Planning to run E30 or another ethanol blend? Enter your tank size, how much fuel is already in it and its ethanol content, and get the exact gallons of E85 and pump gas to add. You can also check what blend you made after a fill-up.
Last updated:
What do you want?
1/4 tank = 25%, empty = 0
%
E10 = 10%; use your last blend if mixed
%
30 for E30, 50 for E50
%
Pump fuels
%
E85 is legally 51–83% ethanol; test it if you can
%
Most US pump gas is E10; ethanol-free is E0
$
per gallon or liter
$
per gallon or liter
Enter your values to see the result.
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How to use the E85 Ethanol Blend Calculator
Enter the tank capacity and how much fuel is in it now (as a gauge percentage or volume).
Enter the ethanol content of the fuel already in the tank (10% for E10, or your last blend).
Set your target blend, such as 30 for E30, and how full you want the tank.
Enter the real ethanol content of your E85 and pump gasoline.
Read how much E85 and gas to add; pump the E85 first, then top off with gasoline.
The ethanol blending formula
Ethanol blends mix by volume. If V0 is the fuel already in the tank at ethanol fraction e0, and you want a final volume Vf at target et, the E85 to add is:
The rest of the space is pump gasoline. Example from empty: a 15 gallon tank to E30 with 80% E85 and E10 gas needs (15 × 0.30 − 15 × 0.10) ÷ (0.80 − 0.10) = 4.29 gallons of E85 and 10.71 gallons of gasoline.
E85 from empty: gallons per 10 gallons of fuel
Target
E85 is 85%
E85 is 80%
E85 is 70%
Stoich AFR (approx.)
E15
0.67
0.71
0.83
13.8
E20
1.33
1.43
1.67
13.5
E30
2.67
2.86
3.33
12.9
E40
4.00
4.29
5.00
12.3
E50
5.33
5.71
6.67
11.8
E60
6.67
7.14
8.33
11.2
E70
8.00
8.57
10.00
10.6
Gallons of E85 per 10 gallons total, with the rest E10 gasoline. Multiply by your tank size ÷ 10. AFR estimated from typical densities (ethanol 0.789, gasoline 0.745 kg/L).
Tips for running ethanol blends
Test your fuel: E85's real ethanol content changes by season and station; a cheap ethanol tester makes the calculation reliable.
Stay consistent: if your tune is for a fixed blend without a flex fuel sensor, mix the same blend every fill-up.
Plan the fill: the more fuel left in the tank, the less the blend can change in one fill-up.
Cold starts: high ethanol blends can be harder to start in cold weather, which is why winter E85 often contains less ethanol.
Frequently asked questions
How do I calculate how much E85 to add for E30?
Work out the ethanol you need in the full tank, subtract the ethanol already there and what the gasoline will add, then divide by the difference in ethanol content between E85 and gas. For a 15 gallon tank at 1/4 full of E10, with E85 that is really 80% ethanol and E10 gas, you add about 4.3 gallons of E85 and 7.0 gallons of gasoline to reach E30.
Is E85 really 85% ethanol?
Not always. Under ASTM D5798, fuel sold as E85 (flex fuel) can contain 51% to 83% ethanol, and suppliers typically lower the ethanol content in winter for cold starts. Many tuners test their E85 with an ethanol content tester or a sensor so the blend calculation is accurate.
What is the stoichiometric air-fuel ratio of E85?
About 9.8:1 by mass for true 85% ethanol, compared with about 14.7:1 for gasoline and 9.0:1 for pure ethanol. That is why an engine needs roughly 30–45% more fuel volume on E85, and why injectors and fuel pumps are often upgraded.
Can I put E85 in any car?
No. Only flex fuel vehicles (FFVs) or engines specifically tuned and fitted with ethanol-compatible fuel system parts should run E85 or high blends. Check your owner's manual or the yellow fuel cap or badge. Using high ethanol blends in a standard car can cause lean running, check-engine lights and fuel system damage.
Why can't I reach my target blend this fill-up?
If the tank already holds a lot of low-ethanol fuel, even filling the rest with E85 may not reach a high target, and vice versa. The calculator shows the possible range and how far to burn the tank down first so the target becomes reachable.
Does the order of filling matter?
No for the final blend, but pumping the smaller amount first (usually E85) and finishing with the other fuel helps you stop at the right amount and mixes the fuels as they enter the tank. Driving a short distance mixes them completely.
Is this calculation exact?
The volumes are a simple volume-weighted mix, which is accurate enough for fuel blending (mixing ethanol and gasoline changes total volume by well under 1%). The AFR and fuel flow figures are estimates using typical densities, and the real result depends on the actual ethanol content of each fuel.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.