defaults, 0.835 g/mL equals 835 kg/m³, a specific gravity of 0.8354 and 37.87° API.
To turn that density into the weight of a fuel load, the fuel weight calculator converts gallons or liters of diesel to pounds.
Where the Index Stops Working
ASTM D4737 applies to fuels with a 90% recovery point below 382°C. When T90 reaches 382°C or higher, the alert warns that the index is only rough for that fuel.
It also warns when Procedure A falls outside 32.5 to 56.5. ASTM notes errors may be greater outside that range, so an engine test or an Ignition Quality Tester result is the better check there.
Density must work out to between 0.600 and 1.100 g/mL at 15°C. The three temperatures must rise in order, with T10 below T50 and T50 below T90, or the tool flags them.
Report Mistakes to Avoid
Mixing °F temperatures with the °C setting throws the Cetane Index Calculator result far off. Set the temperature menu to match the D86 report before typing.
Entering the initial or final boiling point instead of the 10% or 90% recovery point changes the result. Use the recovered temperatures at 10%, 50% and 90%.
Reading API gravity as specific gravity, or the reverse, gives an impossible density. A diesel API is usually a number in the 30s, while specific gravity is below 1.
Cetane Index Questions
What is a good cetane index for diesel fuel?
It depends on the spec you buy to. EN 590 diesel must have a cetane index of at least 46. In the US, highway diesel must meet a D976 index of 40 or a 35% aromatics limit. The default fuel in the Cetane Index Calculator rates 52.2, which clears both.
Does cetane improver raise the cetane index?
No. The index is calculated from density and distillation, and additives like 2-ethylhexyl nitrate barely change either. Improver does raise the cetane number measured in an engine. So an additized fuel can have an engine-tested cetane number above its calculated index.
Why do Procedure A and Procedure B give different results?
They are separate correlations built from different sets of fuels. Procedure A is a nonlinear equation, while Procedure B is a straight-line fit developed for No. 2-D S500 diesel. At the defaults they differ by 2.8 points. Use the one your spec or contract calls for.
Can I use a cetane index calculator for biodiesel?
Not reliably. ISO 4264 covers middle-distillate fuels from petroleum sources, including those with oil sands or oil shale derivatives. Researchers have proposed modified equations built from biodiesel data. For B100 or high blends, a measured cetane number is the safer figure.