Octane Blending Calculator
Mixing two gasolines of different octane ratings gives a blend whose octane number is, to a good first approximation, the volume-weighted average of the two. This calculator takes the octane rating and volume of each fuel and returns the blended octane number, the total blend volume and each fuel's volume fraction. It is ideal for planning a mix to hit a target octane: keep both ratings on the same scale (both anti-knock index, or both RON) and adjust the volumes. Real blends show small non-linear effects, so treat the result as a close first-order estimate.
Octane blending formula
Total volume = volume A + volume B
Blended octane = (octane A * volume A + octane B * volume B) / total volume
Fraction A (%) = volume A / total volume * 100
Fraction B (%) = volume B / total volume * 100
The blended octane is the volume-weighted average: each fuel contributes its octane number in proportion to its share of the total volume. This linear blending rule is the standard first-order model; small synergistic or antagonistic effects can shift the true value slightly.
Octane blending context
- The U.S. pump octane figure is the anti-knock index (AKI), the average of RON and MON.
- Keep both inputs on the same octane scale so the blend is meaningful.
- The blended octane always lies between the two input ratings.
- Real blends show small non-linear effects, so the result is a close estimate.
- Always use fuel of the minimum octane your engine manufacturer specifies.
Octane blending: frequently asked questions
How do you calculate the octane of a fuel blend?
To a good first approximation, the octane number of a blend is the volume-weighted average of the two fuels' octane numbers: each fuel's octane times its volume fraction, summed. This calculator uses that linear blending rule from the volumes and ratings you enter.
What octane numbers should I enter?
Enter the octane rating printed on each fuel. In the United States the pump figure is the anti-knock index, the average of research (RON) and motor (MON) octane numbers. Keep both inputs on the same scale (both AKI, or both RON) so the blend is meaningful.
Is octane blending exactly linear?
Real fuel blending shows small non-linear (synergistic or antagonistic) effects, so the true blended octane can differ slightly from the volume-weighted average. The linear rule is the standard first-order estimate and is accurate enough for planning blends of similar gasolines.
How do I reach a target octane?
Enter the two fuel octane ratings and adjust the volumes until the blended result meets your target. The tool also reports each fuel's volume fraction and the total volume so you can scale the recipe to any batch size.
Why does the blend rating sit between the two inputs?
A volume-weighted average always falls between the lower and higher octane number, weighted toward whichever fuel makes up more of the mix. Blending a high-octane fuel into a lower-octane one raises the result only in proportion to the volume added.
Official sources
- U.S. Federal Trade Commission: Automotive Fuel Ratings (16 CFR Part 306).
- U.S. Department of Energy, Alternative Fuels Data Center: Fuel Properties and Octane.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.