DNA Copy Number Calculator

Convert a DNA mass (ng) and template length (bp) into the number of molecules (copies) for double-stranded DNA, plasmids, and PCR amplicons.

ng
bp
g/mol/bp

Quick Facts

Method
copies = (ng x 1e-9 x 6.022e23) / (bp x 650)
Avogadro's number 6.022e23/mol; 650 g/mol is the average weight of one double-stranded base pair.

Your Results

Calculated
Copy number
-
Number of DNA molecules
Moles of DNA
-
Amount in moles
Molar mass of template
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Length x MW per bp
Copies per microliter
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If the ng amount is in 1 uL

Ready

Enter DNA mass and template length, then calculate.

About the DNA copy number calculation

DNA copy number is the count of individual DNA molecules present in a sample. You cannot weigh single molecules, so the number is derived from two things you can measure: the total mass of DNA (from a spectrophotometer or fluorometer) and the length of each molecule in base pairs (from the plasmid map, amplicon size, or genome length). The calculation converts mass to moles, then moles to molecules using Avogadro's number.

The formula

For double-stranded DNA:

copies = (mass in grams × 6.022×1023) / (length in bp × 650)

Because DNA mass is usually reported in nanograms, multiply the nanogram value by 1×10-9 to get grams first. The three constants and inputs are:

  • 6.022×1023 /mol — Avogadro's number, the count of molecules in one mole.
  • 650 g/mol per bp — the average molecular weight of one base pair of double-stranded DNA, counting both strands, the sugar-phosphate backbone, and associated sodium counterions. Some protocols use 660; the difference is under 2%.
  • length × 650 — the molar mass (g/mol) of the whole molecule. A 3,000 bp plasmid is about 1,950,000 g/mol.

Why copy number matters

Copy number, not mass, is what most molecular assays actually respond to. Quantitative PCR (qPCR and digital PCR) standard curves are built from known copy numbers. Library preparation for next-generation sequencing, gene-cloning ligations, and any experiment that mixes molecules in a defined ratio (such as an insert-to-vector molar ratio) all depend on knowing how many molecules you have, independent of their size.

Single-stranded DNA and RNA

The 650 g/mol constant is specific to double-stranded DNA. For single-stranded DNA use roughly 330 g/mol per nucleotide, and for RNA use about 320–340 g/mol per nucleotide, replacing "bp" with "nt" (number of bases) in the denominator.

Reference points

  • 1 ng of a 1,000 bp dsDNA fragment ≈ 9.3×108 copies.
  • 1 ng of a 3,000 bp plasmid ≈ 3.1×108 copies.
  • 1 ng of the ~4.6 Mbp E. coli genome ≈ 2.0×105 copies.
  • 1 fg (10-6 ng) of a 5,000 bp target ≈ 185 copies — a realistic low-end qPCR standard.

Frequently Asked Questions

What formula converts DNA mass to copy number?
Copies = (mass in grams × 6.022×1023) / (length in bp × 650). Enter mass in nanograms and the tool multiplies by 1×10-9 to convert to grams. The 650 g/mol value is the average molecular weight of one double-stranded base pair, and 6.022×1023 is Avogadro's number.
Should I use 650 or 660 g/mol per base pair?
Both are widely cited average molecular weights for a double-stranded base pair. 650 is the most common convention (used by NEB and many qPCR protocols); 660 appears in some references. The two differ by about 1.5%, well below typical DNA quantification error, so either is acceptable as long as you are consistent. This tool defaults to 650 but lets you override it.
How do I get copies per microliter for a qPCR standard?
Compute copies for the mass contained in one microliter of your stock. If your stock is measured in ng/uL, enter that ng value directly — the result is then copies per microliter. To build a dilution series, divide by 10 for each ten-fold dilution step.
Does this work for single-stranded DNA or RNA?
Not with the default constant. 650 g/mol assumes two strands per base pair. For single-stranded DNA, change the molecular weight to about 330 g/mol per nucleotide and enter the length in nucleotides; for RNA use roughly 320–340 g/mol per nucleotide.

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