ΔE = Δmc^{2}

where ΔE is the change in energy, Δm is the change in mass (mass of products - mass of reactants), and c is the speed of light (3.00 x 10^{8} m/s).

As written, this relation gives the energy change in joules and the mass change in kilograms. Usually small quantities of a sample decay, and the energy change is very large, so it's more common to get an energy change in kilojoules (kJ) corresponding to a mass change in grams. Using the relations

1 kJ = 10^{3} J and 1 kg = 10^{3} g

Einstein's equation may be rewritten

ΔE (in kJ) = 9.00 x 10^{10} Δm (in grams)

For example, to calculate the ΔE in kJ for the radioactive decay of radium:

^{226}_{88}Ra --> ^{222}_{86}Rn + ^{4}_{2}He

when one mole of radium decays, we first calculate Äm for the reaction and then obtain ΔE using the equation.

Δm = mass of 1 mol ^{4}_{2}He + mass of 1 mol ^{222}_{86}Rn - mass of 1 mol ^{226}_{88}Ra

Δm = 4.0015 g + 221.9703 g - 225.9771 g

Δm = -0.0053 g

Note that Δm may be an extremely small quantity, so it is important to know the masses of products and reactants with a high degree of accuracy in order to know the mass difference to two significant figures.

ΔE (in kJ) = 9.00 x 10^{10} x (-0.0053)

ΔE = -4.8 x 10^{8}kJ

ΔE in kJ when one gram of radium (one mole weighs 226 g) decays would be:

ΔE = 1 g Ra x (-4.8 x 10^{8} kJ)/226 g Ra

ΔE = -2.1 x 10^{6} kJ