Atomic mass
The atomic mass or relative isotopic mass refers to the mass of a single particle, and therefore is tied to a certain specific isotope of an element. The dimensionless standard atomic weight instead refers to the average (mathematical mean) of atomic mass values of a typical naturally-occurring mixture of isotopes for a sample of an element. Atomic mass values are thus commonly reported to many more significant figures than atomic weights. Standard atomic weight is related to atomic mass by the abundance ranking of isotopes for each element. It is usually "about" the same value as the atomic mass of the most abundant isotope, other than what looks like (but is not actually) a rounding difference.
C-value
The relative molecular mass may be converted to an absolute value by multiplying it by the atomic mass unit (1 u) in picograms. Thus, 615.8771 is multiplied by 1.660539 × 10 pg. Consequently, the mean mass per nucleotide pair would be 1.023 × 10 pg, and 1 pg of DNA would represent 0.978 × 10 base pairs (978 Mbp).