It is the most common request we get about this calculator, and the answer is a deliberate no. Here is the reasoning, so you can judge it for yourself.
Blending octane is not the octane of the fuel
Neat ethanol has a research octane number around 109. That figure is nearly useless for predicting what happens when you blend it, because ethanol's blending octane — its marginal contribution to a mixture — is much higher at low concentrations, behaving like roughly 152-156 RON at 10-20% by volume, and it falls as concentration rises.
It depends on the fuel you are mixing into
Blending octane is a property of the pair, not of the ethanol. The same ethanol added to two different base gasolines of identical posted octane can produce different results, because the hydrocarbon composition differs. A calculator that knows only "93" does not have enough information to answer.
So linear interpolation is provably wrong
The obvious approach — treat ethanol as 109, treat the gasoline as its pump number, and interpolate by volume — is wrong in both directions. It understates the gain at low blends and overstates it at high ones. Of the shipping calculators we reviewed that display octane, several are demonstrably incorrect by this exact mechanism.
Why not show it anyway, with a warning?
Because the number is what gets screenshotted, quoted in a forum thread, and used to justify a timing decision. A disclaimer does not travel with it. We would rather be the calculator that declined to guess than the one that produced the figure someone leaned on.
What we show instead
The stoichiometric air-fuel ratio of the resulting blend, calculated as a mass-weighted arithmetic mean of the component AFRs. That is derivable from first principles, it is the number a calibration actually uses, and it can be verified — our constants reproduce the figures the tuning industry already recognises: pump E10 at 14.11, and E85 at 85% ethanol at 9.81.
Worth noting: a harmonic-mean form of that calculation circulates online and is wrong by about 0.65 AFR at E30. Our test suite contains a case specifically to catch it.
If this changes
A defensible octane model would need a molar-basis calculation and real inputs for the base fuel's RON and MON, not just its pump sticker. If we build that, it will be because we can stand behind the output — not because it was requested.
Written by the Palm Beach Dyno team — a Ford performance shop running dyno and remote calibration on gasoline, ethanol blends and E85. Last reviewed 4 August 2026.