t = -(T1/2/0.693) * ln(N/No)
Use this equation to find out how long it will take for a given initial activity of radioactive material to decay to a given final activity.
To use the equation:
The information required by the equation is the initial activity (No), the final activity (N), and the half-life of the isotope (T½).
- Select the isotope from the list at the top of the page; this provides the equation with the isotope's half-life. If your isotope is not on the list, you can still use the equation if you manually enter the half-life in the Half-life textbox at the top of the page. You can get half-life values here.
- Enter the initial activity, No. It is not necessary to worry about units here as long as the units for the final activity are the same.
- Enter the final activity, N.
- Click the Calculate button and the decay time is displayed. The time unit will always be the same as the half-life's time unit. If you enter the activities in the wrong order, the answer received will be 0.