Half-Life Calculator
Remaining amount after any number of half-lives.
Enter your values
ⓘ Live results
Results
- Amount remaining
- 25
- Fraction remaining
- 25%
- Half-lives elapsed
- 2
- Decay constant λ
- 0.00012097
- Mean lifetime
- 8266.6426
Working
- 1N = 100 × 0.5^(11460 ÷ 5730) = 25
Quick answer
After each half-life, half the remaining substance decays: N = N₀ × (1/2)^(t/T).
N = N₀ × (½)^(t ÷ t½)
How to use the Half-Life Calculator
- 1Enter the starting amount.
- 2Enter the half-life and elapsed time in the same units.
- 3Read the remaining amount.
Worked example
Using the values the calculator loads with:
Inputs
- Starting amount100
- Half-life5730 units of time
- Time elapsed11460 same units
Results
- Amount remaining25
- Fraction remaining25%
- Half-lives elapsed2
- Decay constant λ0.00012097
- Mean lifetime8266.6426
- N = 100 × 0.5^(11460 ÷ 5730) = 25
Frequently asked questions
What is carbon-14 dating?
C-14 has a 5,730-year half-life, so measuring how much remains in organic material dates it back roughly 50,000 years.
Does half-life change with amount?
No. Radioactive decay is first-order, so the fraction lost per unit time is constant regardless of quantity.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.