Radiocarbon dating estimates the age of once-living material by measuring how much carbon-14 it still contains and comparing that measurement with a calibration curve built from independently dated samples. It can place wood, seeds, charcoal, bone and shell on a calendar timeline, but only inside a known range and only when the sample is clean.
How the carbon-14 clock starts
Carbon-14 forms high in the atmosphere, where cosmic rays knock neutrons out of atoms and those neutrons combine with nitrogen-14. The resulting carbon-14 joins with oxygen to form carbon dioxide, which plants take up during photosynthesis. Animals then take it in through food. While an organism is alive, its carbon-14 level keeps tracking the atmosphere around it.
At death that exchange stops. Carbon-14 decays into nitrogen-14 with a half-life of about 5,730 years, so half of the remaining atoms go in that span and half of what is left goes in the next, as the nist.gov explainer sets out. The shrinking ratio of carbon-14 to stable carbon is the measurement.
Why a raw radiocarbon age is not a calendar date
Atmospheric carbon-14 has never been constant. Production shifts with solar activity and the geomagnetic field, and fossil fuel burning plus atmospheric nuclear testing changed the modern baseline. A raw laboratory figure therefore has to be calibrated.
Calibration matches the sample against material of known age, mainly tree rings counted year by year, along with corals and other archives dated by independent methods. The IntCal20 curve for the Northern Hemisphere reaches back roughly 55,000 years, and the calibration step is what turns a measurement into a calendar range, as the c14.arch.ox.ac.uk guide explains.
This is also why results are reported as ranges with probabilities rather than single years. Parts of the curve are flat, so more than one calendar period can fit the same measurement.
Where the method stops working
The practical ceiling is usually given as around 60,000 years. Beyond that too little carbon-14 survives to be separated from background radiation, notes the uchicago.edu explainer. Material that never took up atmospheric carbon, such as stone, metal, glass or fired pottery, cannot be dated this way at all. In those cases archaeologists date associated organic material instead.
Contamination is the more common problem. A small amount of modern carbon, from handling, conservation treatments or fine roots in a sediment sample, makes old material look younger. Laboratories therefore clean and pretreat samples, and often measure more than one fraction to test whether the results agree. Small or badly degraded samples carry wider uncertainty.
Environment matters as well. Marine organisms and freshwater species in limestone-rich waters can take up older dissolved carbon and return ages that are too old unless a local reservoir correction is applied. The correction depends on the region, which is why the same material type can behave differently in two places.
What a radiocarbon date can and cannot settle
| Material | Usually datable | Main caveat |
|---|---|---|
| Wood, charcoal, seeds | Yes | Old or reused timber, contamination |
| Bone, antler, teeth | Yes, if collagen survives | Conservation glues and consolidants |
| Shell, coral | Yes, with a local correction | Reservoir and hard-water offsets |
| Stone, metal, pottery | No | Date nearby organic remains instead |
| Bulk sediment | Sometimes | Mixed ages, roots, washed-in old carbon |
For archaeology the useful question is often not only how old something is, but whether it is older or younger than something else. A date on a short-lived sample such as a single seed is far more informative than one on a long-lived timber that could have been stored or reused for decades.
A radiocarbon result also dates death, not use. A wooden beam can be centuries older than the building it supports, and a bone can be older than the grave it was placed in. That is why dates are usually combined with stratigraphy and artefact sequences in a statistical model rather than read on their own.
The bottom line
Radiocarbon dating is a clock that starts at death, a calibration curve that corrects for a changing atmosphere, and a set of sample rules that decide whether the number means anything. On clean, short-lived, terrestrial material inside its range, it is one of the most dependable tools in archaeology. Outside those conditions, it can produce confident-looking dates that are badly wrong.