Can I mix 87 and 93 octane in the same tank?

Yes — pump fuels of the same type are designed to be compatible, and the result is simply the volume-weighted average of the two ratings. Half a tank of each gives you 90, which is what midgrade already is at most stations.

How much E85 do I add to make E30?

Starting from E10, roughly one part E85 to one and a half parts E10 lands near E40, and a little under one part to two gives E35. Enter both ethanol percentages here and the tool reports the exact blend for the volumes you have.

Does higher octane fuel give more power?

Only in an engine that can use it. Octane measures resistance to knock, not energy content, so a car tuned for regular gains nothing. An engine with knock sensors and enough boost or compression can advance timing on higher octane and make more power.

Is octane really linear when you blend it?

Close, but not perfectly. Real fuels show small blending interactions, so a lab test may come out a few tenths either side of the arithmetic. That is well inside the tolerance the pump grades are sold to.

Why does the same octane number mean a different fuel in Europe?

Because the test methods differ. RON is measured under light load and MON under heavy load; the US averages them, Europe publishes RON alone. Both describe the same knock resistance on different scales.

Octane Blend Calculator

Work out the octane of a fuel mix, or how much premium to add to reach the rating you want.

The Octane Blend Calculator works entirely in your browser. Fuel volumes, ratings and prices are calculated on your device and are never uploaded.

Work out fuel cost

About Octane Blend Calculator

Octane numbers blend close enough to linearly by volume that a mix of two fuels lands at their volume-weighted average, which makes the arithmetic straightforward once you write it down. This calculator does it both ways. Pour a known amount of one fuel into another and it reports the octane you end up with; name a target instead and it works backwards to the volume of the stronger fuel needed. Ethanol content follows the same volume-weighted rule, so an E85 blend reports the percentage the engine will actually see. Pump prices are optional, and turn the answer into a cost per gallon or litre for the finished mix.

Features

Blends two fuels by volume and reports the resulting (R+M)/2 rating
Reverse mode: the volume needed on top of the tank to reach a target octane
Ethanol percentage of the finished mix, for E85 and flex-fuel blending
Presets for regular, midgrade, premium, E85, ethanol and race fuel
Total cost and effective price per gallon or litre
Works in gallons or litres, with the share each fuel contributes
Warns when a blend passes the ethanol content a non-flex engine can take

How to use the Octane Blend Calculator

Enter how much fuel is already in the tank and its octane rating
Pick the fuel you are adding, or type its rating and ethanol content
Choose whether to mix a set volume or to aim for a target rating
Read the blended octane, the ethanol content and the cost of the mix

Example

Input

10 gal of 87 octane in the tank, adding 93 octane, target 91

Output

Add 20 gal of 93 → 30 gal at 91.0 octane

Reaching 91 from 87 takes twice as much premium as the fuel already in the tank.

Common errors & troubleshooting

The target rating cannot be reached. — Blending only lands between the two ratings you started with. To reach 95 you need a fuel rated above 95 — no amount of 93 will get there from 87.
The octane figure does not match the pump sticker in Europe. — The US quotes the (R+M)/2 anti-knock index, while most of the rest of the world quotes RON, which runs 4 to 5 points higher. 91 AKI is roughly 95 RON.
The ethanol figure looks too high after adding E85. — That is usually correct and worth taking seriously. E85 is mostly ethanol, so even a partial fill pushes the blend well past what a non-flex-fuel engine is approved for.
The blend costs more than buying premium outright. — Compare the effective price per gallon against the pump price for the grade you were aiming at. Blending saves money only when the strong fuel is close in price to the weak one.

Frequently asked questions

Can I mix 87 and 93 octane in the same tank?: Yes — pump fuels of the same type are designed to be compatible, and the result is simply the volume-weighted average of the two ratings. Half a tank of each gives you 90, which is what midgrade already is at most stations.
How much E85 do I add to make E30?: Starting from E10, roughly one part E85 to one and a half parts E10 lands near E40, and a little under one part to two gives E35. Enter both ethanol percentages here and the tool reports the exact blend for the volumes you have.
Does higher octane fuel give more power?: Only in an engine that can use it. Octane measures resistance to knock, not energy content, so a car tuned for regular gains nothing. An engine with knock sensors and enough boost or compression can advance timing on higher octane and make more power.
Is octane really linear when you blend it?: Close, but not perfectly. Real fuels show small blending interactions, so a lab test may come out a few tenths either side of the arithmetic. That is well inside the tolerance the pump grades are sold to.
Why does the same octane number mean a different fuel in Europe?: Because the test methods differ. RON is measured under light load and MON under heavy load; the US averages them, Europe publishes RON alone. Both describe the same knock resistance on different scales.

Related tools

Fuel Cost Calculator — Estimate the fuel cost of a trip from distance, efficiency and price.
Compression Ratio Calculator — Work out static, dynamic and boosted compression ratio from your engine measurements.
Air-Fuel Ratio Calculator — Convert AFR, lambda and phi for gasoline, E85, diesel, methanol and more.
Fuel Economy Converter — Convert between MPG (US), MPG (UK), L/100km and km/L, with trip fuel and cost.
Engine Horsepower Calculator — Estimate engine horsepower from quarter-mile ET or trap speed.
Gas Mileage Calculator — Calculate MPG or L/100km from distance and fuel used, and convert.