Ethanol Fuel Blend Calculator (E10 + E85 Mix, Flex-Fuel)
Ethanol content of a tank after topping up with a different blend, and how much E85 (or E10) to add to land on the percentage you want.
Results
The dashed line marks the value you entered.
What this tool does
Flex-fuel drivers mix blends all the time: half a tank of E10 plus E85 does not give E85, it gives the volume-weighted average. The sums are simple because liquid volumes of petrol and ethanol are close to additive. Two things matter in practice: ethanol holds roughly two thirds of the energy of petrol per litre, so consumption in litres rises as the blend gets richer, and the ethanol content of pump 'E85' varies, notably lower in cold weather. Only use blends your vehicle is approved for.
Formula
E = (V₁ × E₁ + V₂ × E₂) ÷ (V₁ + V₂) · to reach a target: V₂ = V₁ × (E_target − E₁) ÷ (E₂ − E_target)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
vt | Fuel already in the tank | L |
c1 | Ethanol content of that fuel | % |
va | Fuel you add | L |
c2 | Ethanol content of the fuel you add | % |
ct | Ethanol content you want | % |
CF | Ethanol content of the mix | % |
VF | Total volume of the mix | L |
VE | Ethanol in the mix | L |
VN | Fuel to add to reach the ethanol content you want (0 if it cannot be reached) | L |
Worked example
- Fuel already in the tank15 L
- Ethanol content of that fuel10 %
- Fuel you add25 L
- Ethanol content of the fuel you add85 %
- Ethanol content you want30 %
- Ethanol content of the mix56.9 %
- Total volume of the mix40.0 L
- Ethanol in the mix22.75 L
- Fuel to add to reach the ethanol content you want (0 if it cannot be reached)5.45 L
Limitations
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
- Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.