Last updated: February 2, 2026
Radiocarbon dating determines the age of organic materials up to ~50,000 years old by measuring their carbon-14 content. Willard Libby received the 1960 Nobel Prize in Chemistry for developing this method at the University of Chicago in 1947. The American Chemical Society designated it a National Historic Chemical Landmark in 2016.
Radiocarbon dating measures how much carbon-14 remains in dead organic material to calculate when it died.
Carbon-14 is a radioactive isotope produced continuously in the upper atmosphere when cosmic ray neutrons collide with nitrogen-14 atoms. This C-14 mixes with atmospheric CO₂ and enters all living organisms through photosynthesis and the food chain. While alive, organisms maintain equilibrium with atmospheric C-14 levels. When they die, intake stops and the C-14 begins to decay at a known rate.
Carbon-14 has a half-life of 5,730 ± 40 years. After 5,730 years, half the original C-14 remains. After another 5,730 years, one quarter remains. By measuring the remaining C-14, scientists calculate elapsed time since death.
Radiocarbon dating works on any material that was once alive:
There are two main ways to measure carbon-14 in samples:
AMS counts carbon-14 atoms directly by using a particle accelerator. This is the modern, more accurate method that needs only tiny samples (smaller than a grain of rice).
Accuracy: Within +/-30-50 years for samples less than 10,000 years old.
Sample size: 0.5-1 mg (very small)
LSC waits for carbon-14 atoms to decay and counts the radiation they give off. This older method is still used but requires larger samples and takes longer.
Accuracy: Within +/-100-200 years for samples less than 10,000 years old.
Sample size: 1-5 grams (much larger)
| Feature | AMS (Modern Method) | LSC (Older Method) |
|---|---|---|
| Sample size needed | 0.5-1 mg | 1-5 g |
| Accuracy | +/-30-50 years | +/-100-200 years |
| Testing time | 30-40 minutes | 1-3 days |
| Cost per test | $400-1000 | $200-400 |
| Maximum age | ~50,000 years | ~35,000 years |
Raw carbon-14 measurements don't directly give calendar dates. Scientists must convert them using calibration curves based on materials of known age.
The amount of carbon-14 in the atmosphere hasn't always been exactly the same throughout history. It varies slightly due to changes in cosmic ray intensity and other factors. To get accurate calendar dates, scientists compare radiocarbon measurements to materials they can date independently.
Scientists have built calibration curves using materials with known ages:
IntCal20 - The International Standard
The IntCal20 calibration curve combines data from tree rings, corals, lake sediments, and cave formations to provide the most accurate conversion from radiocarbon years to calendar years, extending back 55,000 years.
Calibration has become much more precise over the years. The most recent calibration (IntCal20) is significantly better than older versions:
| Sample Age | Old Calibration (2004) | Current Calibration (2020) | Improvement |
|---|---|---|---|
| 2,000 years ago | +/-80 years | +/-36 years | 55% more precise |
| 10,000 years ago | +/-120 years | +/-76 years | 37% more precise |
| 30,000 years ago | +/-450 years | +/-215 years | 52% more precise |
| 45,000 years ago | +/-1500 years | +/-650 years | 57% more precise |
Some events provide exact yearly markers for calibration. For example, cosmic ray bursts in AD 774-775 and 993-994 caused sudden increases in carbon-14 that can be seen in tree rings worldwide (Miyake et al. 2012, Nature). Nuclear weapons testing in the mid-20th century also created a distinctive "bomb pulse" that serves as a precise modern marker (Hua et al. 2013, Radiocarbon).
Radiocarbon dating has been tested against other independent dating methods many times. The results show consistent agreement between different methods.
Tree rings can be counted to give exact ages going back thousands of years. When scientists radiocarbon date wood samples and compare them to tree ring counts, they match within +/-20 years for samples up to 11,000 years old. The International Tree-Ring Database contains over 4,000 tree ring sequences from 6 continents that confirm radiocarbon accuracy.
This completely different dating method uses the radioactive decay of uranium to thorium. When applied to cave formations and corals, it gives ages that consistently agree with calibrated radiocarbon dates over tens of thousands of years (Cheng et al. 2013, Science).
Some lakes deposit annual layers of sediment that can be counted like tree rings. The Cariaco Basin off Venezuela has a 15,000-year sequence that matches radiocarbon dates within +/-50 years. Lake Suigetsu in Japan has a continuous 52,800-year record that closely matches calibrated radiocarbon ages (Bronk Ramsey et al. 2012, Science).
