Cetane Index Calculator: ASTM D4737 and D976 Diesel CCI

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

What do you want to solve for?

Enter density at 15 °C and the ASTM D86 recovery temperatures.

Calculation method

Procedure A covers No. 1-D grades, No. 2-D S5000 and No. 4-D. Procedure B covers low-sulfur No. 2-D. D976 is the older two-variable equation.

Density or gravity (required)

Distillation temperature unit

T10 – 10% recovery, ASTM D86 (required)

T50 – 50% recovery, ASTM D86 (required)

T90 – 90% recovery, ASTM D86 (required)

+ Advanced options

Cetane Index Formula

The following equations are used in the 2‑variable estimate (from specific gravity and aniline point). For ASTM calculated cetane index calculations, use the ASTM D4737 or ASTM D976 tabs in the calculator above.

API = (141.5) / (SG) - 131.5 DI = (AP₍^ F) \times API) / (100) CNₑₛₜ \approx 0.72 \times DI + 10

Variables:

API is the API gravity (dimensionless)
SG is the specific gravity at 60°F (dimensionless)
AP is the aniline point (use °F in the equation)
DI is the Diesel Index (dimensionless)
CN_est is an estimated cetane number / ignition-quality index (dimensionless)

To estimate ignition quality from specific gravity and aniline point: convert SG to API gravity, compute the Diesel Index, then apply the common correlation CN_est ≈ 0.72×DI + 10.

What is a Cetane Index?

Cetane number describes how readily a diesel fuel ignites in a compression-ignition engine (higher values generally mean a shorter ignition delay and smoother combustion). A cetane index is an estimate of cetane number calculated from measured fuel properties (for example, via ASTM D4737 or ASTM D976) when a direct engine test cetane number is not available.

How to Calculate Cetane Index?

The following steps outline how to estimate ignition quality using specific gravity and aniline point (2‑variable estimate):

Determine the specific gravity at 60°F (SG).
Determine the aniline point (AP) and convert it to °F if needed.
Compute API gravity from SG: API = (141.5 / SG) − 131.5.
Compute Diesel Index: DI = (AP_°F × API) / 100.
Estimate cetane number/index: CN_est ≈ 0.72 × DI + 10.
For ASTM calculated cetane index instead of a 2‑variable estimate, use the D4737 or D976 calculator tabs above.

Example Problem:

Use the following variables as an example problem to test your knowledge.

specific gravity at 60°F (SG) = 0.85

aniline point (AP) = 120°F