Cetane Index Calculator

Enter the specific gravity at 60°F and the aniline point into the calculator to estimate cetane number/ignition quality. For industry-standard Calculated Cetane Index methods, use the ASTM D4737 (4-variable) or ASTM D976 (2-variable) tabs.

Cetane Index API ⇄ SG Converter ASTM D4737 (4-var) ASTM D976 (2-var) Diesel Index

Enter any 2 values to calculate the missing variable (uses a common Diesel Index → cetane estimate correlation; for ASTM CCI use the D4737/D976 tabs)

Specific Gravity at 60°F Aniline Point

Estimated Cetane Number (from SG & Aniline) Preset examples (optional)

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.

\mathrm{API} = \frac{141.5}{SG} - 131.5 \mathrm{DI} = \frac{\mathrm{AP}_{^\circ F}\cdot \mathrm{API}}{100} \mathrm{CN}_{est} \approx 0.72\cdot \mathrm{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

Follow-Up Calculations

Suppose a diesel sample comes out at a cetane index of 52 instead of 42; that usually means the fuel should ignite a little sooner and burn more smoothly. If you’re checking how that change affects injection strategy, the injection timing window helps you match the end of injection to the fuel’s shorter ignition delay, which can reduce diesel knock and rough idle.

If a fleet truck gains even a small efficiency improvement after switching from a 44 cetane fuel to a 50+ cetane blend, the real question becomes how much fuel it burns per unit of work. The fuel burn per horsepower calculation turns that idea into a measurable BSFC value, so you can compare before-and-after fuel quality changes instead of relying on seat-of-the-pants impressions.

A low-speed diesel that runs well on 55 cetane fuel still depends on compression to build enough heat for reliable ignition, especially in cold weather or under load. Use the compression pressure estimate to see how a given ratio translates into cylinder pressure, since a 16:1 engine and a 20:1 engine will behave very differently with the same fuel quality.