DNA Copy Number Calculator: Mass to Copies
What this DNA copy number calculator does
This tool converts a measured DNA mass and a known sequence length into an estimated copy number (how many molecules you have). If you also provide the sample volume, it adds copies per µL and ng per µL to support common lab workflows.
The copy number formula
The calculation follows a standard mass-to-molecules conversion using Avogadro’s constant:
\[ \text{copies}=\left(\frac{m}{M}\right)\times N_A \]
Where:
- \(m\) is the DNA mass in grams.
- \(M\) is the molecular weight in g/mol.
- \(N_A\) is Avogadro’s constant \(6.02214076\times 10^{23}\).
Molecular weight is estimated from length and an average weight-per-base factor:
\[ M = L \times w \]
Common approximations used by many labs:
- dsDNA - \(w \approx 660\ \text{g/mol per bp}\)
- ssDNA - \(w \approx 330\ \text{g/mol per base}\)
- RNA - \(w \approx 340\ \text{g/mol per nucleotide}\)
Copies per µL and concentration
If you provide a volume \(V\) (in µL), the calculator also computes:
\[ \text{copies per µL}=\frac{\text{copies}}{V} \]
And a simple mass concentration:
\[ \text{ng per µL}=\frac{\text{mass (ng)}}{V} \]
How to use the results in practice
qPCR standards and spike-ins
If you need a target number of molecules per reaction, copy number helps you translate from a measured DNA mass to an approximate molecule count. This is useful for creating dilution series, standard curves, and spike-in planning.
Library prep and molarity-style thinking
Many workflows depend on molecule counts (or moles) more than mass alone. While this calculator does not replace a full molarity tool, it provides the essential conversion needed to reason about molecules from mass and length.
Important notes and limitations
- Approximation - The weight-per-base constants are averages; real sequences differ slightly by base composition and modifications.
- Length definition - Use the effective length of the molecule you are counting (for plasmids, the full plasmid length; for amplicons, the amplicon length).
- Volume optional - Setting volume to 0 skips concentration-style outputs and keeps only total molecule estimates.