BioDeviceHub

DNA Copy Number Calculator

Convert a mass of DNA or RNA to the number of molecules it contains, or the reverse, from the template's length. Presets for pUC19, lambda, E. coli and the human genome.

Formula

copies=m×NAL×wˉbase\text{copies} = \dfrac{m\times N_A}{L \times \bar{w}_{\text{base}}}
m=copies×L×wˉbaseNAm = \dfrac{\text{copies}\times L\times \bar{w}_{\text{base}}}{N_A}
NA=6.02214076×1023 mol−1N_A = 6.02214076\times 10^{23}\ \mathrm{mol^{-1}}
NAN_A
the Avogadro constant, exact since the 2019 redefinition of the SI
LL
base pairs (double-stranded) or nucleotides (single-stranded)
wˉbase\bar{w}_{\text{base}}
650 g/mol per bp of double-stranded DNA, 330 per nt of single-stranded DNA, 340 per nt of RNA

How it works

The number of molecules in a sample is its amount in moles times the Avogadro constant, and the amount in moles is the mass divided by the molecular weight. For a long template the molecular weight is the length times the average weight of a base pair.

This is the calculation behind standard curves for absolute qPCR and digital PCR, viral genome copies, and estimates of how many genome equivalents a nanogram of DNA represents. A nanogram of human DNA is only a few hundred haploid genomes, which is why low-input samples are noisy.

Worked example

How many copies of a 5,000 bp plasmid are in 1 ng?

  1. MW = 5,000 × 650 = 3,250,000 g/mol.
  2. Moles = 1 × 10⁻⁹ g / 3,250,000 g/mol = 3.077 × 10⁻¹⁶ mol.
  3. Copies = 3.077 × 10⁻¹⁶ × 6.022 × 10²³.

1.85 × 10⁸ copies, roughly 185 million plasmid molecules per nanogram.

These are the values the calculator opens with, so you can check its output against this example.

Assumptions

  • The preparation is a single species of the stated length. Nicked, linear and supercoiled forms of a plasmid are counted alike.
  • The mass is of the nucleic acid alone, without salts or carrier.
  • For a genome, the length is the haploid size and the copies are genome equivalents, not cells.

Common mistakes

  • Using the plasmid's length when the standard is a linearised or amplified fragment of a different size.
  • Counting copies of a double-stranded molecule when a single-stranded standard is used, or the reverse.
  • Trusting a concentration measured by absorbance for a standard, when the absorbance is partly from contaminants.

Related tools

DNA and RNA Molarity CalculatorConvert ng/µL to nM, pmol per µg and molecules per µLOpen calculatorDNA and RNA Concentration Calculator (A260)Nucleic acid concentration and purity from absorbanceOpen calculatorqPCR ΔΔCt CalculatorRelative gene expression by the 2^−ΔΔCt methodOpen calculatorPCR Master Mix CalculatorComponent volumes for any number of reactionsOpen calculator

Related equipment

Service documentation, failure modes and parts for the instruments this calculation is used with.