Results are presented from a new mathematical model on the mixing of fuel and air. The model is derived from the ideal gas law and the equation of continuity. The equivalence ratio measured within an infinitesimal fluid element of mixture and the time rate of the equivalence ratio are used to quantify the degree and time rare of mixing. The model shows that in addition to the fuel molecular weight, the mixing rate depends on pressure, temperature, density, and their rates. The density rate depends on velocity gradients. The mixing between methane and air is described since the gaseous reactants delineate the droplet heat-up and evaporation for liquid fuels. Results presented here provide the isolated effect of each parameter involved in mixing. Implication of the results on mixing of other fuels and air is discussed. These results are intended to provide important design guidelines for developing high intensity and high efficiency combustors with special focus on low levels of pollutants emission.