A number used to compare the combustion energy of different fuels such as natural gas, SNG, and LPG, and used for comparative purposes of gas burners. Fuel gases with the same Wobbe index can be used interchangeably without a change in the air-to-fuel ratio. It is calculated from the calorific value of the gas divided by the square root of the specific gravity. The SI units are MJ m−3. It is also often still expressed in Imperial units of BTU per standard cubic foot.