DNA Copy Number Calculator
Find out how many copies of a DNA fragment are present in a sample. Enter the total DNA mass in nanograms and the target sequence length in base pairs to get the exact copy number using Avogadro's number.
ng
bp
Number of DNA molecules in the sample
Molecular weight of fragment
Convert ng to grams
Moles of DNA
Copy number (× Avogadro)
- 1
Fragment molecular weight
1,000 bp × 650 g/mol = 6.500e+5 - 2
Convert ng to grams
1 × 10⁻⁹ = 1.000e-9 - 3
Moles of DNA
1.000e-9 ÷ 6.500e+5 = 1.538e-15 - 4
Copy number (× Avogadro)
1.538e-15 × 6.022×10²³ = 926,461,538Multiplying moles by Avogadro's number gives the number of individual molecules.
How does this calculator work?
Copy number = (mass_ng × 6.022×10¹⁴) / (length_bp × 650). Divide the DNA mass by the fragment's molar mass (650 g/mol per base pair) and multiply by Avogadro's number. Enter mass in nanograms and fragment length in base pairs to get the exact molecular count.
Formula
How this is calculated
Each base pair of double-stranded DNA has an average molecular weight of about 650 Daltons (g/mol) — a well-established approximation that works for typical genomic and plasmid DNA. Multiplying the fragment length in base pairs by 650 gives the molar mass of one molecule of the fragment in grams per mole.
Dividing the sample mass (converted from nanograms to grams) by that molar mass gives the number of moles of DNA. Multiplying by Avogadro's number (6.022 × 10²³ molecules/mol) converts moles to the number of individual DNA molecules — the copy number.
This calculation assumes pure double-stranded DNA with a standard 50 % GC content. GC-rich or AT-rich sequences have slightly different molecular weights (GC pairs ≈ 618 g/mol, AT pairs ≈ 652 g/mol), but for most practical purposes the 650 average is accurate to within 5 %. The formula applies equally to plasmid DNA, PCR products, or any defined DNA fragment.
Frequently asked questions
The average molecular weight of a nucleotide is roughly 330 Da, so a base pair (two complementary nucleotides) averages about 650 Da. AT pairs are slightly lighter (~652 Da) and GC pairs slightly heavier (~618 Da after hydration adjustments), but 650 is a reliable working average for mixed-sequence DNA.
For single-stranded DNA, use approximately 330 Da per nucleotide instead of 650, so copy number = (mass_ng × 6.022×10¹⁴) / (length_nt × 330). For single-stranded RNA, use ~340 Da per nucleotide because RNA nucleotides carry a ribose hydroxyl group that adds extra mass.
A 20 µL PCR reaction typically starts with about 100 ng of genomic DNA. For a 1000 bp target from 1 ng of template, the formula gives roughly 900,000 copies — confirming that even 1 ng provides an ample starting concentration for PCR amplification.
TG we-Calculate Editorial Team. (2026). DNA Copy Number Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/dna-copy-number-calculator
TG we-Calculate Editorial Team. "DNA Copy Number Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/dna-copy-number-calculator.
TG we-Calculate Editorial Team, "DNA Copy Number Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/dna-copy-number-calculator
@misc{wecalculate_dna_copy_number_calculator, title = {DNA Copy Number Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/dna-copy-number-calculator}}, year = {2026}, note = {TG we-Calculate} }
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