MMBTU stands for "one million British Thermal Units." It is sometimes stated as "MBTU." A single BTU is the amount of heat energy in fuel required to raise the temperature of one pint of water (454gr.) by one degree Fahrenheit. Natural gas varies in composition and has a variable BTU value, but a rule of thumb is that one cubic foot of natural gas contains about 1,000 BTUs.
Find the BTU value of the gas. This is determined periodically during the refining process by the manufacturer. Natural gas is 70 to 90% methane and up to 20% ethane, with smaller quantities of other gases such as propane and butane. Variations in the composition determine the BTU value.
Convert cubic feet to cubic meters. If your supplier gave you the BTU value of the gas per cubic foot, convert the value to cubic meters as follows:
1 cubic meter = 35.3146667 cubic feet. Multiply this by the BTU value of your gas. If your gas has a value of 1,000 BTU/cubic foot, then one cubic meter will contain 35,314.6667 BTUs.
Calculate the total BTUs in the total number of cubic meters. Multiply the number of cubic meters of gas by the BTUs per cubic meter (from Step 2). For instance, if you have 1,000 cubic meters:
35,314.6667 BTU / cubic meter x 1,000 cubic meters = 35,314,666.7 BTUs.
Convert BTUs to MMBTUs. Since there are 1,000,000 BTUs in an MMBTU, divide the total BTUs by 1,000,000:
35,314,666.7 BTUs / 1,000,000 = 35.314.6667 MMBTUs
Note that your significant figures will depend on how many significant figures are given by the refiner for the BTU value of the gas.