The Wobbe index is an indicator of the interchangeability of fuel gases. It is used to compare the combustion energy output of varying composition fuel gases in appliances. When two fuel gases have identical Wobbe indices for specific valve settings and pressure, their energy outputs are identical.
When comparing energy outputs for appliances, a variation of up to 5 percent in a Wobbe index is considered acceptable for the consumer. The Wobbe index is expressed in BTU per standard cubic foot or megajoules per standard cubic meter. Fuel gases have upper and lower limit ranges within a Wobbe index, rather than a specific value.