The most likley conversions you will need to make are between joules (J) and calories (cal)

To convert energy in calories (cal) to energy in joules (J):
multiply the energy in calories (cal) by 4.18 to give an energy value in joules (J)
energy (J) = 4.18 x energy (cal)

To convert energy in joules (J) to energy in calories (cal):
divide the energy in joules (J) by 4.18 to give an energy value in calories (cal)
energy (cal) = energy (J) ÷ 4.18

Alternatively

To convert energy in joules (J) to energy in calories (cal)
multiply the energy in joules (J) by 0.239 ( since 1 ÷ 4.18 = 0.239) to give energy in calories (cal)
energy (cal) = energy (J) x 0.239

Examples of Conversion Between Calories and Joules

Convert 100 calories to joules

1 calorie = 4.18 joules

100 calories = 100 x 4.18 J = 418 joules

100 cal = 418 J

Convert 12.0 kilocalories to kilojoules

1 kilocalorie = 4.18 kilojoules

12.0 kilocalories = 12.0 x 4.18 kJ = 50.2 kilojoules

^{1} The Ninth International Conference on Weights and Measures (1948) recommended the use of the joule (volt coulomb) as the unit of heat.
The joule is a derived SI unit for the measurement of energy.
The SI base unit for the measurement of energy is kg.m^{2} s^{-2} 1 J = 1 kg.m^{2} s^{-2}

^{2} The joule is named after the English physicist James Prescott Joule.

^{3} The calorie, from the Latin calor meaning heat, was first defined by Nicolas Clément in 1824 as a unit of heat.

^{4}Other units for measuring energy are:

erg (1 J = 10^{7} ergs)

British Thermal Units, BTU (1 J = 9.48 x 10^{-4} BTU)

electronvolts, eV (1 J = 6.24 x 10^{18} eV)

kilowatt hours, kWh (1 J = 2.78 x 10^{-7} kWh)

^{5} National Bureau of Standards defined a calorie as equal to exactly 4.1840 J
You will find this approximated to 1 cal = 4.18 J, even 1 cal = 4.2 J, when used in schools.
Note that this will effect the number of significant figures that you will be able to use in your calculations.

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