Air-Fuel Ratio Calculator
Enter the mass of air and the mass of fuel, with their units, to get the air-to-fuel mass ratio and its inverse. This is an educational ratio calculator and does not give tuning or combustion guidance.
Last updated: September 26, 2026
Frequently asked questions
What is the air-fuel ratio?
The air-fuel ratio (AFR) is the mass of air divided by the mass of fuel in a mixture, written as a number to 1.
Is AFR a mass ratio or a volume ratio?
By definition here it is a mass ratio. A ratio by volume is a different quantity and cannot be swapped in without conversion.
What is a stoichiometric ratio?
It is the AFR at which the fuel would be completely oxidized with no excess air or fuel. It depends on the fuel's composition and the assumed oxidizer, so this tool does not assume any value.
Can I use different units for air and fuel?
Yes. Each has its own unit and the calculator converts both to grams before dividing.
Why is a zero fuel mass rejected?
The ratio divides by fuel mass, so zero would be undefined.
Does this help tune an engine?
No. It is an educational ratio calculator only. It does not provide fuel mixture targets or combustion guidance.