How the E85 mix calculator works
Filling up with E85 is really a mixing problem. Whatever is left in the tank, the E85 from the pump and any gasoline you add all carry some ethanol, and once they mix the tank holds a single blend: the total ethanol divided by the total fuel. The E85 mix tab runs that sum backwards. Tell it how big the tank is, how full it is and what is already in it, and it returns the gallons of E85 and of gasoline that fill the tank to exactly the ethanol content you asked for.
Because the tank ends up full, the two amounts always add up to the room that was left. When a target cannot be reached that way, the calculator says so and gives the highest (or lowest) ethanol content this fill-up can reach. When it can, the table under the result shows both limits: the rest of the tank filled with E85 only and with gasoline only.
Used like this it is an e85 mix calculator for owners who run a fixed blend such as E30 or E50 in a flex-fuel vehicle, or in an engine already built and set up to run ethanol. It does not decide which blend an engine can use: that is up to the vehicle maker, or to whoever set the engine up.
What to enter and where to find it
- Tank size. The usable capacity from the owner’s manual. Fuel gauges are not perfectly linear and a tank is never bone dry, so read the result as a close estimate rather than a lab figure.
- Fuel in the tank now. The gauge reading as a percentage of the tank, so a quarter tank or half a tank becomes the matching percentage. Enter 0 only if the tank really is close to empty: even with the low-fuel light on there is usually some fuel left, so a small figure such as 5 or 10 is closer to the truth than 0 is.
- Ethanol in that fuel. For ordinary pump gas, its ethanol content: E10 is 10%. If the tank already holds a blend, use your flex-fuel sensor reading, a test-kit result or the last answer from this calculator.
- Ethanol in the pump E85. Despite the name, pump E85 is sold anywhere from 51% to 83% ethanol depending on the season and the region. The field starts at 77%, the average a national survey measured in warm-season samples. Test the fuel when you can; the next section explains why.
- Ethanol in the pump gasoline. 10% for E10, the usual US pump gas, or 0 for ethanol-free gasoline.
- Target ethanol content. The blend you want once the tank is full.
How much ethanol is really in E85?
The ASTM specification for E85, D5798, allows anything from 51% to 83% ethanol. Blenders use less ethanol in cold months, because a fuel that is nearly all ethanol has too little vapor pressure to start an engine easily in the cold, and more in warm weather. A 2014 national survey of 91 pump samples, run by NREL for the Coordinating Research Council, measured an average of 77% ethanol in the warmer volatility classes and 71% in winter fuel, with individual samples varying widely from station to station.
The pump does not have to tell you the exact figure either. Under the FTC fuel rating rule, the label on an E85 dispenser may give the ethanol content rounded to the nearest ten or simply show the range, 51%–83% ETHANOL. That is why this calculator asks for the ethanol content instead of assuming it: a flex-fuel sensor reading or an ethanol test kit beats the name on the pump.
Ethanol content calculator: what a mix comes out to
The Ethanol content tab works the other way round, as an ethanol content calculator or ethanol blend calculator. Enter what was already in the tank and how many gallons of E85 and of gasoline you added, and it returns the ethanol percentage of the mix, the total fuel and its stoichiometric ratio. It is the quick check after a fill-up that did not go to plan, such as a pump that clicked off early or a station that ran out of E85 halfway.
E85 is for flex-fuel vehicles only
E85 can only be used in flex-fuel vehicles, which are built at the factory to run on gasoline or any blend up to 83% ethanol. The E85 pump label says it plainly: use only in flex-fuel vehicles, may harm other engines. A yellow fuel cap or a yellow ring around a capless filler, an E85, Flex-Fuel or FFV badge, and the owner’s manual are the usual ways to tell. This calculator is meant for those vehicles and for engines already built and set up to run ethanol; it gives no advice on converting a gasoline-only car.
Ethanol carries about a third less energy per gallon than gasoline, so expect to burn more of it. Flex-fuel vehicles get roughly 15–27% fewer miles per gallon on E85, depending on its ethanol content; the gas mileage calculator shows what your own fill-ups return. Why ethanol raises octane more than a simple average suggests is explained on the octane calculator page.
The E85 blend formula
Everything comes from one ethanol balance, with volumes taken as additive. Ethanol and gasoline change volume very slightly when they mix (if anything they expand a little), far less than the uncertainty in the ethanol content of the pump fuel.
Fuel now = tank size × fuel levelE85 to add = (tank size × target − fuel now × its ethanol − room left × gasoline ethanol) ÷ (E85 ethanol − gasoline ethanol)Gasoline to add = room left − E85 to addEthanol content of a mix = Σ(gallons × ethanol fraction) ÷ Σ gallonsStoichiometric AFR = w × 9.0 + (1 − w) × 14.7, with w the ethanol fraction by mass
The E85 line follows from two conditions: the E85 and the gasoline together fill the room left in the tank, and the ethanol they bring plus the ethanol already there equals the target share of a full tank. Solve the pair and the E85 amount drops out; the gasoline is whatever room remains. If the E85 comes out negative, even a tank topped up with gasoline alone holds more ethanol than the target, and if the gasoline comes out negative, even pure E85 cannot get there. Those are the two cases the calculator reports instead of an answer.
The stoichiometric ratio is the mass of air needed to burn a mass of fuel completely. The air each component needs adds up by mass, not by volume, so the ethanol percentage is first turned into a mass fraction with the densities of ethanol (0.789 kg/L) and gasoline (0.74 kg/L). For E85 at 77% ethanol by volume, ethanol is 78.1% of the fuel by mass and the stoichiometric ratio works out to 10.2:1 against 14.1:1 for E10. The air fuel ratio calculator covers lambda and target mixtures.
