After 50,000 years, only 1 carbon-14 atom in 423 is left. So how do labs date anything?
Every living thing carries a tiny, steady dose of radioactive carbon. At death the intake stops and the clock starts. First guess how much is left in five famous finds. Then run the clock yourself on 400 atoms and see where it stops working.
5,730 yrhalf-life of carbon-14 ("about 5,730 years", Wikipedia)
0.24%left at the 50,000 year limit (computed from the half-life)
Part 1 / Guess first
How much carbon-14 is still in each find?
Set each slider to the percent of the original carbon-14 you think remains. Ages are the midpoint of the range in the source, counted back from 2026.
Line: fraction left = 0.5 to the power (age / 5,730). Teal rings are your guesses, filled dots are the true values.
Part 2 / Run the clock
400 atoms. Drag time forward.
Each atom gets its own random decay moment, so the sample behaves like a real one: the average follows the curve, the count wobbles.
0 yr
left 400 / 400expected 400.0half-lives 0.0
Part 3 / Date a mystery sample
Read the age off the atoms.
A hidden age is drawn. You see only how many of 400 atoms survive. Set your age estimate, then reveal.
atoms left ?
2,000 yr
What to take away
Carbon-14 is a clock with a fading face. Young samples keep most of their atoms, so a count pins the age down well. Beyond roughly 40,000 years only a handful remain, one extra or missing atom shifts the answer by thousands of years, and the signal drowns in contamination. That is why the textbook limit sits near 50,000 years.
Caveat: this page uses plain decay maths. Real labs convert to calendar ages with calibration curves (IntCal), which correct for changes in atmospheric carbon-14.
Sources
Half-life (about 5,730 years), 50,000 year limit, Libby's test on tombs dated 2625 BC: Wikipedia, Radiocarbon dating, page revision of 7 Oct 2026, fetched 25 Oct 2026.
Dead Sea Scrolls, 3rd century BCE to 1st century CE (midpoint used: 75 BCE): Wikipedia, Dead Sea Scrolls, rev. 23 Oct 2026.
Stonehenge sarsen circle, 2600 to 2400 BC, and the skeleton dated 600 to 690 AD: Wikipedia, Stonehenge, fetched 25 Oct 2026.
Lascaux paintings, 17,000 to 22,000 years: Wikipedia, Lascaux, rev. 20 Oct 2026.
Percent remaining, atom counts and the 1 in 423 figure are computed here from the half-life, not taken from a source.