How do you estimate ethanol content?

You can estimate ethanol content by using a graduated test tube, adding water, then adding fuel. After shaking and settling, the water absorbs the ethanol, letting you calculate the exact percentage.

What is an E85 test tube?

An E85 test tube is a graduated cylinder that allows you to easily measure water and fuel volumes to perform an ethanol separation test at home or at the pump.

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Fuel & Efficiency

Ethanol Content Estimator

Estimate ethanol content from specific gravity readings.

Graduated Test Tube Method

Ethanol % = ((Separation Mark - Water Added) ÷ Fuel Sample) × 100

Input the exact values read directly from your graduated ethanol test bottle/cylinder. The tool automatically isolates the added water to find the true fuel percentage.

Initial Water Added (ml):

Settled Separation Mark (ml):

Total Mixture Volume (ml):

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About this tool

The BuildLockr ethanol content estimator helps you calculate your true fuel mix from a physical sample. Use this tool to calculate your exact E85 percentage after doing a water separation test. Whether you use a graduated test tube or measure direct volumes, this E85 ratio calculator gives you the true ethanol rating. This helps drivers tune safely by knowing exactly what is in their gas tank.

How to Estimate Ethanol Content

Using a graduated test tube, add water up to the first marked line (e.g. 10 ml).
Add your fuel sample up to the top marked line (e.g. 100 ml total).
Shake the mixture vigorously for 10-15 seconds.
Let it settle on a flat surface for 1-2 minutes until phase separation is complete.
Read the volume of the water-ethanol separation line. Calculate using the tool above!

Common Ethanol Content Questions

Why is E85 content important?

Pump E85 content can vary from 51% to 85% depending on the season (winter blends have less ethanol) and region. If your tune expects 85% but you get 51%, your engine may run poorly or knock. Always test your fuel.