What the inputs mean
- Tank size and fuel in the tank now: the tool assumes you fill the tank to full, so it adds exactly the empty space.
- Ethanol in the fuel already in the tank: from your last fill. Use the result of the last mix if you have been running a blend.
- Ethanol in your E85: the real figure, not the name. US E85 may hold anywhere from 51% to 83% ethanol by volume under ASTM D5798, and it changes with the season and the station. 85 is the default only because it is the nominal number, so enter what an ethanol content tester reads if you have one.
- Ethanol in your pump gas: an assumption, 10% by default for common US pump gas. Enter 0 for ethanol free gas.
- Target blend: the ethanol percentage you want in the full tank. E30 means 30%.
The formula
Ethanol percentages blend by volume. The blend of a tank is the total ethanol divided by the total fuel:
blend % = (V1 × e1 + V2 × e2 + V3 × e3) ÷ (V1 + V2 + V3)With V0 of fuel at e0 already in a tank of size T, space left R = T − V0, E85 at eH and pump gas at eL, the amount of E85 to add is:
E85 = (T × target − V0 × e0 − R × eL) ÷ (eH − eL)Pump gas is the rest of the space: R − E85. If the answer is below zero or bigger than the space left, the target cannot be reached with these two fuels, and the tool shows the range you can reach.
This treats the volumes as adding up exactly. In reality the total volume changes slightly when ethanol and gasoline mix. The tool ignores that small change.
Worked example
A 15 gallon tank is a quarter full of E10, so it holds 3.75 gallons at 10%. The target is E30, and the E85 really is 85%.
Space left: 15 − 3.75 = 11.25 gallons. E85 = (15 × 30 − 3.75 × 10 − 11.25 × 10) ÷ (85 − 10) = (450 − 37.5 − 112.5) ÷ 75 = 4.00 gallons. Pump gas = 11.25 − 4 = 7.25 gallons.
Check: (3.75 × 10 + 4 × 85 + 7.25 × 10) ÷ 15 = 450 ÷ 15 = 30%.
If that E85 is only 70% ethanol, the same tank needs (450 − 150) ÷ 60 = 5.00 gallons of it and 6.25 gallons of pump gas. That is why the real E85 figure matters more than anything else here.
Once you know the blend, see the stoichiometric ratio for it in the air fuel ratio calculator, the fuel flow it needs in the fuel pump size calculator and the fuel injector size calculator, and how two pump grades mix in the octane mix calculator. Planning boost as well? Try the boost calculator.
Questions
How much E85 do I need for E30?
For a 15 gallon tank a quarter full of E10, with E85 at 85% and pump gas at 10%, add 4.00 gallons of E85 and 7.25 gallons of pump gas. From an empty tank the answer is the same 4.00 gallons of E85 plus 11 gallons of E10.
How much ethanol is actually in E85?
Less than the name suggests, quite often. Under ASTM D5798, US E85 may hold 51% to 83% ethanol by volume, and it varies by season and station. An ethanol content tester tells you what you bought.
What if my E85 is only 70% ethanol?
You need more of it. In the same 15 gallon tank, a quarter full of E10, E30 takes 5.00 gallons of 70% E85 and 6.25 gallons of pump gas, instead of 4.00 and 7.25.
What blend do 5 gallons of E85 and 10 gallons of E10 make?
With E85 at 85%: (5 × 85 + 10 × 10) ÷ 15 = 35%, so about E35. With E85 at 70% the same mix is (350 + 100) ÷ 15 = 30%.