What is the DNA copy number formula?

copies = (mass_g × 6.022 × 10²³) ÷ (length_bp × average_bp_weight). Average base-pair weight is 660 g/mol for dsDNA and 330 g/mol per nucleotide for ssDNA.

What units should I enter for mass?

Enter mass in nanograms (ng). The calculator automatically converts ng to grams internally.

What is the average base-pair molecular weight used?

For dsDNA the calculator uses 660 g/mol per base pair (two complementary nucleotides). For ssDNA it uses 330 g/mol per nucleotide.

Can I use this for plasmid DNA copy number?

Yes — enter the total plasmid size in bp (e.g. 5,500 for a 5.5 kb vector) and select dsDNA. The result is the number of plasmid molecules in your specified mass (enter 1 ng to find molecules per ng).

How do I get copy number from a Ct value?

Use this tool to convert your standard's mass to copies, then build a qPCR standard curve (log copies vs Ct). Read unknown copies off the regression line at the measured Ct.

Can I use this for RNA?

The ssDNA setting is a close approximation for single-stranded RNA because the average nucleotide weight is similar (~340 g/mol for RNA vs 330 g/mol for DNA). For high-precision RNA work, adjust slightly.

Why does copy number change with DNA length?

Longer DNA molecules are heavier. The same mass of DNA therefore contains fewer copies of a long molecule than a short one.

DNA Copy Number Calculator

Calculate the number of DNA molecules in a sample from mass (ng), fragment length (bp), and strand type (single or double).

DNA Copy Number

DNA Mass (ng)

DNA Length (bp or nt)

Strand Type

Double-stranded (dsDNA) Single-stranded (ssDNA)

Enter mass and length to calculate copy number

Copied!

Summary

This calculator determines how many DNA molecules are present in a sample using the sample mass in nanograms, the DNA length in base pairs, and whether the DNA is single-stranded (ssDNA) or double-stranded (dsDNA). It applies Avogadro's number and the average nucleotide molecular weight to give an accurate copy count.

How it works

Enter the DNA mass in nanograms (ng) — the calculator converts to grams internally.
Enter the fragment length in base pairs (bp); for plasmids enter the total plasmid size.
Select ssDNA or dsDNA so the right average nucleotide weight is used (330 vs 660 g/mol).
The tool multiplies total moles (mass ÷ molar mass) by Avogadro's number (6.022 × 10²³) to get molecule count.
Read the total copy number in your sample (enter 1 ng if you need copies per ng) for fast scaling of qPCR standard dilutions.

Use cases

Prepare accurate molar quantities of DNA for cloning and ligation reactions.
Determine copy number for qPCR standard curve construction.
Calculate the number of plasmid molecules before transfection.
Estimate template copy number for digital PCR experiments.
Convert a sequencing library mass to molecule count for accurate pooling.
Cross-check the IDT or Science Primer copy number calculations on your own.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu