What the inputs mean
- Octane: the number on the pump for each fuel. Use one scale for all of them. US pumps show (R+M)/2, the average of the research octane number (RON) and the motor octane number (MON). Many other countries show RON. The same fuel has a different number on each scale, so a RON figure and an (R+M)/2 figure cannot go into one calculation.
- Amount: how much of each fuel goes in, in gallons or liters. Only the ratio matters, so any unit works as long as both use it.
The formula
mixed octane ≈ (V1 × O1 + V2 × O2) ÷ (V1 + V2)This is a volume-weighted average: each fuel counts in proportion to how much of it is in the mix. To reach a target, solve for the share of the higher octane fuel:
share of higher fuel = (target − lower octane) ÷ (higher octane − lower octane)Why the result is approximate
Octane does not blend exactly in a straight line. How two fuels interact depends on what is in them, and the gap is largest when one of them contains ethanol. The volume-weighted average is the common way to estimate it, and this tool applies no correction factor, because any single factor would be a guess. Treat the answer as a close estimate, not a measured octane number.
Worked example
5 gallons of 87 octane mixed with 5 gallons of 93:
(5 × 87 + 5 × 93) ÷ 10 = 900 ÷ 10 = about 90.
Going the other way, 12 gallons at 91 octane from 93 and 87: share of 93 = (91 − 87) ÷ (93 − 87) = 4 ÷ 6 = 66.7%. That is 8 gallons of 93 and 4 gallons of 87.
The result is maths from the numbers you enter. The octane your engine needs is set by its maker or its tune, so check the owner's manual or your tuner, not this estimate.
Mixing E85 into gasoline? The E85 mix calculator works out the ethanol blend, and the air fuel ratio calculator gives its stoichiometric ratio. For fuel system sizing, see the fuel pump size calculator and the fuel injector size calculator. The boost calculator covers boost and power.
Questions
What octane do I get mixing 87 and 93?
Equal amounts give about 90. Two parts 87 to one part 93 give about 89. These are volume-weighted estimates: real blending is not exactly linear.
How much 93 do I need to make 91 octane?
Two thirds of the mix. For 12 gallons that is 8 gallons of 93 and 4 gallons of 87, by the volume-weighted estimate.
Is mixed octane exactly the average?
No. The volume-weighted average is a common approximation. Octane does not blend exactly linearly, especially when ethanol is involved, so the real number can be somewhat different.
Can I compare RON with the octane on US pumps?
Not directly. US pumps show (R+M)/2, while many other countries show RON, and the same fuel gets a different number on each scale. Use one scale for every fuel you enter.