Convert between plasmid DNA mass (ng) and copy number. Calculate molar equivalents for cloning, transfection, and qPCR standard preparation.
For Research Use Only. This calculator uses the average molecular weight of 660 g/mol per base pair for dsDNA. Actual values may vary based on sequence composition and DNA form (supercoiled, nicked, linear).
Enter parameters and click Calculate to see results
Knowing the copy number of a plasmid preparation is essential for many molecular biology applications. Whether you're setting up a ligation reaction, preparing qPCR standards, or optimizing transfection conditions, understanding the relationship between DNA mass and molecule count helps ensure reproducible results.
copies = (mass × NA) / MWAmount of DNA in grams (convert ng to g by multiplying by 10-9)
Avogadro's number: 6.022 × 1023 molecules/mol
Molecular weight: plasmid size (bp) × 660 g/mol
Double-stranded DNA (dsDNA) consists of two complementary strands. Each nucleotide has an average molecular weight of approximately 330 Da. Since each base pair includes one nucleotide on each strand, the combined weight is about 660 Da (or g/mol) per base pair.
Note: This is an average value. The exact molecular weight varies slightly depending on the GC content of your sequence (A, T, G, and C have slightly different molecular weights). For most practical purposes, 660 g/mol per bp is sufficiently accurate.
Calculate molar ratios between insert and vector DNA. Typical insert:vector ratios of 3:1 to 5:1 (molar) optimize ligation efficiency.
Prepare standard curves with known copy numbers. Serial dilutions of a plasmid with calculated copy number create reliable quantification standards.
When co-transfecting multiple plasmids, molar equivalents ensure equal representation regardless of plasmid size differences.
Optimal assembly reactions require specific molar ratios of fragments. Convert ng to pmol for accurate fragment mixing.
| Plasmid Size (bp) | MW (kDa) | Copies per 1 ng | fmol per 1 ng |
|---|---|---|---|
| 1,000 | 660 | 9.12e+8 | 1.515 |
| 3,000 | 1,980 | 3.04e+8 | 0.505 |
| 5,000 | 3,300 | 1.82e+8 | 0.303 |
| 8,000 | 5,280 | 1.14e+8 | 0.189 |
| 10,000 | 6,600 | 9.12e+7 | 0.152 |
Confusing mass ratios with molar ratios
100 ng of a 10 kb plasmid contains far fewer molecules than 100 ng of a 3 kb insert. Always convert to molar amounts for proper ratios.
Using 330 Da/bp for dsDNA
330 Da is for single-stranded DNA or RNA. Double-stranded DNA uses ~660 Da/bp (both strands combined).
Ignoring DNA purity and form
Spectrophotometric measurements can be affected by contaminants. Supercoiled, nicked, and linear forms may also behave differently in downstream applications.
Convert between mass, molarity, and concentration for DNA/RNA samples.
Calculate DNA and reagent volumes for transfection master mixes.
Plan serial dilution series for qPCR standards and assays.
Calculate volumes and masses for buffer and reagent preparation.
From copy number to molar ratios, get accurate calculations for your cloning, transfection, and qPCR experiments
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