Fuel blend planning
E85 calculator for E30, E50, and custom blends
Enter the fuel in the tank and the ethanol content at the pump, then get the E85 and pump-gas amounts for the blend your calibration uses.
Enter the fuel left in the tank, its ethanol content, the fuel available at the station, and your target blend. Enter 30 for E30, 50 for E50, or any supported target. The result gives you the high-ethanol fuel to add first and the pump gas that finishes the fill.
The target must match the calibration in the car. Use the actual E85 content from a tester, sensor, or posted pump value - a fuel labeled E85 is not always 85 percent ethanol.
Fill-up calculation
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Add these amounts
How much E85 do you need for E30?
The old fuel is part of the next blend. If the tank has E10 left from the last fill, pouring in a fixed amount of E85 will not automatically make E30. This calculator starts with the fuel and ethanol already present, then splits the remaining tank space between high-ethanol fuel and pump gas.
There is no universal E30 amount. The answer changes with tank capacity, fuel level, the blend already in the tank, and whether the E85 pump tests at E60, E70, or E85. Enter those numbers here and use the result as the fill-to-full plan for that specific starting point.
E85 blend calculator formula
The math is a volume-weighted ethanol balance: ethanol already in the tank plus ethanol from both fuels you add must equal the target percentage of the full tank. That is why the calculator needs three ethanol inputs - current fuel, high-ethanol fuel, and pump gas - rather than assuming every fill starts empty on E10.
Use the ethanol percentage you actually bought
The E85 label describes a high-ethanol fuel category, not a fixed blend at every station. In the United States, the DOE's E85 specification allows 51 to 83 percent ethanol depending on season and location. That matters when the tune has a defined ceiling: the result for E70 and the result for E85 are different even when the pump label is the same.
What the E number means
The number after E is the ethanol percentage by volume in the finished fuel: E30 is 30 percent ethanol, E50 is 50 percent, and E85 is 85 percent. A higher E number changes the amount of fuel the engine needs. An E30 map, a flex-fuel calibration, and a pump-gas map are separate fuel strategies, not interchangeable labels.
Keep the blend inside the tune's range
Confirm that the calibration, injectors, pump, and fuel-pressure system support the blend you are mixing. On a tuned WRX, follow the ethanol range and logging process supplied with the exact map; our WRX flex-fuel guide explains why fuel content has to remain part of the calibration plan. If the sensor reading or pump test is questionable, use a known-safe blend until the fuel is confirmed.
E85 calculator questions
Can you mix E85 and 91 or 93 octane gas?
Yes. The finished ethanol percentage depends on the amount and ethanol content of each fuel. Use the pump-gas ethanol value shown at the station or specified for your area, then keep the final blend inside the range supported by the calibration.
Why can't the calculator make my target blend?
A partly full tank only has so much room for the next two fuels. If the target is above or below what those fuels can produce in the remaining space, start with less fuel in the tank, choose a different target, or use fuel with a different ethanol percentage.
Should you add E85 or pump gas first?
Add the calculated high-ethanol fuel first, then top off with the calculated pump-gas amount. That order makes it easier to follow the result at the pump and leaves the final blend based on the amounts entered above.