What it does
Every radioactive sample loses the same fraction of its activity in the same time. After one half-life half is left, after two a quarter, and so on:
A = A₀ × 2^(−t / T½) = A₀ × e^(−λt), with λ = ln 2 / T½.
Labs use this all day: to know what a vial of F-18 will read at injection time on a scanner, how much P-32 is left in a stock from last month, or what a check source reads this year. This tool does it from dates and times, so you never have to count the hours by hand.
- Decay correction from a reference activity and time to any other date and time, forwards or backwards.
- Decay factor, time elapsed in hours or days, and how many half-lives that is.
- When it falls to a given activity, as a date and time.
- Decay table at any step (every 15 min, every day) for up to 200 rows, ready to print or download.
- Specific activity of the pure isotope in Bq/g and Ci/g, and molar activity in GBq/µmol and Ci/mmol.
- Your sample’s molar activity and the labelled fraction, from its activity and amount.
The list holds 33 nuclides used in research, imaging, teaching sources and natural radioactivity, with half-lives from the NNDC and DDEP evaluations. Choose “Your own half-life” for anything else.
How to use it
- Pick the nuclide.
- Type the activity on the label or certificate and its unit, and the reference date and time.
- Pick the date and time you want the activity for. A time before the reference works too.
- Optionally type an activity to find when the sample will fall to it.
- Set the decay table step and number of rows, and download it as a PDF, Word file or image.
Worked example: F-18
A vial holds 370 MBq (10 mCi) of F-18 at 08:00. The half-life is 109.77 minutes.
- At 10:00, 120 minutes later, that is 1.093 half-lives, so the activity is 370 × 2^(−120/109.77) = 173.4 MBq, 46.9 % of the reference.
- It falls to 100 MBq at 3 h 27 min after the reference, at about 11:27.
For C-14 (half-life 5,700 years) the carrier-free molar activity is 62.7 mCi/mmol, and for tritium 29.0 Ci/mmol, the familiar upper limits for labelled compounds. A sample of a C-14 compound with 50 mCi/mmol is therefore about 80 % labelled.
Questions
Which half-life values are used? The recommended values of the US National Nuclear Data Center and the Decay Data Evaluation Project, rounded as shown in the list. A year here is 365.25 days.
Why does the specific activity use the mass number? The exact isotopic mass differs from the mass number by less than 0.1 %, far below other uncertainties in a real sample.
Can I use this for patient doses? No. It is for lab work, teaching and checking numbers. Clinical activities must come from a calibrated dose calibrator and your local procedures.
Is anything sent anywhere? No. It all runs in your browser and works offline once the page has loaded.
Where the numbers come from
- Half-lives: NNDC (ENSDF, US Brookhaven National Laboratory) and DDEP recommended values.
- Atomic weights: IUPAC/CIAAW standard atomic weights (abridged values).
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-08.