DNA Copy Number Calculator
How many copies of a DNA molecule are in a given mass — the number qPCR standards, plasmid dilutions and library preps are built on.
A base pair of double-stranded DNA weighs about 660 g/mol (330 for a single-stranded base), so a fragment of L base pairs has a molar mass of 660 × L.
How the dna copy number calculator works
A base pair of double-stranded DNA weighs about 660 g/mol (330 for a single-stranded base), so a fragment of L base pairs has a molar mass of 660 × L. Divide the mass by that for moles, multiply by Avogadro’s number for copies. One nanogram of a 3 kb plasmid is about 300 million copies.
Formula: copies = mass (g) × 6.022 × 10²³ ÷ (length × 660)
Worked examples
| Inputs | Copies | Note |
|---|---|---|
| 1 ng of a 3 kb plasmid | 304,148,523 | 3.04 × 10⁸ copies |
| 10 ng of a 200 bp amplicon | 45,622,278,485 | 4.56 × 10¹⁰ |
| 0.5 ng of a 60 nt oligo | 15,207,426,162 | 1.52 × 10¹⁰ |
FAQFrequently asked questions
Why 660 g/mol per base pair?
The average molar mass of a nucleotide pair including the sugar-phosphate backbone, less the water lost in polymerization; 330 for a single nucleotide. Exact values depend on sequence, but 660 is the standard approximation.
How do I make a qPCR standard curve?
Calculate the copies per µL of your stock, dilute it serially (usually tenfold) to cover the expected range, and run each dilution to build the curve.
Does plasmid size matter?
Yes — enter the whole plasmid length, vector plus insert, because the mass you measured is the whole molecule.
Can I use ng/µL from a spectrophotometer?
Yes, but check the purity ratios: contaminants absorb at 260 nm and inflate the mass, which would overstate the copy number.
Where these figures come from
- NCBI Bookshelf — Molecular Biology of the Cell — growth kinetics and molecular conventions
- National Human Genome Research Institute — the US genomics research institute
Last checked: September 2026. Formulas are the standard textbook forms; assumptions are stated on each page because they are where these models break.