What decay law does this calculator use?

It uses the standard exponential decay formula: A(t) = A₀ × 2^(−t / t½), where A₀ is initial activity, t is elapsed time, and t½ is the half-life. This is mathematically equivalent to A(t) = A₀ × e^(−λt) where λ = ln(2) / t½.

What units of activity are supported?

You can enter activity in Becquerels (Bq), kiloBecquerels (kBq), MegaBecquerels (MBq), GigaBecquerels (GBq), or Curies (Ci). The result is displayed in the same unit you selected.

How accurate are the preset half-lives?

Preset half-lives are sourced from the IAEA and NNDC evaluated nuclear data and are accurate to at least 4 significant figures for the listed isotopes. For research-grade work, always verify against the latest evaluated data files.

Can I use this for secular equilibrium or decay chain calculations?

This tool covers single-step decay only. For decay chains with multiple daughter products, a dedicated chain ingrowth calculator is needed.

What does "fraction remaining" mean?

Fraction remaining is A(t) / A₀, expressed as a decimal between 0 and 1, or equivalently as a percentage. After one half-life it is 0.5 (50%), after two half-lives it is 0.25 (25%), and so on.

Radioactive Decay Calculator

Name: Radioactive Decay Calculator
Availability: InStock
Author: Nham Vu

Enter an isotope, initial activity, and elapsed time to calculate remaining activity, fraction remaining, and number of half-lives elapsed.

Decay Parameters

Isotope / Half-Life Preset

Initial Activity (A₀)

Elapsed Time

Decay Results

Enter parameters and click Calculate to see results.

Results copied to clipboard

Summary

Enter an isotope, initial activity, and elapsed time to calculate remaining activity, fraction remaining, and number of half-lives elapsed.

How it works

Select a common isotope from the preset list or choose "Custom" to enter any half-life.
Enter the initial activity (in Bq, kBq, MBq, GBq, or Ci) and elapsed time (in seconds, minutes, hours, days, or years).
The calculator applies N(t) = N₀ × 2^(−t / t½) to compute the remaining activity.
Results show remaining activity (in the same unit), the fraction and percentage remaining, and the number of half-lives elapsed.
Use the "Copy Results" button to copy all values to your clipboard.

Use cases

Verify radioisotope activity at time of use for medical physics and PET imaging.
Plan safe handling and disposal schedules for radioactive waste.
Calculate decay corrections for calibration sources in radiation detection.
Support nuclear engineering coursework and exam preparation.
Estimate remaining activity of industrial isotopes (e.g., Am-241 in smoke detectors).
Cross-check decay chains in nuclear fuel cycle analysis.

Frequently Asked Questions

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Last updated: 2026-05-23 · Reviewed by Nham Vu