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DNA Copy Number Calculator

Convert nanograms of DNA (plasmid, genomic or amplicon) into molecule copies from the fragment length — and back from copies to mass — for qPCR standards and dilution series.

Convert

DNA mass

Copies wanted (copies mode)

Length

Strandedness

Volume

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How it works

copies = mass (g) × N_A ÷ (length × 660 g/mol per bp); single-stranded uses 330 g/mol per nucleotide

A base pair of double-stranded DNA averages 660 g/mol (sodium salt), so the molar mass of any fragment is just 660 × length. Dividing the mass by that and multiplying by Avogadro’s number gives copies — the calculation behind every qPCR standard curve and every “how much plasmid is 10⁸ copies” question. Note how small the numbers get: a single copy of a 5 kb plasmid weighs 5.5 attograms.

Worked example

10 ng of a 5,000 bp plasmid: 5,000 × 660 = 3.3 × 10⁶ g/mol, so 10 ng is 1.82 × 10⁹ copies — 3.6 × 10⁷ per µL in 50 µL.

Frequently asked questions

Should I use 650 or 660 g/mol?

Either is within 2%; 660 is the sodium-salt convention most calculators use. Exact sequence weights differ slightly.

Does plasmid supercoiling matter?

Not for mass–copy conversion, though supercoiled and linear standards can amplify differently in qPCR.

Related calculators

PCR Amplification Yield Calculator

Copies and nanograms of PCR product after n cycles from template copies or template mass, with amplification efficiency, the plateau set by product, primer and dNTP limits, the cycle a gel band appears, cycles needed for a target amount and a cycle-by-cycle table.

DNA Molecular Weight Calculator

Molecular weight of DNA or RNA from its length or from the exact sequence — dsDNA, ssDNA or RNA, 5′ end chemistry, linear or circular — with mass ↔ pmol ↔ copy-number conversions, molarity from ng/µL, base composition and the A260 extinction coefficient of an oligo.

Serial Dilution Calculator

Compute the concentration at each tube of a serial dilution series from a starting concentration, a fixed fold factor per step and the number of steps.

For teaching and planning — verify against your lab protocol and instrument documentation.