The geeky bit for gas engineers
Your Calorific value can be found on your gas bill, the entry should be in "MJ per m3" - The default 38.736 is provided as the UK average of "10.76J/kW" which most engineers will use. This calculator will treat the kW value as Gross and you may need to divide by 0.9 to get the m3/hr value for what we'd presume to be "useful" energy used (Depending on your application).