Radioactive Decay Calculator with Half-Life Analysis & Plotting
Published: Jul 15, 2025
Source: Ionactive Radiation Protection Resource
Ionactive Radioactive Decay Calculator
This radioactive decay calculator allows you to calculate how activity changes over time using half-life. It supports forward decay, reverse calculations, and time-to-decay between two activity levels.
The tool is suitable for radiation protection, nuclear medicine, research, environmental protection and teaching applications. It also includes visual decay plots to help illustrate how activity reduces over multiple half-lives. Additional resource after the calculator explains how this decay expression is used:
\[
A(t) = A_0 \cdot e^{-\lambda t}
\]
If you are looking for effective half life canulations the consider this Ionactive resource: Effective Half Life Calculator.
Radioactive Decay Calculator
Release notes
Version 3.0 (April 2026). This latest version of the Ionactive Radioactive Decay Calculator has undergone further improvements including full plotting implementation to help visualise radioactive decay, and the addition of more radionuclides. 'Time to decay from activity A to B' is still included which can assist in questions such as 'how long before a HASS source is no longer a HASS source', or 'how long will it take to decay store some radioactive waste down to VLLW levels' [where HASS = High Activity Sealed Source and VLLW = Very Low Level Waste].
Radioactive Decay Calculator with Half-Life Analysis & Plotting Outline
This interactive radioactive decay calculator allows the user to determine the remaining radioactivity of a material at a future point in time — or, working backwards, to estimate its original activity at an earlier time (reverse calculation). It also allows calculations to determine the time taken to decay from one activity to another lower activity. Once the calculation runs the results are presented in text form and also as an interactive plot - this plot is can be explored and manipulated in all sorts or ways to display additional useful data.
We have loaded the calculator with the most likely radionuclides of interest (e.g. from medicine, industry, research and public curiosity).
Radioactive decay is a predictable, exponential process governed by a particular radionuclides' decay physics, and hence its half‑life (time taken for the activity to reduce by half).
The calculator is used in the following way:
- Choose the calculation mode from the drop down menu (Activity at future time 'forward', Original activity from later value 'reverse' or Time to decay from activity A to activity B).
- Choose time input (elapsed time, or start and end dates).
- Choose the radionuclide. If you are just looking for half-life data, then selecting a radionuclide will display this data below your selection.
- Choose the activity unit (becquerels or curies) and the unit multiplier (e.g. micro, milli, base, kilo, mega, giga etc).
- Select graph controls, the default is keep it simple, selecting 'on' opens up additional graphing functions.
- Enter the known activity (either current or known later activity depending on calculation mode selected).
- Enter time elapsed (in minutes, hours, days, years etc). If you have selected start & end dates (from time input menu) then you can enter dates instead.
- Click calculate to see the answer, press reset to start a new calculation from scratch.
- The calculator assumes that radioactivity of < 1 Bq is no greater than background.
If you have selected 'Time to decay from activity A to activity B' from the calculation mode menu, the inputs change slightly and now you can enter initial activity (A) and target activity (B). In this mode you can also optionally choose to add a calculation start date. This can be useful, for example when you know the activity on a certain day and want to know the date when the decay has reached a desired level of activity. This could be useful in nuclear medicine where you bag up waste of a known (or assessed) activity, and want a disposal date when it has likely reached a desired threshold (e.g. < 40 KBq per item and / or < 400 kBq per 0.1 m3 of waste - VLLW in the UK). Another use would be assessing when a HASS source (High Activity Sealed Source) decays to a level where it is no longer HASS.
Ionactive Radioactive Decay Calculator plot functionality
For this release we now include interactive radioactive decay curves from within the calculator. With the following examples we have extracted the plot from the calculator so you can clearly see how they work. If you want to fully manipulate the plots then input the desired parameters into the calculator, click calculate and then have a play. The following are interactive so mouse over them and see what you come up with. If you manipulate them and get lost, just double click on the plot and it will bring you back to the default display.
First up is P-32 - here we are looking at the decay of 37 MBq of P-32 over a period of 100 days.
The next example is I-131 - here we are looking at the decay of 5 GBq of I-131 over a period of 32 days.
The final example is F-18 - here we are looking at the decay of 400 MBq of F-18 over a period of 1 day (expressed in minutes).
Practical uses of the calculator
Here are some practical examples of how you might use this calculator.
- You have a sealed Ir‑192 source for industrial radiography today and want to know its expected activity six months from now.
- You may discover an old labelled Co‑60 source and want to estimate its original activity when it was first supplied (based on how much remains today).
- You may want to check how quickly a F‑18 PET radiopharmaceutical loses activity in the event of a spill (noting that dose rate is proportional to activity in this case).
- You may wish to calculate the time required before a Co-60 HASS source drops below the HASS threshold.
- You may wish to calculate how long radioactive waste will take to decay down to the UK VLLW levels.
- You are looking at historical records for a laboratory decommissioning and Environmental Agency permit surrender, and wish to know if Ca-45, disposed via a sink (to drain), could still be present in a worse case scenario (i.e. retained in sink trap).
What is going on under the hood of this calculator?
The radioactive decay law is most commonly written as follows (for activity):
\[
A(t) = A_0 \cdot e^{-\lambda t}
\]
where:
- \( A_0 \) is the initial activity
- \( A(t) \) is the activity at time \( t \)
- \( \lambda \) is the decay constant
The above expression is not used directly in the Ionactive calculator, since we are already working with half-life (expressed in hours, days, years etc). This calculator instead uses this equivalent form:
\[
A(t) = A_0 \cdot 2^{-t/T_{1/2}}
\]
where:
- \( T_{1/2} \) is the half‑life of the isotope
Since the calculation is based on the radionuclide (i.e. half‑life) you select from the dropdown selection, there is no need to calculate or know the decay constant (\( \lambda = \ln(2) / T_{1/2} \)). For many users this is more intuitive since the idea that the activity halves every \( T_{1/2} \) is easier to relate too.
Both forms are equivalent and describe the same exponential decay process — they’re just expressed differently depending on which parameter (\( \lambda \) or \( T_{1/2} \)) you start from.
If you’re more familiar with \( e^{-\lambda t} \), then remember that:
\[
2^{-\frac{t}{T_{1/2}}} = e^{-\lambda t}, \quad \text{where} \quad \lambda = \frac{\ln(2)}{T_{1/2}}
\]