What unit is used to measure heat?

There are several.

The calorie is used by scientists.

The B.T.U (B.Th.U. in UK) or British Thermal Unit is used by engineers.

The BTU (in UK only) or Board of Trade Unit is used by bureaucrats.

Note. The Calorie (otherwise kilocalorie) is used to measure the heat content of foodstuffs or fuels. Note also that the capitalisation of Calorie is the only indication that it is 1000 times bigger than the calorie.