All calculatorsDNA Copy Number Calculator
Convert DNA mass or concentration into copy number (molecules) in seconds — supports dsDNA vs ssDNA (plus plasmids), bp length, unit conversions, copies/µL + total copies, optional copies per reaction, step-by-step, quick picks, and a mini log-scale visual.
Background
Copy number is “how many DNA molecules you have.” We convert mass → moles using molar mass (estimated from length in bp), then moles → molecules using Avogadro’s number. If you know the exact molar mass (g/mol), you can override the estimate for extra accuracy.
How to use this calculator
- Choose a mode: concentration, mass, or molarity.
- If using mass/concentration, enter DNA length and whether it’s dsDNA or ssDNA (or choose plasmid).
- Optional: turn on MW override to use an exact g/mol value.
- Optional: turn on copies per reaction for qPCR/ddPCR workflows.
- Click Calculate for results, plus optional steps and visual.
How this calculator works
- Molar mass from length: MW ≈ bp × 660 (dsDNA/plasmid) or MW ≈ bases × 330 (ssDNA)
- MW override (optional): Use an exact MW (g/mol) if you already know it.
- Moles: n = mass(g) / MW(g/mol)
- Copies: copies = n × NA where NA = 6.022×10²³
- Copies/µL: copies/µL = copies / volume(µL)
- Copies per reaction: copies/reaction = (copies/µL) × template(µL)
Formula & Equation Used
dsDNA / plasmid molar mass estimate: MW ≈ bp × 660 g/mol
ssDNA molar mass estimate: MW ≈ bases × 330 g/mol
Copies from mass: copies = (mass(g) / MW) × 6.022×10²³
Copies from molarity: copies = (C(mol/L) × V(L)) × 6.022×10²³
Copies per reaction: copies/reaction = (copies/µL) × template(µL)
Example Problem & Step-by-Step Solution
Example 1 — Plasmid copies in a tube
You have 5 ng/µL of a 3000 bp plasmid (circular), and a total volume of 50 µL. Find total copies.
- Molar mass: MW ≈ 3000 × 660 = 1.98×10⁶ g/mol.
- Total mass: 5 ng/µL × 50 µL = 250 ng = 2.50×10⁻⁷ g.
- Moles: n = 2.50×10⁻⁷ / 1.98×10⁶ ≈ 1.26×10⁻¹³ mol.
- Copies: copies = n × 6.022×10²³ ≈ 7.6×10¹⁰.
Note: topology (circular vs linear) doesn’t change MW — it’s just a helpful label.
Example 2 — qPCR copies per reaction
You have a 150 bp dsDNA amplicon at 10 ng/µL. You add 2 µL template into a 20 µL qPCR reaction. Find copies per reaction.
- Molar mass: MW ≈ 150 × 660 = 9.90×10⁴ g/mol.
- Concentration to grams: 10 ng/µL = 1.0×10⁻⁸ g/µL.
- Moles per µL: n/µL = (1.0×10⁻⁸) / (9.90×10⁴) ≈ 1.01×10⁻¹³ mol/µL.
- Copies per µL: copies/µL ≈ 1.01×10⁻¹³ × 6.022×10²³ ≈ 6.1×10¹⁰.
- Copies per reaction: copies/rxn = (6.1×10¹⁰) × 2 ≈ 1.22×10¹¹.
- (Optional) Copies per µL of reaction: (1.22×10¹¹) / 20 ≈ 6.1×10⁹.
Example 3 — MW override (exact g/mol)
You have DNA at 10 ng/µL and you know the exact molecular weight is 1.98×10⁶ g/mol. You have 20 µL. Find total copies (using MW override).
- Total mass: 10 ng/µL × 20 µL = 200 ng = 2.00×10⁻⁷ g.
- Moles: n = 2.00×10⁻⁷ / 1.98×10⁶ ≈ 1.01×10⁻¹³ mol.
- Copies: copies = 1.01×10⁻¹³ × 6.022×10²³ ≈ 6.1×10¹⁰.
This bypasses the 660/330 approximation (useful for modified constructs or vendor-provided MW).
Frequently Asked Questions
Q: Do I need DNA length (bp)?
For mass-based inputs (ng/µL or ng), yes — unless you use MW override. For molarity inputs (nM), you can compute copies without bp.
Q: Is 660 g/mol per bp always correct?
It’s a widely used approximation for dsDNA. Actual molar mass depends slightly on base composition and modifications, so MW override is helpful if you already know the exact g/mol.
Q: Does plasmid topology change copy number?
No — circular vs linear doesn’t change the mass→moles→copies math. Topology is included as a label for reporting.
Q: Why are results huge?
Molecules are tiny — even nanograms can be tens of billions of copies, especially for short fragments.
