Several aspects of radiocarbon measurement have built-in uncertainties. Every laboratory must factor out background radiation that varies geographically and through time. The variation in background radiation is monitered by routinely measuring standards such as anthracite (coal), oxalic acid, and certain materials of well-known age. The standards offer a basis for interpreting the radioactivity of the unknown sample, but there is always a degree of uncertainty in any measurement. Since decay-counting records random events per unit time, uncertainty is an inherent aspect of the method.

Most laboratories express the uncertainty at one standard deviation (± 1 sigma), meaning that there is a probability of about 67% that the true age of the sample falls within the stated range, say ± 100 years. Most laboratories consider only the counting statistics, i.e., the activity of the sample, the standards, and the background, when establishing the 1-sigma limits. However, some laboratories factor in other variables such as the uncertainty in the measurement of the half-life. Two laboratories, the Geological Survey of Canada and the University of Waterloo, follow an unconventional practice by reporting 2-sigma errors, implying a probability of about 95% that the true age of the sample falls within the stated range. Some laboratories impose a minimum value on their error terms.