Nucleic acids · Molar amounts
DNA copy number calculator.
Convert dsDNA mass and length to pmol and copies, or a copy number back to mass.
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How to convert DNA mass to copy number
The molar mass of dsDNA is about 660 g/mol per base pair, so a molecule of N bp weighs N x 660 g/mol. Then moles = mass / (N x 660) and copies = moles x 6.022 x 1023 (Avogadro's constant, exact since 2019). Promega's BioMath writes the same relation as mass in pg divided by 660 pg/pmol per base pair.
Worked example. 10 ng of a 3,000 bp plasmid: molar mass 1,980,000 g/mol, so 10 x 10-9 / 1,980,000 = 5.05 x 10-15 mol = 5.05 fmol = 3.04 x 109 copies. A useful anchor is that 1 ug of a 1 kb fragment is about 1.52 pmol.
This is the standard way to set standard-curve copy numbers for qPCR or to match molar amounts for cloning. For insert and vector amounts see the ligation ratio calculator.
Frequently asked questions
What mass per base pair should I use?
660 g/mol per base pair is the usual average for dsDNA. Some references use 650, and Thermo's molecular-weight table uses about 607 per base pair (sodium salt), which gives 1.65 pmol per ug of 1 kb instead of 1.52. Pick one and be consistent; the box is editable.
How many copies in 1 ng of DNA?
It depends on length. For 1 kb dsDNA, 1 ng is about 1.5 fmol, or about 9.1 x 10^8 copies.
Does this work for single-stranded DNA or RNA?
Not directly. Single-stranded DNA and RNA have about half the mass per nucleotide of a base pair, so change the mass-per-unit box and the length accordingly.
What is Avogadro's constant?
6.022 140 76 x 10^23 per mole, an exact defined value (NIST CODATA).
Sources
Checked
- Promega BioMath, DNA mass to pmol (660 pg per pmol of base pairs)
- NIST CODATA, Avogadro constant (6.022 140 76 x 10^23 per mol, exact)
- Thermo Fisher, DNA and RNA molecular weights and conversions (1 A260 dsDNA = 50, ssRNA = 40, ssDNA = 37 ug/mL)
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Research use and scope
This is arithmetic for research-bench planning, not clinical, diagnostic or safety advice. Conversion factors are standard approximations; strain, instrument, reagent and protocol differences matter. Check results against your own protocol, kit manual and lab supervisor, and follow your institution's biosafety rules.