Half Life Calculator
Find the remaining quantity of a radioactive substance after decay, or calculate how long it takes to reach a given amount.
Half-Life Calculator
Any unit: grams, moles, atoms, %, Bq…
Must use the same unit as the half-life above.
Must be less than the initial quantity.
Common Isotopes
Enter values and click Calculate to see results.
Remaining Quantity
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Decay Constant (λ)
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Half-Lives Elapsed
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Fraction Remaining
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Decay Table
| Half-Lives (n) | Elapsed Time | Remaining Qty | % Remaining |
|---|
Summary
Find the remaining quantity of a radioactive substance after decay, or calculate how long it takes to reach a given amount.
How it works
- Choose a calculation mode: remaining quantity, elapsed time, or number of half-lives.
- Enter the initial quantity and the half-life period (with units).
- For remaining quantity mode, enter the elapsed time.
- For elapsed time mode, enter the final remaining quantity.
- Click Calculate to see the result, decay constant, and a decay table.
Use cases
- Determine how much Carbon-14 remains in an archaeological sample.
- Calculate residual radioactivity of a medical isotope (e.g., Iodine-131).
- Verify nuclear waste decay timelines for safety planning.
- Solve physics and chemistry homework problems on radioactive decay.
- Understand exponential decay behavior in classroom demonstrations.
- Check how many half-lives have elapsed between two measured quantities.
- Compute the decay constant for a given isotope.
- Estimate when a radioactive source falls below a safe activity threshold.
Frequently Asked Questions
Last updated: 2026-06-13 ·
Reviewed by Nham Vu