DNA Copy Number Calculator (Copies from ng)

This calculator converts a mass of double-stranded DNA into a number of copies, as used when setting up qPCR standards. Enter the mass in nanograms, the length in base pairs and the average mass per base pair; you get the number of copies.

Formula

Copies = mass (g) × Avogadro’s number ÷ (length in bp × mass per bp). Mass in g = ng × 10⁻⁹; Avogadro’s number = 6.02214076 × 10²³ per mol.

• mw: about 650 g/mol per base pair is a common figure for double-stranded DNA; some protocols use 660

Worked Example

10 ng of a 5,000 bp plasmid at 650 g/mol per bp.

1. Molar mass = 5,000 × 650 = 3,250,000 g/mol
2. Moles = 10 × 10⁻⁹ ÷ 3,250,000 = 3.077 × 10⁻¹⁵ mol
3. Copies = 3.077 × 10⁻¹⁵ × 6.022 × 10²³ = 1,852,966,388 copies (about 1.85 × 10⁹)

How to read the result

For a standard curve, make tenfold dilutions from this stock. For single-stranded DNA use about 330 g/mol per base; for RNA about 340 g/mol per nucleotide.

Limits and common mistakes

• The result is only as accurate as the concentration measurement.
• Use the length of the whole molecule (plasmid plus insert), not just the target region.

Frequently Asked Questions

Why 650 g/mol?

It is the average mass of one base pair of double-stranded DNA (sodium salt), rounded.

How many copies in 1 ng of a 3 kb plasmid?

About 3.1 × 10⁸.

Can I use it for RNA?

Yes, with about 340 g/mol per nucleotide and the length in nucleotides.

What is Avogadro’s number?

6.02214076 × 10²³ per mole.

in ng | DNA mass | ng | 10 | 0
in bp | Template length | bp | 5000 | 1
in mw | Average mass per base pair | g/mol | 650 | 1 | 1000
out copies | Number of copies | copies | ng * 1e-9 * 6.02214076e23 / (bp * mw) | 0