Chemistry

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Half-Life Calculator.

Calculate half-life and elapsed age from a decay constant and remaining fraction.

On-device calculationNo signup
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Set your values

Results update as you type.

halfLifeYears: 5,776.226505

halfLifeYears

5,776.226505
elapsedYears: 5,776.226505

elapsedYears

5,776.226505

Educational chemistry arithmetic only. Keep units consistent and verify assumptions against your laboratory or course specification.

Use Cases

Radioactive decay estimation

Estimate the half-life and age of a radioactive sample when you know its decay constant and how much remains relative to the original amount.

Example: If decay constant is 0.05 1/year and remaining fraction is 0.5, half-life is about 13.86 years.

Academic and lab calculations

Quickly compute half-life and elapsed time for chemistry or physics problems involving exponential decay, without manual log calculations.

Example: For a decay constant of 0.1 1/year and remaining fraction 0.25, elapsed age is about 13.86 years.

Frequently Asked Questions

What does the half-life calculator do?
This calculator computes the half-life and the elapsed age (time since the start) of a substance based on the decay constant (in 1/year) and the remaining fraction (as a fraction between 0 and 1). It uses the relationship between these quantities to provide quick estimates.
How is half-life calculated from decay constant?
Half-life (t½) is calculated using the formula t½ = ln(2) / λ, where λ is the decay constant. The calculator uses the decay constant you input (in 1/year) to compute the half-life in years.
What is the elapsed age?
Elapsed age is the time that has passed since the substance had its original amount, given the remaining fraction. It is calculated using the decay equation: t = -ln(remaining fraction) / λ. The result is in years.

Tips & Common Mistakes

Tips

  • Ensure the decay constant is in units of 1/year to get results in years.
  • The remaining fraction should be between 0 and 1 (e.g., 0.5 for 50% remaining).
  • For a remaining fraction of 0.5, the elapsed age equals the half-life.
  • Use the calculator for educational or reference purposes; for precise applications, consult a professional.

Common Mistakes to Avoid

  • Entering the remaining fraction as a percentage (e.g., 50) instead of a decimal (0.5).
  • Using a decay constant with incorrect units (e.g., 1/day) without converting to 1/year.
  • Assuming the elapsed age is the same as the half-life unless the remaining fraction is exactly 0.5.

Last updated: August 13, 2026