Starlight Tools

Octane Mixing Calculator

Blend two fuel grades toward a target octane, find how much fuel to add to an existing amount, or estimate the octane of known quantities. All values stay in this browser tab.

Planning estimate—not a certified fuel ratingUse the same scale for every octane value: AKI with AKI, RON with RON, or MON with MON. Real fuels can blend nonlinearly, particularly when their ethanol or other oxygenate content differs. Confirm compatibility and your vehicle manufacturer's fuel requirements.

Enter the fuel blend

Do not mix values from different scales.

Use the same unit for both quantities.

Usually the lower-octane fuel.

Usually the higher-octane fuel.

Must fall between fuels A and B.

Total amount after blending.

Privacy: values are calculated locally. They are not uploaded, stored, or included in analytics events.

Target blend amounts

Octane blending formulas

The calculator uses a simple volume-weighted model for two fuels:

estimated blend octane = (octane A × volume A + octane B × volume B) ÷ (volume A + volume B)

For a target final amount, the share of fuel B is:

fuel B fraction = (target − octane A) ÷ (octane B − octane A)

To add fuel B to a known amount of fuel A:

fuel B to add = volume A × (target − octane A) ÷ (octane B − target)

The equations work whether fuel B is higher or lower octane, provided the target lies between the two source ratings. Only ratios matter, so any single consistent volume unit can be used.

How to plan an octane blend

  1. Choose the job. Build a final batch, add to fuel already in a tank, or check two known amounts.
  2. Match the octane scale. U.S. pump labels normally show AKI; many other countries show RON. Do not enter one fuel as AKI and another as RON.
  3. Use actual source ratings. Read the pump or supplier documentation. Do not treat an ethanol percentage such as E85 as an octane number.
  4. Enter volumes in one unit. The calculation uses proportions, so gallons or liters produce the same blend ratio.
  5. Check suitability before mixing. Confirm allowed ethanol content, lead status, fuel type, storage, and minimum octane with the vehicle and fuel suppliers.

AKI, RON, MON, and real blending behavior

  • AKI: the Antiknock Index used on U.S. retail gasoline labels. The FTC defines it as (RON + MON) ÷ 2, also written (R+M)/2.
  • RON: Research Octane Number, widely displayed as the pump grade in many countries.
  • MON: Motor Octane Number, measured under a different test procedure and typically lower than RON for the same commercial gasoline.
  • Octane is not energy content: it describes knock resistance, not how much chemical energy is in a gallon or liter.

Why the result is approximate: Department of Energy and EPA-indexed research documents show that RON and MON can blend nonlinearly. Alcohols such as ethanol may have a blending effect that depends on concentration and base-gasoline composition. A simple average is useful for rough planning between comparable finished fuels, but it cannot certify the finished blend.

Octane mixing FAQs

What formula does this calculator use?

It multiplies each fuel's octane by its share of the total volume and adds the contributions. For example, 5 US gal of 87 AKI plus 10 US gal of 93 AKI gives a linear estimate of 91 AKI.

Can I mix AKI and RON values?

No. Use one rating method throughout. An 87 AKI U.S. pump grade is not the same rating as 87 RON. If you know both RON and MON for a fuel, AKI is their arithmetic average.

Is octane blending perfectly linear?

No. This is a volume-weighted planning estimate. Actual RON and MON can be nonlinear, especially where ethanol or other oxygenates differ, and depend on the composition of both fuels.

Can a target be above both source fuels?

Not in this model. A weighted average lies between the two source ratings. Use a source fuel with a rating beyond the target or obtain tested supplier blend data.

Does higher octane always make more power?

No. Higher octane increases resistance to knock. It only enables a performance change when the engine design, controls, or calibration can take advantage of that resistance.

Can I use E85 as “85 octane”?

No. E85 names an ethanol-content range, not an octane rating. Obtain the fuel's actual AKI, RON, or MON from the supplier, and use E85 only in a compatible vehicle and fuel system.

Are my values tracked?

No. The calculation runs locally in your browser and does not upload, store, or attach inputs to analytics events.

Limits and fuel-safety disclaimer

  • The model assumes additive volumes and a linear octane response. It does not account for blend chemistry, test uncertainty, temperature-related volume change, or seasonal fuel formulation.
  • It does not check ethanol or methanol content, leaded-fuel legality or catalyst compatibility, vapor pressure, volatility, energy content, detergent package, storage stability, or material compatibility.
  • A calculated rating does not override the owner's manual, ECU calibration, racing organization rules, road-fuel law, fuel supplier documentation, or a certified laboratory result.

Safety disclaimer: Gasoline vapors are highly flammable and harmful. Do not mix or transfer fuel near flames, sparks, hot surfaces, or unsuitable electrical equipment. Use approved containers and procedures, prevent static discharge and spills, and follow local law and manufacturer guidance. Never experiment with incompatible fuels in a vehicle.

Methodology and sources

Last reviewed: August 2, 2026. Definitions and the limitation on linear blending were checked against U.S. government guidance and research indexes:

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