Plasmid DNA mass to molecule count
DNA molecule count can be estimated by dividing DNA mass by the approximate molar mass of one plasmid and multiplying the resulting moles by Avogadro’s constant.
This tool reports molecules in the entered purified DNA sample. It does not measure biological plasmid copy number per cell, which requires cellular and assay information.
How to calculate plasmid DNA copies
- Enter DNA mass: Use the mass of purified plasmid DNA in µg, ng, or pg.
- Enter plasmid length: Supply the complete plasmid length in base pairs.
- Confirm the mass convention: Use 660 g/mol/bp as a common dsDNA average or enter a justified alternative.
- Calculate: Generate the estimated number of plasmid molecules in the sample.
Formula and variables
Approximate the plasmid molar mass from its base-pair length, convert sample mass to moles, then convert moles to molecules.
Molecules = mass(g) × NA / [length(bp) × average mass(g/mol/bp)]- NA — Avogadro constant
- Entities per mole (6.02214076 × 10²³ mol⁻¹)
- L — Plasmid length
- Double-stranded DNA length (bp)
- Mb — Average base-pair molar mass
- Approximate average molar mass per dsDNA base pair (g/mol/bp)
Copies in a 100 ng plasmid sample
Estimate molecules in 100 ng of a 3,000 bp plasmid using 660 g/mol per base pair.
- DNA mass
- 100 ng
- Plasmid length
- 3,000 bp
- Average mass
- 660 g/mol/bp
- Plasmid molar mass ≈ 3,000 × 660 = 1.98 × 10⁶ g/mol
- Molecules ≈ (100 × 10⁻⁹ / 1.98 × 10⁶) × NA
Result: The sample contains approximately 3.04 × 10¹⁰ plasmid molecules.
This is a theoretical count before accounting for purity, damage, or measurement uncertainty.
Understanding your results
What copy count represents
The result is the estimated number of intact-equivalent dsDNA molecules represented by the entered mass.
- Longer plasmids contain fewer molecules at the same mass.
- A concentration input must first be multiplied by volume to obtain total mass.
- Spectrophotometric contamination can inflate mass and calculated copies.
Assumptions
- The sample is double-stranded plasmid DNA.
- The entered mass represents the plasmid and not contaminating nucleic acids or salts.
- The selected average molar mass per base pair is appropriate.
Limitations
- Does not calculate plasmid copies per cell or genomic copy-number variation.
- Does not correct for purity, topology, fragmentation, extraction recovery, or pipetting error.
- Sequence-specific molar mass can differ from the average base-pair approximation.
Common mistakes
- Entering concentration without multiplying by sample volume.
- Using insert length instead of total plasmid length.
- Confusing molecules in a tube with copies per bacterial cell.
- Mixing ng, µg, and pg.
Practical use cases
PCR template planning
Estimate starting plasmid molecules from a measured DNA mass.
Standards and dilutions
Estimate a theoretical stock copy count before preparing serial dilutions.
Frequently asked questions
Is this plasmid copy number per cell?
No. It estimates DNA molecules from purified mass; cellular copy number requires a cell-based measurement and normalization method.
Why is 660 g/mol per base pair used?
It is a common average molecular-weight approximation for double-stranded DNA. The calculator allows another value when needed.
Can I use DNA concentration?
Multiply concentration by the relevant sample volume to obtain mass, then enter that mass.
Sources and review
- PCR Setup—DNA Copy Number — Thermo Fisher Scientific. Accessed 2026-07-13.
- Biomath DNA Calculators — Promega. Accessed 2026-07-13.
Reviewed 2026-07-13.