Calculate how much E85 and pump gas to blend for a target like E30 or E50, top off a partly full tank, and get the ethanol percent and stoichiometric AFR.
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E85 Ethanol Blend Formula
Blending E85 with pump gas is a volume-weighted ethanol balance. The finished ethanol percentage is the average of the two fuels, weighted by how much of each you pour in. The calculator solves that balance in three directions.
To mix a fresh batch to a target blend, first find what share of the batch must be E85, then split the total volume:
E85 fraction = (Et - Eg) / (Ea - Eg) Volume of E85 = V * E85 fraction Volume of pump gas = V - Volume of E85
To top off fuel already in the tank and fill it to full at a target blend:
E85 to add = (C*Et - Vc*Ec - (C - Vc)*Eg) / (Ea - Eg) Pump gas to add = (C - Vc) - E85 to add
To find the blend you actually made from two known amounts:
Resulting E% = (Va*Ea + Vb*Eg) / (Va + Vb)
Every mode also estimates the stoichiometric air fuel ratio of the finished blend, scaled from gasoline down to pure ethanol:
Stoich AFR = 14.7 - 5.7 * (E / 100)
Variables:
- Et is the target ethanol percent you want in the finished fuel (30 means E30)
- Ea is the ethanol percent of your E85, nominally 85 but 51 to 83 in practice
- Eg is the ethanol percent of the pump gas you blend in, usually 10 for E10 or 0 for ethanol free
- Ec is the ethanol percent of the fuel already sitting in the tank
- V is the total volume you are mixing, C is the tank capacity, and Vc is the fuel already in the tank
- Va and Vb are the volumes of E85 and pump gas you added
- E is the ethanol percent of the finished blend, and Stoich AFR is the estimated air fuel ratio at lambda 1.00
The mix mode answers the most common question, how many gallons of each fuel to pour into an empty can or tank to hit a number like E30. The top off mode is for a car that already has fuel in it: it accounts for the volume and ethanol content already present, so the numbers it gives you actually land on the target once the tank is full. The resulting blend mode works backward from what you poured, which is handy for checking a mix or logging what went in. The air fuel ratio output is there because ethanol needs more fuel than gasoline, so the richer your blend the lower its stoichiometric ratio, and that number is the lambda 1.00 target your fueling has to reach.
Blend Ratios and Seasonal E85 Content
This table lists the common target blends made from E85 at 85 percent and E10 pump gas, with the amount of each fuel per 10 gallons and the estimated stoichiometric air fuel ratio of the result. The ratio is the value most blend calculators leave out, and it is what you set your fueling target to.
| Target blend | E85 per 10 gal | Pump gas per 10 gal | Est. stoich AFR |
|---|---|---|---|
| E20 | 1.33 gal | 8.67 gal | 13.56:1 |
| E30 | 2.67 gal | 7.33 gal | 12.99:1 |
| E40 | 4.00 gal | 6.00 gal | 12.42:1 |
| E50 | 5.33 gal | 4.67 gal | 11.85:1 |
| E60 | 6.67 gal | 3.33 gal | 11.28:1 |
| E70 | 8.00 gal | 2.00 gal | 10.71:1 |
| E85 | 10.00 gal | 0.00 gal | 9.86:1 |
Pump E85 is rarely a true 85 percent. ASTM D5798 allows 51 to 83 percent ethanol so the fuel starts in cold weather, so the same pump changes through the year. Enter the value you measure with a test kit or flex fuel sensor for an accurate blend, and use this table as a guide to what to expect by season.
| Season | Typical ethanol content | Why it changes |
|---|---|---|
| Summer, warm months | 79% to 83% | Heat supplies volatility, so ethanol runs near the top of the range. |
| Spring and fall | 70% to 79% | Transition blends as suppliers adjust for milder temperatures. |
| Winter, cold months | 51% to 70% | More gasoline is added to raise vapor pressure for cold starting. |
Example Problems
Example 1: Mixing a fresh batch.
You want 15 gallons of E30 in a fuel jug from E85 at 85 percent and E10 pump gas. The E85 fraction is (30 – 10) / (85 – 10) = 20 / 75 = 0.267. Volume of E85 is 15 * 0.267 = 4.0 gallons, and pump gas is 15 – 4.0 = 11.0 gallons. The estimated stoichiometric AFR is 14.7 – 5.7 * 0.30 = 12.99:1.
Example 2: Topping off a tank.
Your 16 gallon tank holds 4 gallons of E10 and you want E40, with your E85 tested at 80 percent. E85 to add is (16*40 – 4*10 – (16 – 4)*10) / (80 – 10) = (640 – 40 – 120) / 70 = 6.86 gallons. Pump gas to add is (16 – 4) – 6.86 = 5.14 gallons. Filling with those amounts brings the full tank to E40 at about a 12.42:1 stoichiometric ratio.
Frequently Asked Questions
What ethanol content should I enter for my E85?
Enter the real ethanol percentage of the fuel you are pumping, not a flat 85. Retail E85 legally ranges from 51 to 83 percent under ASTM D5798 and drops in winter for cold starting, so assuming 85 will make your finished blend leaner than you planned. A cheap ethanol test kit or a flex fuel sensor reading gives you the number to type into the fuel content field. When you do not know it, 70 is a safer default than 85 for most of the year.
Why does the calculator show an air fuel ratio?
Ethanol carries oxygen, so it needs less air to burn completely than gasoline does. That is why the stoichiometric ratio falls from about 14.7:1 on straight gas to roughly 9.8:1 on E85. The value shown is the lambda 1.00 target for your blend, the ratio your fueling has to reach for a complete burn. Richer blends demand more fuel volume, which is the main reason bigger injectors and pumps come up when people move to higher ethanol. Treat the number as a close estimate, since exact stoichiometry depends on the specific gasoline.
Can I run a homemade E85 blend in any car?
Flex fuel vehicles are built to run any ethanol blend up to E85, so mixing your own is fine for them. A standard gasoline car is designed for up to E10 or E15, and its fuel system and tune are not set up to add the extra fuel a high ethanol blend needs, so a lean condition is the risk. Small blends like E20 to E30 are often used on tuned cars with supporting fuel system changes, but going higher without a proper tune and hardware is where engines get into trouble. Match the blend to what the car and its tune can actually deliver.
