Octane Calculator

How the octane mix calculator works

An octane rating measures how well a fuel resists knock, the uncontrolled ignition of the last part of the mixture that a hot, high-compression or boosted engine can suffer. It says nothing about how much energy the fuel holds. When two fuels are mixed, the usual first estimate of the result is the average of their ratings weighted by volume: equal amounts land halfway between the two, and a smaller share of the second fuel moves the blend proportionally less.

The Octane mix tab applies that rule to any two fuels, as an octane mix calculator. The Race fuel blend tab runs it backwards: give it the rating of your pump gas, the rating of the race fuel and the octane you want, and it returns how much of each to pour. Treat both results as estimates. The rule is a fair first approximation for two gasolines, and it is known to be wrong for ethanol, as explained further down.

How much octane an engine needs is set by the engine itself: compression ratio, boost, ignition timing and combustion chamber design. The compression ratio calculator and the turbo size calculator deal with those; this page only deals with the fuel.

What to enter and where to find it

• Octane of each fuel. On a US pump it is the number on the yellow label, the anti-knock index or (R+M)/2. Race fuel makers print their ratings on the drum or the spec sheet, sometimes as RON, sometimes as MON and sometimes as (R+M)/2. Use the same method for both fuels, or the answer means nothing.
• Amount of each fuel. Gallons or liters, as long as both fuels use the same unit. Only the proportion matters for the rating.
• Target octane and total fuel. On the Race fuel blend tab, the rating you want and how much finished fuel you are mixing, such as a full tank or a single can.

AKI vs RON: what the pump number means

The number on a US pump is not a single test result. The FTC fuel rating rule defines it as the average of two laboratory ratings, the research octane number (RON, ASTM D2699) and the motor octane number (MON, ASTM D2700), both found by comparing the fuel with reference fuels in a single-cylinder test engine with adjustable compression. The motor method runs hotter and faster, so MON comes out lower. Their average, (R+M)/2, is what the label calls the minimum octane rating, also known as the anti-knock index or AKI.

European pumps show RON instead, so the same fuel reads higher there. US regular, posted at 87, typically measures 91–93 RON, about 4–6 points more, and 93 AKI premium is roughly 98 RON. A RON figure and an AKI figure for the same fuel are both correct; they are different scales.

The posting rule matters when you compare numbers. A station must show the rating as a whole or half number equal to or less than the rating certified for that fuel, so the posted number is a floor, not an exact measurement. The Energy Information Administration counts 85 up to, but not including, 88 AKI as regular, 88 to 90 as midgrade and anything above 90 as premium. Older engines need less octane at altitude, so in some mountain states every grade is posted lower; modern engines already correct for air pressure and are calibrated for the rating in the owner’s manual.

Race fuel blend: how much to add

Topping up pump gas with race fuel is the usual reason to blend octane on purpose, usually for a dragstrip or track day where the engine is set up for more octane than the pump offers. The Race fuel blend tab gives the share of race fuel that lifts the pump gas to your target and turns it into gallons for the amount you are mixing.

Because the estimate is linear, the race fuel share grows in step with the octane you ask for: closing half the gap between the two fuels takes a half-and-half mix, and closing all of it means straight race fuel. A small bump needs only a splash; a target close to the race fuel’s own rating needs nearly a full tank of it.

Most US pump gas contains ethanol, and some race fuels are oxygenated, so the real blend may not follow the straight line exactly. The next sections explain the formula and where it goes wrong. For a number you can rely on, the blend has to be tested in a lab.

The octane blend formula

The calculator uses the linear blending rule by volume, the usual first estimate when two fuels are mixed:

1. Blend octane ≈ (gallons₁ × octane₁ + gallons₂ × octane₂) ÷ (gallons₁ + gallons₂)
2. Race fuel share = (target − pump gas octane) ÷ (race fuel octane − pump gas octane)
3. Race fuel = total fuel × race fuel share; pump gas = total fuel − race fuel
4. AKI = (RON + MON) ÷ 2

The first line is a weighted average: each fuel counts in proportion to its share of the volume. The second is the same line solved for the share, so the two tabs always agree; put the race fuel answer back into the Octane mix tab and it returns the target. The last line is the definition the pump label uses.

None of this is exact chemistry. Octane numbers are measured, not calculated, and the way two fuels interact depends on what they are made of. Refiners blend with blending octane values measured for their own components rather than with this simple average, and those values change with the base fuel.

Where the linear estimate breaks down

Ethanol is the clearest exception. US regular is usually made by adding 10% ethanol to a blendstock rated 84 AKI, and the finished E10 is sold as 87 AKI. For a straight-line average to explain that, the ethanol would have to rate 114 AKI: more than even its research octane number of 109, while its AKI sits below its RON. Count ethanol at its full RON and the linear estimate for that E10 is 86.5, still short of the real fuel.

Research on ethanol blends explains why. Octane rises roughly in proportion to the share of ethanol molecules, not the share of volume, and an ethanol molecule is much smaller than an average gasoline molecule. A modest volume share is therefore a large share of the molecules, so the first gallons of ethanol raise octane the most and each further gallon adds less. Plotted against volume the curve bends, and a straight line between gasoline and ethanol reads low for every blend in between.

So when one of the fuels is E85 or ethanol, read the answer as a conservative floor: the real blend will usually rate higher. The e85 calculator works out the ethanol content of such a mix. Between two hydrocarbon fuels with no ethanol in them, such as ethanol-free gasoline and an unoxygenated unleaded race gasoline, the straight line is closer, but it is still an approximation, and it drifts further the more the two fuels differ in makeup. E85 pumps show the ethanol content, not an octane rating, so any octane you enter for E85 is itself an assumption. Leaded race fuels are another case: the octane gained from lead falls off quickly as more is added, so a leaded fuel diluted with pump gas does not follow a straight line either. Read the result as a rough guide and go by the race fuel maker’s own blending data when it has any.

Worked example

Two common jobs, worked out with the same functions that run the calculator. Both fuels are rated the same way in each case.

Typical octane mix results

The table applies the linear rule to two common premium pump ratings and two common race fuel ratings, all as AKI. Read it as a quick guide to how far a given share of race fuel moves the number, with the caveats above.

The pattern holds for any pair: every extra tenth of race fuel closes another tenth of the gap between the two ratings, which is why the columns with the bigger gap climb faster.

Common mistakes

• Mixing rating methods. A race fuel quoted as MON cannot be blended on paper with a pump AKI: for the same fuel MON reads lower than RON, and AKI sits between them. Get both ratings on the same scale first.
• Expecting octane to add power. Octane resists knock; it does not add energy. Fuel with more octane than the engine needs does not make more power by itself.
• Trusting the estimate when one fuel is ethanol or E85. Adding ethanol or E85 raises octane more than the linear estimate says, so the calculator undersells those blends, and E10 pump gas in the mix makes any estimate less certain. An exact figure needs a lab test.
• Reading the pump number as exact. The posted rating is a minimum, rounded down to a whole or half number, so the fuel in the tank may rate slightly higher than the label.
• Comparing US and European numbers directly. A European RON figure and a US AKI figure for the same fuel differ by several points: US regular at 87 AKI is about 91–93 RON.