Ethanol Blend Calculator
Mix E85 and pump gas to hit a target ethanol percentage — for a fresh batch or a top-off of a partly full tank — or work out the ethanol content of a mix or a water test, with stoichiometric AFR, extra fuel flow, energy content and cost.
How it works
Mix a batch: E85 = (target − pump%) ÷ (E85% − pump%) × total · Top off: E85 to add = (final volume × target − fuel in tank × its % − fuel added × pump%) ÷ (E85% − pump%) · Check a mix: % = (E85 vol × E85% + pump vol × pump%) ÷ total vol · Water test: % = (water layer after − water before) ÷ fuel sample · Stoich AFR ≈ [(1−x)·0.74·14.7 + x·0.79·9.0] ÷ [(1−x)·0.74 + x·0.79]
Blending two fuels of known ethanol content to a target is a mixing balance: the ethanol each fuel brings must add up to the ethanol in the finished fuel. For a fresh batch that gives the share that must be E85; for a top-off the fuel already in the tank joins the balance, so you only add what the remaining room needs — and if the target cannot be reached with the room left, the calculator tells you the range that can. Because pump gas is usually already E10 and E85 itself is not always 85% ethanol (seasonal blends can run much lower), the calculation uses each fuel’s real percentage; the water-test mode turns a tube tester reading into that percentage, and the check mode tells you what blend you already made. For tuners the results add an estimated stoichiometric air-fuel ratio, how much more fuel by volume the injectors must deliver than on pump gas, and the energy per gallon — ethanol carries about a third less energy than gasoline, so range falls as the ethanol share rises. Optional prices compare the cost with filling on pump gas.
Worked example
To brew 10 gallons at 30% ethanol from 85% E85 and 10% pump gas: E85 = (30 − 10) ÷ (85 − 10) × 10 = 2.67 gal, leaving 7.33 gal of pump gas; the blend’s stoichiometric AFR is about 12.9:1 and it needs about 8% more fuel flow than E10. To top off a 16-gallon tank that holds 4 gallons of E10 to E40 with E85 that tests at 80%: E85 to add = (16 × 40 − 4 × 10 − 12 × 10) ÷ (80 − 10) = 6.86 gal, then 5.14 gal of pump gas.
Sources: U.S. DOE Alternative Fuels Data Center — Fuel properties comparison · U.S. DOE Alternative Fuels Data Center — Ethanol fuel basics (flex fuel up to 83% ethanol)
Frequently asked questions
Is E85 always 85% ethanol?
No. E85, also sold as flex fuel, can contain 51–83% ethanol, and the content is lower in winter for cold starting. Use the water-test mode with a tube tester, or read a flex-fuel sensor, and enter the real figure as the E85 content.
How do I top off a tank that already has fuel in it?
Choose “Top off tank”, enter the tank size, how much fuel is in it and that fuel’s ethanol content. The calculator works out how much E85 and pump gas to add for the target. If the target is out of reach with the room left, it shows the range you can hit — burn more fuel first or adjust the target.
Why does the pump gas percentage matter?
Most regular gasoline is already E10, so it contributes 10% ethanol of its own. Treating it as pure gasoline would make the final blend richer in ethanol than intended. Enter 0 only for ethanol-free gas.
How much more fuel does an ethanol blend need?
Ethanol needs less air per unit of fuel to burn completely (about 9:1 by mass versus 14.7:1 for gasoline), so the injectors must flow more. E30 needs about 8% more volume than E10, and E85 around 40% more than pure gasoline — check that the injectors and pump have headroom.
Will my fuel economy drop?
Usually yes. Ethanol holds about 76,000 BTU per gallon against about 116,000 for gasoline, so E30 has around 7% less energy than E10. Real mileage depends on the engine and tune; some turbocharged engines recover part of it with more timing.
Why do people blend their own ethanol mix?
Ethanol has a high effective octane and cools the intake charge, which can support more boost or timing. Tuners blend to a specific content (say E30 or E50) to balance those gains against shorter range and harder cold starts.
Related calculators
Educational estimate. Real-world results vary with conditions, tuning and measurement.