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Volumetric blending assumes octane blends linearly on volume: blend octane is the volume-weighted average of component octanes. Real blending is non-linear — aromatics and oxygenates show blending octane numbers different from their neat values — so refineries use blending indices calibrated from test data.
Volumetric octane blending
Blend octane = (V_A x RON_A + V_B x RON_B) / (V_A + V_B)
Octane response depends on interactions between hydrocarbon classes. Ethanol in particular blends with an effective octane well above its neat value at low concentrations, then tails off.
RON is measured under mild conditions, MON under more severe ones. AKI, used in North America, is the average of the two and typically 8-10 numbers below RON for the same fuel.