Octane Blend Calculator (Mixing Fuels & Octane Boosters)

How it works

blend octane = (octane_A × volume_A + octane_B × volume_B) ÷ total · fraction of B for a target = (target − octane_A) ÷ (octane_B − octane_A) · ethanol % blends by volume; stoich AFR ≈ 14.7 − 5.7 × ethanol fraction

Octane rating measures a fuel's resistance to knock, and when two gasolines are mixed the rating of the blend sits at the volume-weighted average — close enough to linear that the arithmetic works for pump fuels, though ethanol behaves a little better than the straight line. That makes topping up a tank with a few litres of race fuel or E85 a calculable way to reach a target octane for a tuned engine, and it also exposes how little a bottle of octane booster does. Ethanol brings a second variable: it carries less energy and burns at a richer stoichiometric ratio, so blends beyond about fifteen percent need fuel-system headroom and a tune, and the calculator reports the ethanol content of the mix alongside the octane.

Worked example

To turn a 60 L tank into 94 octane using 91 pump fuel and 100-octane race fuel, the race fuel must be (94 − 91) ÷ (100 − 91) = one third of the volume: 40 L of 91 plus 20 L of 100. With E10 pump fuel and ethanol-free race fuel the blend ends up at 6.7% ethanol. Adding 20 L of E85 (≈ 105 octane, 85% ethanol) to 40 L of 91 instead would give about 95.7 octane at 35% ethanol — flex-fuel territory.

Frequently asked questions