Calculate diesel cetane index from density at 15 °C and D86 T10/T50/T90 recovery temperatures using ASTM D4737 Procedure A or B or the D976 equation.

Estimate for applicable petroleum distillates without cetane improver; not a measured cetane number or fuel certification. Use density measured/corrected to 15°C and D86 recovery temperatures corrected to standard pressure. Biodiesel blends are outside D4737 scope.

Cetane Index Formula

Use density D in g/mL at 15°C and ASTM D86 recovery temperatures T10, T50 and T90 in °C, corrected to standard barometric pressure. This calculator implements D4737-21 A/B and the D976 two-variable equation; it does not use aniline point or Diesel Index.

For Procedure A, define B = exp[−3.5(D − 0.85)] − 1, X = T10 − 215, Y = T50 − 260 and Z = T90 − 310.

CI = 45.2 + 0.0892X + (0.131 + 0.901B)Y + (0.0523 − 0.420B)Z + 0.00049(X² − Z²) + 107B + 60B².

Procedure B: CI = −399.90D + 0.1113T10 + 0.1212T50 + 0.0627T90 + 309.33.

D976: CI = 454.74 − 1641.416D + 774.74D² − 0.554T50 + 97.803(log10 T50)².

What is a Cetane Index?

A cetane index estimates ignition quality from fuel properties; it is not a measured D613 cetane number. D4737 excludes biodiesel blends and does not measure the effect of cetane improver. Procedure A includes No. 2-D S15; Procedure B applies to No. 2-D S500. Verify the applicable standard and grade.

How to Calculate Cetane Index?

Select the equation and solve mode, then enter the visible required properties. SG or API gravity at 60°F is not an exact substitute for density at 15°C. Use laboratory density or a suitable temperature correction.

Inverse calculations return all mathematical candidates within density 0.70–1.05 g/mL or T50 120–400°C; A/B also require T10 ≤ T50 ≤ T90. These search bounds do not establish method validity or a blending recommendation. Optional thresholds are numeric comparisons only; a measured-minus-calculated difference does not establish additive treatment.

Example: D = 0.850 g/mL, T10 = 215°C, T50 = 260°C and T90 = 310°C give CI = 45.2 by Procedure A and about 44.29 by Procedure B. Holding temperatures fixed and solving the A equation for target 45.2 gives D = 0.850 g/mL.