The full form of MBtu is 1,000 British thermal units. It’s used on Academic & Science ,Units in Worldwide

One Thousand British thermal units (MBtu) is a unit of measure equal to 1000 British thermal unit (Btu). M in MBtu stands for 1000 (Roman numeral value, M=1000). MBtu is used as a unit of measurement for comparing the energy content of natural gas and other fuels. The terms is also widely used in the power, steam generation, heating and air conditioning industries.

Note:

MBtu does not mean One Million British Thermal Units. People often get confused because M in metric system usually indicate one million (1,000,000 or 10⁶), but M in MBtu is taken as the roman numberal value, M=1000. MMBtu is abbreviation for One Thousand Thousand British thermal units or One Million British thermal units.