Three isotopes of carbon are found in nature; carbon-12, carbon-13 and carbon-14.
Carbon-12 accounts for ~99.8 % of all carbon atoms, carbon-13 accounts for ~1% of carbon atoms while ~1 in every 1 billion carbon atoms is carbon-14.
The radiocarbon age is determined by the equation where -8033 represents the mean lifetime of 14C (Stuiver and Polach, 1977), Asn is the activity in counts per minute of the sample and Aon is the counts per minute of the modern standard.
A variant of this equation is also used when the samples are analysed by AMS.
This method needs less than 1 mg of carbon and directly measures the abundance of the individual ions of carbon (14C, 12C and 13C).