The rate of radioactive decay is measured by an isotope's half-life, which is the time it takes for half of a radioactive isotope to decay into a different isotope. This means that after the half-life period, only one-half of the isotopic material will remain. Although the concept is simple enough, the formal formula is slightly more complex, because it involves natural logarithms. To simplify the calculation, you can create an Excel spreadsheet to run the numbers for you.

Open Microsoft Excel.

Enter the isotopes half-life in cell "A1." Half-life can be any unit of time, such as microseconds or years. However, to keep units consistent, use days. To convert years to days, multiply by 365.25 with the formula "=years_325.25". To convert seconds, divide by (24_60_60) with the formula "=seconds/(24_60*60)".

Enter the length of time, for which you wish to measure the radioactive decay, in cell "A2." Use units of days.

Enter the formula "=EXP(-LN(2)*A2/A1)" in cell "A3." The answer gives you the fraction, in decimal form, of the remaining isotope.

Right-click cell "A3," select "Format Cells", click "Percentage" in the Category list of the Number tab, and click "OK." This converts the answer into a percentage.