The heating value of a sample gas is calculated by a microcontroller (12) from the heating value of a reference gas, and from flow ratios determined as the gas is consumed by catalytic combustion. In a preferred embodiment, chambers (5, 14) of fixed volume are charged to a predetermined pressure with a reference gas and a sample gas, respectively, and flowed to the catalyst at variable, changing flow rates as pressure decays. During the discharge cycle, a pressure transducer (13) senses the decaying pressure and this information is input to the microcontroller (12), which computes molar flow rates and which also senses the power level of combustion through a bridge circuit (24) in the catalytic apparatus (8, 10). Based on a ratio of molar flow rates and a ratio of corresponding power levels, a heating value of the sample gas is calculated by the microcontroller (12) and output to a visual display or other output device.