Explicit approximate equations for estimating the conversion factor of fuel-nitrogen into nitric oxide are presented. They depend on the fuel-nitrogen mole fraction, the initial nitric oxide mole fraction, and the kinetics-equilibrium mole fraction of nitric oxide. This last parameter expresses a limiting value of fuel-nitrogen conversion; it includes the complex nitrogen chemistry and depends thus on combustion conditions. Experimental results demonstrate that the kinetics-equilibrium mole fraction for fuel-lean and high-temperature conditions can be well estimated by the chemical-equilibrium mole fraction, but for lower temperatures the kinetics-equilibrium mole fraction has to be described by other correlations.