How do you convert 1 mmbtu to kilogram?

One mmbtu is the common usage designation of 1 million british thermal units, also called ten therms or one dekatherm in US or English heat content measure. It does not convert to kg but it does convert to calories (metric system) or Joules/ gram or KJ/Kg in the SI system.

1 btu = 252 calories so 1 mmbtu = 252,000,000 calories or 252 Mcal

1 calorie equals 4.1868 j/g so 252 Mcal =1055. 0736 MJ/g or 1,055,073,600 KJ/Kg

(1 J/g = 1 KJ/Kg)

therefor 1 mmbtu = 1,055,073,600 KJ/Kg