For radioactive elements, a half life is the time it takes for half of the substance to disintegrate. For example, if you started with 100g of radium, after one half life, the amount drops to 50g -- the rest becomes other elements. After a second half life, the amount drops to 25g. To use the half life calculation, you need to know the number of half lives that pass.
Divide the amount of time that has passed by the half life of the substance to find the number of half lives. For example, if your substance has a half life of 1 month and the total time equals 12 months, divide 12 by 1 to find that 12 half lives have passed.
Raise 0.5 to the power of the number of half lives. For this example, raise 0.5 to the 12th power to get 0.000244141.
Multiply the result by the amount of the substance you started with to find the amount remaining. Completing the example, if you started with 4,000 g, multiply 4,000 by 0.000244141 to find you have 0.98 g remaining of the original radioactive substance.