Chemistry

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Radioactive Decay Calculator.

Calculate remaining and decayed amount from half-life and elapsed time.

On-device calculationNo signup
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Set your values

Results update as you type.

Remaining amount: 25 amount

Remaining amount

25.000000amount
Decayed amount: 75 amount

Decayed amount

75.000000amount

Educational formula calculator. Keep units consistent and verify assumptions against your laboratory or course specification.

Use Cases

Determine remaining isotope in a sample

Useful for researchers or students to find how much of a radioactive substance remains after a certain period, given its half-life.

Example: If you start with 100 grams of a substance with a half-life of 5 years, after 10 years, 25 grams remain.

Estimate decayed amount for safety or disposal

Helps in planning storage or disposal by calculating how much radioactive material has decayed over time.

Example: Calculate the decayed amount of a medical isotope after a few hours to ensure safe handling.

Frequently Asked Questions

How does the radioactive decay calculator work?
It uses the exponential decay formula: remaining amount = initial amount * (1/2)^(elapsed time / half-life). You input the initial amount, elapsed time, and half-life, and it computes the remaining and decayed amounts.
What units can I use for time and amount?
The calculator accepts any consistent units for time (e.g., seconds, years) and any unit for amount (e.g., grams, moles). Ensure that elapsed time and half-life are in the same time unit for accurate results.
Can I use this calculator for any radioactive isotope?
Yes, as long as you know the half-life of the isotope. The calculator is generic and works for any substance that undergoes exponential decay, including carbon-14, uranium-238, and medical isotopes.

Tips & Common Mistakes

Tips

  • Ensure elapsed time and half-life are in the same unit (e.g., both in years) to avoid errors.
  • For very long or short times, use scientific notation to maintain precision.
  • Double-check your initial amount unit; the result will be in the same unit.
  • Remember that the decayed amount is simply the initial amount minus the remaining amount.

Common Mistakes to Avoid

  • Using different time units for elapsed time and half-life without converting.
  • Forgetting that the decayed amount is not the same as the remaining amount; it's the difference.
  • Assuming the calculator accounts for decay chains or multiple isotopes; it only handles single exponential decay.

Last updated: August 13, 2026