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DNA copy number calculator

Copy number from mass or concentration and length, for qPCR standards.

Molecule

Copy number

How is DNA copy number calculated?

copies = mass (g) × 6.022 × 10²³ ÷ (length × weight per base)
copies/µL = concentration (g/µL) × 6.022 × 10²³ ÷ (length × weight per base)

Weight per base: 660 g/mol for double-stranded DNA (per base pair), 330 g/mol for single-stranded DNA, 340 g/mol for RNA (both per nucleotide).

Worked example. A 3,000 bp dsDNA plasmid has a molecular weight of 3,000 × 660 = 1.98 × 10⁶ g/mol. On the Mass tab, 500 ng of it is1.52 × 10¹¹ copies. On the Concentration tab, a stock at50 ng/µL holds 1.52 × 10¹⁰ copies/µL. On theTarget copies tab, 1 × 10⁹ copies weigh 3.288 ng.

When do you need copy number instead of mass or concentration?

A qPCR standard curve is built from a dilution series with a known number of template molecules per reaction, not a mass, since amplification efficiency is defined per copy. Converting a plasmid or gBlock preparation from ng/µL to copies/µL is the usual first step before that dilution series, and the same conversion checks how many template molecules are in a transformation, a ligation or an in vitro transcription reaction.

To go the other way and work with mass, moles and molarity directly, see thenucleic acid mass to moles converter.

Frequently asked questions

How do I convert mass to DNA copy number?

Multiply the mass in grams by Avogadro’s number, then divide by the total molecular weight of the molecule (its length in bases times the weight per base). The calculator does this for whatever mass unit you enter.

Why is dsDNA 660 g/mol per base but ssDNA is 330?

A base pair of double-stranded DNA carries two nucleotides, one on each strand, at roughly 330 g/mol each, so a dsDNA base pair averages about 660 g/mol. Single-stranded DNA has only one nucleotide per position, hence 330. RNA nucleotides carry an extra oxygen on the ribose sugar, which brings the average to about 340 g/mol.

What does copies/µL mean and when do I need it?

It is the number of DNA or RNA molecules in each microliter of a solution at a known concentration, for example a NanoDrop reading in ng/µL. This is the figure a qPCR standard curve is normally built from, usually as a dilution series down from a known copies/µL stock.

How much DNA do I need for a target copy number?

Pick the Target copies tab and type the number of copies you want, for example 1e9. With the same length and molecule type, the calculator gives the mass in nanograms. This is the usual way to prepare a qPCR standard, working from 10⁹ copies down to 10 copies per reaction.

Does this work for a whole plasmid or genome, not just a short oligo?

Yes, enter the total length of the plasmid, amplicon or genome and switch the unit beside it to kb for anything long. A 3 kb plasmid is entered as 3000 bases or as 3 kb; the underlying calculation is the same regardless of length.