Calculate gross or net Wobbe index, calorific value, gas relative density, or a two-gas comparison. Use matching temperature, pressure and heating-value basis. Energy-unit conversion does not change the volume reference conditions; Btu means International Table Btu.

Use matching temperature, pressure and heating-value basis. Selecting gross/net does not convert your data.

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Wobbe Index Formula

The following example problem outlines the steps and information needed to calculate the Wobbe Index.

WI = Vc / (Gs)1 / 2

Variables:

  • WI is the Wobbe Index (same units as Vc, e.g., Btu/ft³ or kJ/m³)
  • Vc is the calorific value per unit volume on your selected gross/HHV or net/LHV basis (e.g., Btu_IT/ft³) 
  • Gs is the specific gravity (relative density) of the fuel gas relative to air at the same reference conditions (dimensionless) 

To calculate a Wobbe index, divide calorific value on the selected gross/HHV or net/LHV basis by the square root of gas relative density. A matching index alone does not establish safe appliance interchangeability.

How to Calculate Wobbe Index?

The following steps outline how to calculate the Wobbe Index.

  1. First, select Wobbe index and the heating-value basis, then enter the calorific value and its energy-density unit. 
  2. Next, determine the specific gravity of the fuel (relative to air). 
  3. Next, gather the formula from above = WI = Vc / (Gs)^1/2.
  4. Finally, calculate the Wobbe Index.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

HHV value for the fuel (Btu/ft³) = 1050

specific gravity of the fuel = .61

WI = 1050 / √0.61 ≈ 1344.387239 Btu_IT/ft³.