C035 – ISO 6976/GPA 2145:2003 Calorific Value and Relative Density


This calculates calorific values, density, relative density and Wobbe index from a gas composition. Results are calculated for the composition treated as both a real and an ideal gas, inferior (net) and superior (gross) calorific value and Wobbe index are displayed in each case.


FLOCALC Calculation reference F049

Kelton calculation reference C035

Kelton calculation reference C111

International standard, Natural Gas – Calculation of calorific values, density, relative density and Wobbe index (ISO 6976:1995)

GPA Standard 2145-03 Table of Physical Constants for Hydrocarbons and Other Compounds of Interest to the Natural Gas Industry


Hydrocarbon Tail

  • No Tail
  • C6 Tail
  • C7 Tail
  • C9 Tail

Select the appropriate hydrocarbon tail. Where a tail is selected the percentage split of each component is entered, this is generally obtained from laboratory analysis.

Molecular Weights

  • Rounded
  • Full Precision

This option gives the choice to use the rounded molecular weights as shown in Table 1, or calculate the molecular weights with full precision using the elemental molars masses shown beneath Table 1 of ISO 6976.


This performs the calculations from C111 when set for 2003. It calculates gas properties at ISO standard conditions (15°C/15°C & 1.01325bar)