Radiocarbon ages are not the same as calendar time due to past fluctuations of carbon-14 in the atmosphere. Calibration is necessary to convert radiocarbon ages into calendar equivalents (calibrated ages). For example a radiocarbon age of 4520 ± 30 BP calibrates to 3356−3101 cal BC with the IntCal13 calibration curve at two sigma (95.4 %). The two sigma range is generally preferred since there is a 95% probability that the true age of the sample falls within this range. In most publications the raw radiocarbon age should be reported as well as the calibrated age range.

Our reporting process includes an automatic attempt at calibrating your radiocarbon result with an internationally agreed calibration curve. No account will be taken of any marine component in your sample material. No attempt will be made to determine if your sample lies within the limits of the calibration curve. Thus, the automatic calibration result may contain warnings of the sample age being unsuitable for the calibration curve.

In addition to reporting radiocarbon ages we also report a quantity called F14C. Roughly speaking, this is the same as fraction modern, but with C13 fractionation and background corrections clearly implied.

The convention in reporting radiocarbon ages is that samples dating to a more recent time than 1950 AD are reported as "Greater than Modern", although their F14C is still reported. Samples whose F14C is less than the background are reported as "Greater than" the age equivalent value of the background uncertainty.

To use our on-line radiocarbon calibration program please go to There are links to additional information on radiocarbon calibration in the CALIB manual at and a downloadable version for MS Windows.

As marine samples require special consideration of regional reservoir effects in calibration, the calibrated age ranges sent with your date certificate have been calculated assuming the samples are terrestrial Northern Hemisphere samples using the IntCal13 curve. For more information on marine reservoir corrections please see our website at