Radioisotope Activity Calculator

Calculator Inputs

Isotope or sample label Optional label for the result card and exports.

Decay data mode Choose the radioactive decay parameter you know.

Starting quantity mode Use whichever starting quantity is available.

Half life value

Half life unit

Decay constant value

Decay constant unit

Initial activity value

Initial activity unit

Sample mass value

Mass unit

Molar mass (g/mol)

Number of atoms Scientific notation is accepted.

Elapsed time value

Elapsed time unit

Output activity unit

Graph end value Leave blank to auto-size the decay chart.

Graph time unit

Graph points

Example Data Table

Isotope	Half Life	Initial Activity	Elapsed Time	Resulting Activity
I-131	8 days	5 MBq	16 days	1.25 MBq
Tc-99m	6 hours	800 MBq	12 hours	200 MBq
C-14	5730 years	1000 Bq	5730 years	500 Bq
Cs-137	30.17 years	3.5 GBq	60.34 years	0.875 GBq

Formula Used

1) Decay constant from half life
λ = ln(2) / T_1/2

2) Activity from nuclei count
A = λN

3) Remaining nuclei after time t
N(t) = N₀ e^-λt

4) Activity after time t
A(t) = A₀ e^-λt

5) Atoms from mass
N₀ = (m / M) × N_A

6) Decayed fraction
f_decayed = 1 - e^-λt

Here, λ is the decay constant, T_1/2 is half life, N is the number of undecayed nuclei, A is activity, m is sample mass, M is molar mass, and N_A is Avogadro’s constant.

How to Use This Calculator

Select whether you know half life or decay constant.
Choose the starting quantity type: activity, mass, or atoms.
Enter the sample value and all required units.
Provide the elapsed time for the decay period.
Pick the output activity unit for the final display.
Set graph duration and graph points, or leave auto sizing.
Click Calculate Activity to display results above the form.
Use the CSV and PDF buttons to export the calculated summary.

FAQs

1) What does this calculator actually compute?

It computes radioactive activity after elapsed time, remaining atoms, decayed atoms, remaining fraction, percent decayed, half lives elapsed, and optional remaining mass for a decaying sample.

2) Can I start with sample mass instead of activity?

Yes. Choose the mass mode, enter sample mass and molar mass, and the calculator converts that mass into atoms using Avogadro’s constant before computing activity.

3) Which activity units are supported?

The calculator supports Bq, kBq, MBq, GBq, Ci, mCi, μCi, and nCi. Internally, it converts activity to becquerels for consistent calculations.

4) What is the difference between half life and decay constant?

Half life is the time required for activity or nuclei count to drop by half. The decay constant measures the probability of decay per unit time.

5) Why does the graph curve downward?

Radioactive decay follows an exponential law. That means the sample loses the same fraction over equal time intervals, so the activity falls rapidly at first and then more gradually.

6) Can I use years, days, or hours together?

Yes. Half life, elapsed time, graph time, and decay constant units are converted automatically. You can mix supported units without doing manual conversions first.

7) Does this replace laboratory or medical calibration?

No. This page is useful for educational, planning, and estimation work. Clinical dosing, shielding, and legal compliance must always follow validated instruments and professional procedures.

8) Why are some results shown in scientific notation?

Radioisotope calculations often involve extremely large atom counts or very small decay constants. Scientific notation keeps those values readable without losing scale information.