DNA Copy Number & pmol ⇄ ng Calculator

Understanding the DNA copy number calculator — concept and principle

Lab protocols usually give DNA amounts in nanograms, but reactions work on molecules. Building a qPCR standard curve, setting up a ligation, transfecting plasmids or loading a sequencing library all require converting mass into picomoles or into the number of copies. This calculator does both conversions from the mass, the length and whether the DNA is double- or single-stranded.

100 ng of a 5 kb plasmid is only 0.03 pmol — yet almost 20 billion molecules.

How it is calculated

Molecular weight (g/mol) = length × 617.96 + 36.04 for double-stranded DNA (length in bp). For single-stranded DNA the calculator uses 308.97 per nucleotide with the same end term.
pmol = ng × 1000 ÷ MW. (ng ÷ MW gives nmol; multiplying by 1000 gives pmol.)
Copies = ng × 10⁻⁹ ÷ MW × 6.02214076 × 10²³, where the last number is the Avogadro constant.

The principle

A mole is a fixed number of molecules, 6.02214076 × 10²³. Dividing a mass by the molar mass gives moles, and multiplying moles by the Avogadro constant gives the count of molecules. For DNA the molar mass grows almost linearly with length: an average base pair of the sodium-salt form weighs about 618 g/mol, and a small correction accounts for the chain ends. The key consequence is that, for the same mass, a longer molecule means fewer copies: 100 ng of a 500 bp fragment holds ten times as many molecules as 100 ng of a 5000 bp plasmid.

Worked example

Inputs: 100 ng of double-stranded DNA, 5000 bp.
MW = 5000 × 617.96 + 36.04 = 3,089,836.04 g/mol.
pmol = 100 × 1000 ÷ 3,089,836.04 = 0.03236 pmol (32.4 fmol).
Copies = 100 × 10⁻⁹ ÷ 3,089,836.04 × 6.02214076 × 10²³ = 19,490,162,850, about 1.95 × 10¹⁰ copies.

Each point is the calculator re-run with only “Length (bp, or nt if single-stranded)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Length (bp, or nt if single-stranded)	Copies
1,250	77,957,923,490
2,500	38,979,871,040
3,750	25,986,782,760
5,000	19,490,162,850
6,250	15,592,166,650
7,500	12,993,492,420
10,000	9,745,138,259

How to read the result

For qPCR standards, copies per microlitre are what matter: divide the copy number by the volume the DNA is dissolved in, then prepare a ten-fold dilution series. For ligation and Gibson-type assembly, protocols are written in pmol or fmol, so compare fragments by moles rather than nanograms. If the result seems off by a factor of 1000, check whether you entered µg instead of ng, or kb instead of bp.

Common mistakes

Entering kilobases (5) instead of base pairs (5000).
Choosing double-stranded for a single-stranded oligo or cDNA, which halves the copy count.
Using a concentration that is itself inaccurate; copy numbers are only as good as the ng value from A260 or a fluorometric assay.

Keep in mind: The molecular weight uses average nucleotide masses, so it is an approximation for a specific sequence. For short single-stranded oligos, calculate MW from the exact base composition, and remember that copy numbers below a few copies per reaction follow Poisson statistics.

Frequently asked questions

How many copies are in 1 ng of a 1 kb dsDNA fragment?

MW ≈ 618,000 g/mol, so 1 × 10⁻⁹ ÷ 618,000 × 6.022 × 10²³ ≈ 9.7 × 10⁸ copies.

How do I convert pmol back to ng?

Multiply: ng = pmol × MW ÷ 1000.

Does a supercoiled plasmid have a different molecular weight?

No. Supercoiling changes shape, not mass, so the same formula applies; only quantification by absorbance or dyes may behave differently.

Sources: New England Biolabs, NEBioCalculator and technical reference on DNA molecular weight · Bureau International des Poids et Mesures, The International System of Units (SI Brochure), definition of the mole

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

Your numbers