All D14C values are normalized to the base value of -25.0 per mille with respect to the standard carbonate (VPDB).
D14C is calculated using: Figure 1: Decay curve for C14 showing the activity at one half-life (t/2).
The terms "%Modern", or "pm C" and D14C are shown related in this diagram along with the Radiocarbon age in years BP (Before 1950 AD).
A time-independent level of C14 activity for the past is assumed in the measurement of a CRA.
The activity of this hypothetical level of C14 activity is equal to the activity of the absolute international radiocarbon standard.
Beukens (1994) for instance has stated that this means the limit of the range for his Isotrace laboratory is 60 000 yr which is very similar to the conventional range.
Figure 1: This gif shows the comparison in radioactivity between a sample, or unknown (green area) , a modern standard (dark blue) and a background (small red peaks) derived from beta decay. A radiocarbon measurement, termed a conventional radiocarbon age (or CRA) is obtained using a set of parameters outlined by Stuiver and Polach (1977), in the journal Radiocarbon.
If a sample age falls after 1950, it is termed greater than Modern, or Where Aabs is the absolute international standard activity, 1/8267 is the lifetime based on the new half life (5730 yr), Y = the year of measurement of the appropriate standard.