Disclosed are methods, systems, and computer-readable mediums for determining combustion properties of wellhead gas. A methane concentration of a gas mixture is measured. A non-methane hydrocarbon concentration of the gas mixture is measured. An inert concentration of the gas mixture is calculated. A first surrogate for the non-methane hydrocarbon concentration is selected, where the first surrogate is selected as propane if the methane concentration is greater than a first threshold, and where the first surrogate is selected as pentane if the methane concentration is less than or equal to the first threshold. A methane number of the gas mixture is determined, where the methane number is based on the methane concentration, the first surrogate, and the inert concentration.