About the Decay Correction Calculator
This tool estimates how radioactive activity changes over a selected elapsed time using the physical half-life of the isotope. It is useful for nuclear science, radiopharmacy, radiation safety training, and lab planning when you need forward decay or back-correction from a later measurement.
How to use this calculator
- Enter the reference or measured activity and choose its unit.
- Enter the physical half-life and choose the half-life time unit, or select an optional isotope preset.
- Enter the elapsed time between the reference time and the later time.
- Choose forward decay or back-correction as the calculation type.
- Click Calculate to view the activity, fractions, factors, half-lives elapsed, and decay constant.
- Click Reset to restore the default values.
How it works
The calculator uses exponential radioactive decay. It converts the half-life and elapsed time to seconds, then finds the number of elapsed half-lives as t divided by T½.
For forward decay, the remaining activity is A = A0 × 2^(-t/T½). The remaining fraction is the same decay factor, and the decayed fraction is 1 minus that remaining fraction.
For back-correction, the calculator treats the entered activity as a later measured activity and divides by the remaining fraction, equivalent to multiplying by the correction factor 1 / 2^(-t/T½), to estimate the original activity at the reference time.
The calculation assumes physical decay only. It does not include biological clearance, production, shielding, detector geometry, or calibration effects, and the results are educational estimates rather than radiation safety, clinical, regulatory, or professional advice.
Example calculation
Suppose the reference activity is 10 MBq, the half-life is 6.01 hours, and the elapsed time is 4 hours in forward decay mode. The elapsed half-lives are 4 / 6.01 = 0.6656, so the remaining fraction is 2^-0.6656 = about 0.6304. The activity after 4 hours is 10 × 0.6304 = about 6.304 MBq, with about 36.96% decayed.
Frequently asked questions
What is decay correction?
Decay correction adjusts a radioactive activity measurement from one time to another using the isotope’s half-life and the elapsed time.
What is the difference between forward decay and back-correction?
Forward decay starts with an original activity and estimates the later activity. Back-correction starts with a later measured activity and estimates the earlier original activity.
Does the calculator convert between MBq, mCi, and other activity units?
No. The selected activity unit is carried through to the result; the calculator does not convert the entered activity into another activity unit.
Why is the correction factor greater than 1?
For any positive elapsed time, less activity remains later, so estimating the earlier activity requires multiplying the later measurement by a factor greater than 1.
Are isotope presets editable?
Yes. Selecting a preset fills the half-life value and unit, but you can still edit the half-life fields afterward.