DNA Copy Number Calculator
Enter the DNA mass (ng), fragment length (bp), and mass per base pair to compute the number of DNA copies, for qPCR absolute quantification and standard curves.
Input Data
Results
At a glance:Enter the DNA mass (ng), fragment length (bp), and mass per base pair to compute the number of DNA copies, for qPCR absolute quantification and standard curves.
Formula
Copy number = (mass × 6.022×10²³) ÷ (length × 10⁹ × g/bp).
mass in ng → ×10⁻⁹ g; 1 mol = 6.022×10²³ molecules (Avogadro's number).
$$N = \dfrac{m \times N_A}{L \times 10^{9} \times M_{bp}}$$How to Use
- Enter the DNA mass in ng and the fragment length in bp.
- Set the mass per bp (default 650 for dsDNA).
- The calculator returns the copy number.
Reference: common DNA template lengths and copy numbers (at 150 ng, 650 g/bp)
| Template type | Length (bp) | Copies (approx.) |
|---|---|---|
| E. coli genome | 4,700,000 | 3.0×10⁷ |
| Plasmid (pUC19-class) | 2,700 | 5.2×10¹⁰ |
| PCR product | 1,000 | 1.4×10¹¹ |
| Short fragment | 300 | 4.6×10¹¹ |
Copy number is proportional to DNA mass and inversely proportional to fragment length: at the same mass, shorter fragments yield more molecules.
Case Studies
E. coli genome copy number
A 4,700,000 bp genome, take 150 ng, 650 g/bp.
Copies = (150 × 6.022×10²³) ÷ (4,700,000 × 10⁹ × 650) ≈ 2.96×10⁷ molecules.
From this highest point, make 10-fold serial dilutions (with the serial dilution calculator) to build a qPCR absolute-standard curve.
Plasmid copy number
A 3,000 bp plasmid, take 50 ng, 650 g/bp.
Copies = (50 × 6.022×10²³) ÷ (3,000 × 10⁹ × 650) ≈ 1.54×10¹⁰ molecules.
At the same mass, a short plasmid holds far more molecules than long genomic DNA, reflecting that copy number is inversely proportional to length.
FAQ
Should I use 650 or 660 g/mol per bp?
Both are common approximations. 660 g/mol is the slightly stricter average for double-stranded DNA per base pair; 650 g/mol is the more widely used simplified value. The two differ by about 1.5% in copy number, negligible for most applications; just stay consistent within one experiment.
Why is copy number inversely proportional to fragment length?
Because a longer DNA molecule is heavier per molecule (molecular weight scales with length), so fewer molecules fit in a fixed mass. This is also why qPCR must record template length: ng alone cannot compare the actual starting molecules of different-length templates.
Should mass be single- or double-stranded?
This formula assumes double-stranded DNA (dsDNA), where 650–660 g/mol per bp already includes both strands. If your sample is single-stranded DNA or RNA, the average molar mass per nucleotide differs (about 330 g/mol), so use the corresponding coefficient, or the copy number will be wrong.
How do I get the DNA mass (ng)?
Use a NanoDrop conversion from A260 absorbance (see the DNA/RNA Concentration Calculator), or fluorometric quantification with Qubit. Once you have concentration (ng/μL), multiply by the sample volume (μL) to get mass (ng), then enter it here.
How do I use such large copy numbers?
For absolute quantification, convert copy number to copies per reaction volume, or take the base-10 logarithm as the X-axis of the standard curve. After 10-fold serial dilutions, each point decreases 10-fold in copies, covering several orders of magnitude of quantification range.
Related Tools
References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.