DNA copy number calculator.
Find how many molecules are in a DNA sample. Enter the amount in nanograms and the template length in base pairs, and the calculator returns the copy number, molecular weight and moles.
Amount and length
LiveBased on an average base pair weight of 650 g/mol and Avogadro’s number. Useful for qPCR standards, cloning and sequencing.
Copy number
1.85e+10
molecules in the sample
Assumes double-stranded DNA at 650 g/mol per base pair. Calculations run in your browser; nothing you enter is stored.
How it works
From mass to molecules
To count DNA molecules you go from mass to moles to copies. The molecular weight of a double-stranded template is its length in base pairs times about 650 g/mol per pair. Dividing the mass by that weight gives moles, and multiplying by Avogadro’s number gives the number of copies.
For 100 ng of a 5,000 bp template, the molecular weight is 3,250,000 g/mol, which works out to roughly 1.85 times ten to the tenth copies.
Reference
Length to molecular weight.
Molecular weight of a double-stranded template at 650 g/mol per base pair.
| Length (bp) | Molecular weight |
|---|---|
| 100 bp | 65,000 |
| 500 bp | 325,000 |
| 1,000 bp | 650,000 |
| 5,000 bp | 3,250,000 |
| 10,000 bp | 6,500,000 |
The full guide
The complete guide to DNA copy number.
Why copy number matters, how it is calculated, and how to use it for qPCR standards.
Why copy number matters
Many experiments need the number of molecules, not just the mass. qPCR standard curves, digital PCR, cloning and library prep all depend on knowing how many copies of a template you have. Two samples with the same mass can hold very different copy numbers if the templates differ in length.
Shorter templates pack more copies per nanogram, since each molecule weighs less.
How it is calculated
The molecular weight of double-stranded DNA is its length in base pairs times 650 g/mol, the average weight of a base pair. Divide the mass in grams by that weight to get moles, then multiply by Avogadro’s number, 6.022 times ten to the twenty-third, to get copies.
Putting it together: copies equal the amount in ng times 6.022e23, divided by length times 650 times one billion.
Making qPCR standards
To build a standard curve you often need a known copy number, then a serial dilution. Calculate the copies in your stock, then dilute ten-fold across the range you expect in your samples. Each step is then a known order of magnitude.
Working in copies rather than mass makes results comparable across templates of different sizes.
Assumptions and limits
The 650 g/mol figure is an average for double-stranded DNA; single-stranded DNA or RNA differ, and exact sequence composition shifts it slightly. For most quantification this average is close enough.
The estimate also assumes your mass measurement is of intact, full-length template, so quantify carefully first.
The formula
Mass to
copies.
Molecular weight is length times 650. Copies are mass over weight, times Avogadro’s number.
DNA concentration ›# Copy number
MW = length(bp) × 650
copies = ng × 6.022e23 / (bp × 650 × 1e9)
# worked example
100 ng, 5000 bp → 1.85e10 copiesQuestions
Copy number questions.
How do I calculate DNA copy number?
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Copies equal the amount in ng times Avogadro’s number, divided by the length in bp times 650 times one billion. For 100 ng of a 5,000 bp template that is about 1.85e10 copies.
What molecular weight per base pair is used?
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An average of 650 g/mol per base pair for double-stranded DNA. So a 5,000 bp template weighs about 3,250,000 g/mol.
Why do shorter templates have more copies?
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Because each molecule weighs less, so a fixed mass contains more of them. Copy number is inversely proportional to template length for the same mass.
What is this used for?
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Mainly qPCR and digital PCR standards, plus cloning and sequencing, where you need a known number of molecules rather than just a mass.
Is this DNA copy number calculator free?
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Yes. It is completely free with no sign-up, and every calculation runs locally in your browser, so nothing you enter is stored or sent anywhere.
About the developer
Jean Borg
Jean builds and maintains every calculator on freecalculators.pro from Malta, with a focus on tools that are fast, free and show their working. The DNA copy number calculator uses 650 g/mol per base pair and Avogadro’s number, for lab estimates.