Absolute quantification by qPCR depends on comparing an unknown sample against a standard curve of known molecule counts, not just known mass. Converting a DNA fragment's mass into its actual copy number — and then into a dilution series — is the calculation that bridges a spectrophotometer or Qubit reading to a usable standard curve.

Why copy number matters more than mass alone

Two DNA samples with the same mass can contain very different numbers of molecules if their fragment lengths differ — a short 200 bp fragment has far more molecules per nanogram than a 10,000 bp fragment of the same mass, since each individual molecule weighs less. For qPCR standard curves, what matters is the actual molecule count (copy number), because Ct values track how many target copies are present, not how many nanograms. This calculator converts mass and length into copy number using Avogadro's number, the same constant used for any mole-to-molecule conversion in chemistry.

Why dsDNA and ssDNA use different average masses

Double-stranded DNA length is measured in base pairs, where each unit already accounts for both complementary strands, so the standard approximation of 660 g/mol per base pair reflects the combined mass of both strands. Single-stranded DNA length is measured per individual base, so its average mass (330 g/mol per base) is roughly half that of a base pair — it's the mass of just one strand's worth of nucleotide. Using the wrong strandedness setting for your sample type will produce a copy number that's off by roughly a factor of two.

Building a serial dilution series for a standard curve

A qPCR standard curve is built by amplifying several known-concentration dilutions of a reference template — often a linearized plasmid or PCR product — and plotting their Ct values against log(copy number). A 10-fold serial dilution across 6–8 points is the conventional choice, since it spans several orders of magnitude while keeping the number of reactions manageable. Once you know the neat concentration in copies/µL, each subsequent standard is simply the previous one diluted by the same factor, which is exactly what the qPCR Standard tab calculates.