Worked example
Three fill-ups worked out with the same functions that run the calculator.
Mixing E30 from a quarter tank
A flex-fuel car with a 16-gallon tank shows a quarter tank, 25% on the gauge, of E10 (10% ethanol). The owner wants E30, and the station’s E85 tests at 77% ethanol:
- Fuel in the tank now 16.0 gal × 25% = 4.00 gal, room for 12.00 gal
- Ethanol the full tank needs 16.0 gal × 30% = 4.80 gal
- Ethanol already there 4.00 gal × 10% = 0.40 gal
- E85 to add (4.80 − 0.40 − 12.00 × 0.10) ÷ (0.77 − 0.10) = 4.78 gal
- Gasoline to add 12.00 − 4.78 = 7.22 gal
- Ethanol by mass 0.30 × 0.789 ÷ (0.30 × 0.789 + 0.70 × 0.74) = 0.314
- Stoichiometric AFR 0.314 × 9.0 + 0.686 × 14.7 = 12.91:1
So about 4.8 gallons of E85 go in first and 7.2 gallons of E10 top the tank off. The table under the calculator shows the limits of this fill-up: E85 alone would give 60.3% ethanol and gasoline alone 10.0%.
Checking a fill-up after the fact
Another owner had 3 gallons of E10 left, pumped 6 gallons of E85 at 77% and then 5 gallons of E10:
- Total fuel 3.0 + 6.0 + 5.0 = 14.0 gal
- Ethanol in it 3.0 × 0.10 + 6.0 × 0.77 + 5.0 × 0.10 = 5.42 gal
- Ethanol content 5.42 ÷ 14.0 = 38.7%
- Ethanol by mass 0.387 × 0.789 ÷ (0.387 × 0.789 + 0.613 × 0.74) = 0.402
- Stoichiometric AFR 0.402 × 9.0 + 0.598 × 14.7 = 12.41:1
The tank now holds 14.0 gallons at 38.7% ethanol, and its stoichiometric ratio is 12.4:1 by mass.
The same fill-up with winter E85
Had the E85 been winter fuel at the survey average of 71% ethanol:
- Total fuel 3.0 + 6.0 + 5.0 = 14.0 gal
- Ethanol in it 3.0 × 0.10 + 6.0 × 0.71 + 5.0 × 0.10 = 5.06 gal
- Ethanol content 5.06 ÷ 14.0 = 36.1%
- Ethanol by mass 0.361 × 0.789 ÷ (0.361 × 0.789 + 0.639 × 0.74) = 0.376
- Stoichiometric AFR 0.376 × 9.0 + 0.624 × 14.7 = 12.55:1
The same gallons now give 36.1% ethanol instead of 38.7%. That gap is the reason to test the fuel, or read the flex-fuel sensor, when you are aiming for an exact blend.
Typical ethanol blends and their stoichiometric AFR
The table runs common blends through the same formulas. The E85 rows use the limits of the specification and the survey averages quoted above, which shows how far apart two tanks of fuel sold under the same name can be.
| Blend | Ethanol by volume | Ethanol by mass | Stoichiometric AFR | Fuel volume vs. E0 at stoich |
|---|---|---|---|---|
| E0, ethanol-free gasoline | 0% | 0.0% | 14.70:1 | — |
| E10, US pump gasoline | 10% | 10.6% | 14.10:1 | +3.6% |
| E15 | 15% | 15.8% | 13.80:1 | +5.5% |
| E30 | 30% | 31.4% | 12.91:1 | +11.6% |
| E50 | 50% | 51.6% | 11.76:1 | +21.0% |
| E85 at the ASTM D5798 minimum | 51% | 52.6% | 11.70:1 | +21.5% |
| E85, winter survey average | 71% | 72.3% | 10.58:1 | +32.7% |
| E85, warm-season survey average | 77% | 78.1% | 10.25:1 | +36.5% |
| E85 at the ASTM D5798 maximum | 83% | 83.9% | 9.92:1 | +40.5% |
| Pure ethanol (reference) | 100% | 100.0% | 9.00:1 | +53.2% |
The last column is how much more fuel, by volume, an engine needs at the stoichiometric ratio than on ethanol-free gasoline for the same amount of air. At the warm-season average it is about 36% more than ethanol-free gasoline, or about 32% more than E10 pump gas, which is why ethanol blends need extra capacity from the injectors and the pump; the fuel injector calculator sizes injectors for it.
Common mistakes
- Taking the name literally. E85 is a product name, not a measurement. Pump fuel runs from 51% to 83% ethanol and winter fuel usually carries less (the survey average was 71%), so a mix worked out from the name alone ends up with less ethanol than planned.
- Forgetting the fuel already in the tank. Even a quarter tank of E10 shifts the result noticeably. Read the gauge before you start pumping and enter it.
- Pumping the gasoline first. Pump the E85 first and top off with gasoline last. If the gauge reading was a little off, the error ends up in the gasoline amount and the tank is still full, which is what the calculation assumes.
- Mixing up volume and mass. Pumps, sensors and test kits give ethanol by volume, while the stoichiometric ratio needs the share by mass. Plugging a volume percentage straight into a mass-weighted average puts the ratio off slightly, by up to about a tenth of a point in mid-range blends.
- Using E85 in a car that is not a flex-fuel vehicle. The calculator assumes a flex-fuel vehicle or an engine already set up for ethanol. The E85 pump label itself reserves the fuel for flex-fuel vehicles.
Last updated October 29, 2026 by Fernando Margareto, Founder & Editor.