t = (T_{1/2}/0.693) *
ln(A/Ao)
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
(A_{o}), the final activity (A), and the halflife of the
isotope (T_{1/2}).
 Select the isotope from the list at the top of the page; this
provides the equation with the isotope's halflife. If your
isotope is not on the list, you can still use the equation if you
manually enter the halflife in the Halflife
textbox at the top of the page.
 Enter the initial activity. 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.
 Click the Calculate button and the decay
time is displayed. The time unit will always be the same as the
halflife unit.
An example:
You purchase 9.25 MBq of P32. How long will it take to decay to
2.0 MBq?
 Select Phosphorus 32 from the isotope list.
14.26 and d show up in the
Halflife and Halflife time
unit textboxes.
 Enter 9.25 in the Initial activity
(A_{o}) textbox.
 Enter 2.0 in the Final activity
(A) textbox.
 Click the Calculate button and the answer
31.5068... d is received.
 You can then use the time calculator to find out the date 32
days from now.
If you enter the activities in the wrong order, the answer
received will be 0.
