It is calculated on the assumption that the atmospheric radiocarbon concentration has always been the same as it was in 1950 and that the half-life of radiocarbon is 5568 years.
For this purpose `present' refers to 1950 so you do not have to know the year in which the measurement was made.
Once calibrated a radiocarbon date should be expressed in terms of cal BC, cal AD or cal BP.
The cal prefix indicates that the dates are the result of radiocarbon calibration using tree ring data.
To extend this method further we must use the fact that tree ring widths vary from year to year with changing weather patterns.
By using these widths, it is possible to compare the tree rings in a dead tree to those in a tree that is still growing in the same region.
See also ORAU's Explanation of Radiocarbon Results.
This method will tell you the years in which the radiocarbon concentration of tree rings is within two standard deviations of your measurement (e.g.
By using dead trees of different but overlapping ages, you can build up a library of tree rings of different calendar ages.
This has now been done for Bristlecone Pines in the U. A and waterlogged Oaks in Ireland and Germany, and Kauri in New Zealand to provide records extending back over the last 14,000 years.
between 2940BP and 3060BP for the measurement 3000 -30BP).
A slightly different method is now more often used which is called the `probability method'.