About this calculator
Find the amount remaining, the starting amount, the time or the half-life itself.
How to use it
- Choose what to find, enter the other values with their units, and read the answer in every common unit.
The formula
N = N₀ × (½)t ÷ t½; t = t½ × log₂(N₀ ÷ N); λ = ln 2 ÷ t½
- N₀
- starting amount
- N
- amount left
- t½
- half-life
Worked example
Carbon-14 (half-life 5,730 years) after 11,460 years
- Two half-lives have passed.
- 25% remains: 100 → 25.
What the result means
After 10 half-lives less than 0.1% remains.
Assumptions
- First-order decay.
Limitations
- Carbon dating needs calibration against tree rings for accurate ages.
Frequently asked questions
What is half-life?
The time for half the amount to decay.
How does carbon dating work?
Living things hold a steady level of carbon-14, which decays after death with a half-life of 5,730 years.
What is the decay constant?
λ = ln 2 ÷ half-life.
Last reviewed on 6 October 2026. Found a mistake? Tell us.