Copies = (mass × 6.022e23) / (length × 650 × 1e9)
Copy number = (mass in grams × Avogadro's number) / (DNA length in bp × 650 Da/bp). For example, 1 ng of a 5 kb plasmid: copies = (1e-9 × 6.022e23) / (5000 × 650) = 1.85 × 10⁸ copies. This calculation is essential for: qPCR standard curves (serial dilutions of known copy numbers), transfection (moles of DNA per cell), cloning (insert:vector molar ratios), and next-gen sequencing library quantification. For PCR, 10³-10⁵ template copies are typically needed. For transformation, 1-10 ng of plasmid provides billions of copies. The calculation assumes dsDNA; for ssDNA, use 330 Da/nt instead of 650 Da/bp.