Gas
Wobbe index
- where
- Wobbe index (J/m³; the calculator shows MJ/m³)
- calorific value on stated basis (J/m³; any unit you pick is converted)
- gas relative density to air
Explain this equation
Works out the Wobbe index of a gas, the main figure for whether different gases will give a burner the same heat input. It is used when comparing gas qualities or reading appliance gas data.
cv is the calorific value, the energy in each cubic metre. SG is the relative density of the gas compared with air: below 1 means lighter than air.
Gas flowing through a fixed injector at a fixed pressure has a volume flow that falls with the square root of its density. Heat input is that flow times cv, so it is proportional to cv / √SG. Two gases with the same Wobbe index give about the same heat input through the same injector.
Sense check: 39.5 MJ/m³ and SG 0.6 gives about 51 MJ/m³, typical of UK natural gas. A heavier gas with the same cv has a lower Wobbe index.
Watch out: this is a property calculation only. It does not permit changing an appliance to a different gas.
Enter your values to see the result.
Good to know
A gas property calculation, not permission to interchange fuel or adjust an appliance. CV and relative density must use consistent reference conditions.
Sources
Results are arithmetic only. Worked examples are not recommended design settings. This library does not provide automatic gas pipe selection, live gas tightness tests, purging procedures, combustion acceptance limits, installation approval or commissioning certification. Manufacturer data and the applicable current method remain essential.