Annual ice layers in Greenland and Antarctica can be counted back tens of thousands of years. Organic material trapped in these ice cores can be radiocarbon dated, and the results agree with the ice layer counts within +/-100 years over 40,000 years (see NOAA Paleoclimatology Ice Core Data).
Objects from recorded history provide tests of radiocarbon dating:
The Shroud of Turin
In 1988, three independent laboratories (University of Arizona, Oxford University, and ETH Zurich) tested samples from the Shroud of Turin. All three labs got consistent results showing the cloth was made between 1260-1390 CE, not 2,000 years ago as some claimed. This matched historical records showing the shroud first appeared in the 1350s (Damon et al. 1989, Nature).
Otzi the Iceman
When this mummified body was found in the Alps in 1991, radiocarbon dating showed he died 5,200 +/- 40 years ago. This age was confirmed by:
- Tree ring dating of his wooden axe handle
- Archaeological analysis of his copper axe and stone tools
- Comparison with similar artifacts from the Copper Age
- DNA analysis showing genetic markers from Neolithic Europeans
Chauvet Cave Paintings
When this French cave was discovered, art experts thought the paintings were 15,000-17,000 years old based on their style. However, radiocarbon dating of charcoal from the torches and paints gave dates of 30,000-32,000 years - much older than expected. These dates were confirmed by:
- Uranium-thorium dating of mineral deposits over the paintings
- Thermoluminescence dating of heated sediments on the cave floor
- Light-exposure dating of sediment layers
This case demonstrated that radiocarbon dating can yield results that differ from those predicted by other methods.
Radiocarbon laboratories worldwide regularly test identical samples to make sure they're getting consistent results. These blind tests show that labs generally agree very well:
| Testing Program | Year | Number of Labs | Labs That Agreed |
|---|---|---|---|
| SIRI | 2013 | 72 | 95% |
| VIRI | 2007-2010 | 69 | 93% |
| FIRI | 1999-2001 | 92 | 90% |
| TIRI | 1995 | 74 | 88% |
Modern radiocarbon labs follow strict procedures to ensure accuracy:
These procedures ensure labs maintain accuracy within +/-20-30 years for samples less than 5,000 years old and +/-100-400 years for samples approaching 40,000 years old.
Radiocarbon dating has limitations and potential complications that scientists account for:
Radiocarbon dating only works up to about 50,000 years. After that, too little carbon-14 remains to measure accurately. For older samples, scientists use other dating methods like potassium-argon or uranium-series dating.
If modern or ancient carbon contaminates a sample, it can make the results wrong. Labs use careful cleaning procedures to remove contamination:
Marine organisms and those living in certain lakes can appear older than they really are because they incorporate "old carbon" from the water. Scientists have studied these effects and developed corrections (Marine20 calibration):
What an organism ate can affect its radiocarbon age, especially if it consumed marine or freshwater organisms. Scientists routinely test for this using chemical analysis of carbon and nitrogen isotopes, then apply corrections as needed.
Several objections to radiocarbon dating appear frequently. Here are the claims and scientific responses:
Trace amounts of C-14 detected in ancient carbon (coal, diamonds, graphite) are below the method's detection limit. At these levels, background radiation and instrument noise produce apparent signals indistinguishable from actual C-14. This is why the method has a ~50,000 year upper limit—not evidence that ancient materials are young.
This is correct, and scientists have known this since the 1960s. Atmospheric C-14 varies with solar activity, Earth's magnetic field strength, and ocean circulation. This is precisely why calibration curves exist. Tree-ring calibration revealed these variations and provides corrections. The IntCal curves incorporate these fluctuations back to 55,000 years.
Radioactive decay rates are governed by nuclear physics and are independent of temperature, pressure, or chemical environment. Decay constants have been measured under extreme conditions (high pressure, extreme temperatures) with no measurable variation. The same physics that enables nuclear power and medical imaging underlies radiocarbon decay.
Some living mollusks and aquatic plants show apparent ages of hundreds or thousands of years due to the reservoir effect—they absorb "old" carbon from water. This is a known phenomenon with established corrections (Marine20). Terrestrial organisms in equilibrium with atmospheric CO₂ do not show this effect.
Radiocarbon dating has been tested and refined for over 75 years:
The convergence of radiocarbon dates with multiple independent methods—each based on different physical principles—provides the primary evidence for the method's reliability.