 Radioactive decay

 
This calc gives the remaining activity (as a fraction of the original activity) of a sample with known halflife after a given time. A simple exponential decay model is used.
 
Isotope halflifes range from immeasurably short times for very unstable isotopes to incredibly long ones (millions of years even). This calc can also be used to calculate the amount of decay for any other exponential (firstorder) process with a known halflife.  