Air–fuel ratio, abbreviated as AFR, a dimensionless number, is the mass ratio of air to fuel mixture present in an internal combustion engines. Because it is a ratio, it is unitless. Fuel doesn't burn on its own. It has to be mixed with air. AFR tells you how many parts of air are mixed with each part of fuel. For example, a 12:1 AFR (or just 12) means the mixture is 12 parts air to one part fuel. Gasoline engines typically run around 14.7 AFR.
Air–fuel ratio calculator
Air–fuel ratio formula