Ethanol Fuel Blend Calculator:
Dial In Your Flex Fuel Mix in Seconds
I run a flex fuel kit in my 2002 BMW M3, which means every time I fill up I'm juggling three numbers: how much fuel is already in the tank, what ethanol percentage that fuel is, and what percentage I actually want to be running before I drive off. Most ethanol calculators online are American, imperial-only, and assume you're filling from empty — none of that matched how I actually use mine. So I built the tool you've found on this page.
Punch in your tank capacity (litres or US gallons — your call), your current fuel level, your current ethanol blend, your target blend, and the ethanol content of the fuel you're about to add. The calculator tells you exactly how much to add to land on your target, and if there isn't enough room left in the tank to get there by topping up alone, it'll tell you how much to drain first and how much to add after, so you hit the number exactly instead of guessing and checking with a refractometer after the fact.
It works for any flex fuel setup, in any vehicle — anyone blending pump gas with E85 (or straight ethanol) to hit a target percentage will get the same math out of it.
How it works
The calculator uses a standard volumetric blending formula to solve for how much of your new fuel to add. It accounts for how much room is actually left in your tank, and if your target isn't reachable by topping up alone, it works out the drain-then-refill numbers so you can still hit it exactly.
If you're interested in more details of the specs of the car I'm running my flex fuel kit in, here is its current build sheet.
FAQ
Q. How much ethanol is in E85?
Less than the name suggests. Pump E85 in North America typically runs anywhere from 51% to 83% ethanol, depending on the season and your region — refiners blend in more gasoline during cold months so the fuel still starts reliably. If you need an exact number for tuning purposes, check the pump label or test the fuel yourself with a refractometer rather than assuming 85%.
How do I calculate an ethanol blend for flex fuel?
You need four numbers: how much fuel is already in your tank and what ethanol percentage it is, how much fuel you're adding and what ethanol percentage that is, and what percentage you're trying to land on. The math is a straightforward volumetric blend — plug those numbers into the calculator above and it'll tell you exactly how much to add.
What ethanol percentage should I run?
That depends entirely on your engine, your tune, and what you're doing with the car — there's no universal answer. Flex fuel setups are typically tuned for a range (say 30-50% for a mild pump-gas-plus-E85 mix, or higher for dedicated E85 tunes), and your tuner or ECU documentation should specify the target for your specific setup. This calculator helps you hit whatever number you're aiming for — it doesn't tell you what that number should be.
Can I use this if my tank isn't empty?
Yes — that's actually the case it's built for. Enter your current fuel level and current ethanol mix, and it'll calculate the blend based on what's already in the tank plus what you're adding, rather than assuming you're starting from zero.
What if there isn't enough room in my tank to reach my target?
The calculator will tell you. If topping up alone can't get you there because the tank's too full, it'll show you the closest you can get by filling the remaining space, plus a second option: how much to drain first, then how much to add after, to land on your target exactly.
Does this work in litres or US gallons?
Both — use the unit toggle at the top of the calculator to switch, and it'll convert your tank capacity automatically.