Radioactive Activity Calculator
Compute remaining activity after decay, decay constant, percent remaining, and optional time needed to reach a target activity.
What this radioactive activity calculator does
This tool uses the standard radioactive decay equation to estimate how activity changes over time. If you know an isotope’s half-life and starting activity, the calculator returns the remaining activity at any elapsed time. It can also estimate the time needed to decay to a chosen target level.
Core equation used
The calculator applies:
A(t) = A₀ × 2-t/T½
- A(t): activity after time t
- A₀: initial activity
- T½: half-life
It also computes the decay constant:
λ = ln(2)/T½, and the number of elapsed half-lives:
n = t/T½.
How to use the calculator
Step 1: Enter starting values
Input initial activity and choose the unit (Bq, kBq, MBq, GBq, Ci, mCi, or µCi). Then enter the half-life and its time unit.
Step 2: Enter elapsed time
Add the elapsed time and choose its unit. The calculator automatically converts between time units behind the scenes.
Step 3: Optional time-to-target
If you also provide a target activity, the calculator estimates how long decay would take to reach that target.
Worked example
Suppose a sample starts at 500 MBq with a half-life of 30 days, and you want the activity after 90 days:
- Elapsed half-lives: 90 / 30 = 3
- Remaining fraction: (1/2)3 = 1/8
- Remaining activity: 500 × 1/8 = 62.5 MBq
The calculator performs these steps instantly and also reports percentages and decay constant.
Unit notes and conversion context
Activity is measured in disintegrations per second:
- 1 Bq = 1 decay/second
- 1 Ci = 3.7 × 1010 Bq
- 1 mCi = 3.7 × 107 Bq
- 1 µCi = 3.7 × 104 Bq
This is an activity calculator, not a dose calculator. Radiation dose depends on geometry, shielding, energy spectrum, exposure path, and biological factors.
Common mistakes to avoid
- Mixing time units (for example, half-life in days but elapsed time in years) without conversion.
- Confusing activity with absorbed dose or dose equivalent.
- Entering target activity larger than the initial activity when computing time-to-target.
- Using rounded half-life values that are too coarse for precision work.
Frequently asked questions
Does this support half-life in years?
Yes. You can use seconds, minutes, hours, days, or years for both half-life and elapsed time.
Can I use curies instead of becquerels?
Yes. The calculator supports Bq, kBq, MBq, GBq, Ci, mCi, and µCi.
Is this useful for decay chains?
Not directly. This calculator models single-isotope exponential decay. Parent-daughter chains require coupled equations.
Summary
If you need a clean, quick radioactive decay equation tool with half-life conversion and activity unit conversion, this calculator provides a straightforward workflow. Enter initial activity, half-life, and time; get remaining activity and related metrics instantly.