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Dictionary of Units / Measurements

Energy or Work

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 pounds-force      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 #

kilogram-force 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 #

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