If you want to use a calculator with better number formatting, then go to the FORMATTED DISPLAY

The only disadvantage of that calculator is that it will not work with all browsers. If that is the case, return to this page.

For instructions, scroll to the middle of the page.

Radioactive Decay Calculator

Time 1 >>>>>>>>>
Time 2 >>>>>>>>>
Day 0 Activity   >>>

Choose Isotope >>

Half-Life >>>>>


The "Time 1" and "Time 2" input boxes are for inputting the start and end times of the interval you want to be calculated. Time 1 subtracted from Time 2 equals a difference that when divided by ten, yields the increments into which the interval will be divided.

(Time 2   –   Time 1)   ÷   10   =   Increment

For example, if you enter Time 1 as 50 and Time 2 as 70, your results will be in 2 day increments. (50, 52, 54, ... 70).

The "Day 0 Activity" is for inputting the radioactivity on the reference date and NOT for the activity at the start of the chosen interval. (Referring to the above example, this would NOT be the Day 20 activity).

As for the isotope "dropbox", we have chosen 70 of the most popular isotopes used in chemistry and medicine. They are listed in increasing mass number order. Choose the isotope, click the "Calculate" button and you have your calculations. Notice that the half-life of the isotope is displayed beneath the "Calculate" button.

Now, let's suppose you disagree with our half-life value OR you want calculations for an isotope we haven't listed. You can't use the boxes beneath the 'Calculate' button - these are output only. However, going to the drop-down menu, the last two choices are 'Other (Days)' and 'Other (Years)'. Make your selection from these two and then click "Calculate". An input 'prompt box' will appear and then you can input the half-life. Click the 'OK' on the prompt and your calculations will appear. DO NOT click the "Calculate" button again.

We hope this calculator will save you the time that you previously spent on these laborious calculations. If this calculator has been a help to you, please let your colleagues know and please send us an E-Mail (see Home Page). Also, if you have any problems to report, again please notify us.

All Half-Life values (except C 14, P 33, S 35, Ca 45, Rb 86 and Nb 95) were obtained from

The National Institute of Standards and Technology
Radionuclide Half-Life Measurements
M. P. Unterweger, D. D. Hoppes, F. J. Schima, and J. S. Coursey


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