There is a lot of room for confusion in some of the units used here. The calorie can take 5 different values and, while these do not vary by very much, for accurate work it is necessary to specify which calorie is being used.
The 5 calories are known as the
 International Table calorie = cal(IT)
 thermochemical calorie = cal(th)
 mean calorie = cal(mean)
 15 degree C calorie = cal(15C)
 20 degree C calorie = cal(20C).
Unless a clear statement is made saying otherwise, assume the IT calorie is being used.
As a further complication, in working with food and expressing nutritional values, the unit of a Calorie (capital C) is often used to represent 1000 calories, and again it is necessary to specify which calorie is being used for that.
The British thermal unit (Btu) can also take different values and they are named in a similar way to the calorie, that is Btu (IT), (th), etc. Also note that the therm is 100 000 Btu so its exact size depends on which Btu is being used.
The S I unit of energy or work is the joule. To change any of these other units of energy or work into their
equivalent values in joules use the operation and conversion factor given. Those marked with # are
exact. Other values are given to an appropriate degree of accuracy.
British
thermal
units(IT)x
1055.056
Btu
(th)
x
1054.350
Btu
(mean)
x
1055.87
calories

cal
(IT)
x
4.1868
#

cal
(th)
x
4.184
#

cal
(mean)
x
4.190
02

cal
(15C)
x
4.185
80

cal
(20C)
x
4.181
90
Calorie
(food)
x
4186
(approx.)
centigrade
heat
units
x
1900.4
ergs
divide
by
10
000
000
#
foot
poundsforce
x
1.355
818
foot
poundals
x
0.042
140
gigajoules
[GJ]
x
1000
000
000
#
horsepower
hours
x 2
684
520
(approx.)
joules
[J]
1
kilocalories
(IT)
x
4186.8
#
kilocalories
(th)
x
4184
#
kilogramforce
metres
x
9.806
65 #
kilojoules
[kJ]
x
1000
#
kilowatt
hours
[kWh]
x 3
600
000
#
megajoules
[MJ]
x 1
000
000
#
newton
metres
[Nm]
x 1
#
therms
x
105
500
000
(approx.)
watt
seconds
[Ws]
1
watt
hours
[Wh]
x
3600
# 
