DNA Copy Number Calculator
Enter your DNA mass and fragment or plasmid length in base pairs to estimate the number of DNA molecules (copies) present, assuming double-stranded DNA.
Last updated: September 25, 2026
Frequently asked questions
Where does the 650 g/mol per base pair figure come from?
It's a commonly used average molecular weight for one base pair of double-stranded DNA (accounting for both strands, including the sugar-phosphate backbone and paired bases), derived from the average molecular weights of the four possible base pairs. It's an average, not an exact figure for any specific sequence, since actual molecular weight varies slightly with the sequence's GC content.
Does this work for RNA or single-stranded DNA too?
This calculator's default assumption (650 g/mol per unit) is calibrated for double-stranded DNA specifically. Single-stranded nucleic acids and RNA have different average molecular weights per nucleotide, so applying the dsDNA figure to those sample types would give an inaccurate copy number estimate.
Why does the result use scientific notation?
DNA copy numbers are often extremely large (billions to trillions of molecules even in a small mass), so scientific notation (like 1.234 × 10¹²) keeps the result readable and precise without an unwieldy string of digits - it's the standard way this kind of large number is reported in molecular biology.
How is this different from the DNA Concentration Calculator?
The DNA Concentration Calculator determines how concentrated your sample is (mass per volume, like ng/µL) from an absorbance reading. This DNA Copy Number Calculator instead estimates how many individual DNA molecules are present in a given mass, based on the molecule's length - concentration and copy number are related but distinct measurements, and this tool answers the 'how many molecules' question specifically.
Does plasmid size affect how many copies I get from the same mass?
Yes, significantly - for the same total DNA mass, a shorter plasmid or fragment contains many more individual molecules (copies) than a longer one, since each individual molecule of a longer sequence weighs more. This is why copy number calculations always require the DNA length, not just the mass.
Is this calculation exact enough for quantitative PCR (qPCR) standard curves?
It provides a reasonable starting estimate for preparing standards, but actual qPCR quantification accuracy depends on many additional factors (pipetting precision, DNA purity, exact GC content, and more) beyond this idealized calculation - many labs also verify standard concentrations empirically rather than relying solely on a calculated estimate.