About this tool
The Octane Blending Calculator handles the two questions that come up when mixing fuels. First, given two fuels of known octane and volume, the resulting octane blends linearly by volume: result = (V1 × O1 + V2 × O2) / (V1 + V2). So blending 10 gallons of 91 with 5 gallons of 100 octane race fuel gives (10×91 + 5×100) / 15 ≈ 94 octane. Octane here is the AKI (R+M)/2 pump rating, which blends closely enough to linear for practical mixing.
Second, target mode works backwards: given the two fuels' octanes, a desired target and a total tank volume, it finds how much of each fuel you need. The fraction of fuel 1 is (target − O2) / (O1 − O2), then V1 = total × fraction and V2 = total − V1. The target must fall between the two octane values — you cannot blend two fuels to a number higher or lower than both — and the tool flags that case. Volumes are shown in both US gallons and litres.
This is the standard way to stretch expensive race fuel, top off with an octane number in mind, or estimate a blend with E85 or an octane booster treated as a high-octane component. Real fuels can blend slightly non-linearly and E85 changes air-fuel requirements, so treat the result as a close estimate and tune accordingly. All math runs locally in your browser.