how do you work the gas rate of an appliance?

Answer 1

Ensure that no gas is to be used except by your appliance.
Take a reading of the Gas Meter.
Have a stop-watch or clock handy.
Note the exact time and start your appliance.
When finished, as you shut down the appliance, make a note of the time.
Re-read the gas meter and divide the gas reading by the time it was in use.
The result should give Cubic Feet, or Cubic Metres used in hours, or minutes or seconds. (ft³ or m³ / hr (or min or sec).

Answer 2

The appliance will be rated in BTUs / Hr.
Get the BTU.s /cu. ft. off the bill and work from there.