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Ethanol Calculator

Ethanol Calculator

This calculator is useful if you are mixing your own E85 at home or if you want to add just a few gallons of gas to your tank.


Ethanol Blend Calculator: For mixing fuels when filling up at the gas station.

This calculator is useful to get the right ethanol mixture when fueling your car at the gas station. If you are running a specific ethanol content this calculator is for you. If you know your current ethanol content and your target ethanol content this calculator will tell you how much E85 and pump gas to put in your gas  tank to get your desired ethanol content with a full tank.

How to use this calculator

  1. Input how many gallons of gas your car holds (see common tank sizes below)
  2. Enter the ethanol content from the pump gas you will be using in this mixture. The ethanol content of 91oct coming from the gas station typically has an ethanol content of 9.8%.
  3. Enter the ethanol content from the E85 gas you will be using in this mixture. The ethanol content of the E85 coming from the gas station will vary throughout the year. In the summer months (May-September), when it’s always above freezing, you will typically find 78-83%. In the winter months (October-April), you will typically find 68-72%. It is not very common to find E85 that is actually 85% here in Minnesota.
  4. Enter your target ethanol mix.
  5. Enter your current fuel tank level – i.e., if you have a quarter tank left, that’s 25%.
  6. Lastly, enter the ethanol content of the fuel currently in your gas tank.

Gas tank sizes for common vehicles are as follows:

  • Subaru WRX/STI (all years) – 15.9 gallons
Gas tank size
Gasoline Ethanol Percentage
E85 Ethanol Percentage
Target Ethanol Mix
Current Fuel Level Percentage
Current Ethanol Mix
E85 to Add
Pump Gas to Add
Resulting Mix
0.0% Ethanol