DNA Copy Number Calculator — Convert ng of DNA to Copies per µL

Welcome to the DNA Copy Number Calculator!

Use this calculator for your PCR runs to convert nanograms (ng) of DNA to its copy number. You can also use the calculator the other way around — determine the DNA stock solution needed for a specific copy number per microliter (copies/µL). Choose between double-stranded DNA (dsDNA), single-stranded DNA (ssDNA), or single-stranded RNA (ssRNA). Switch between Metric (ng/µL) and US (ng/mL) units with a single click.

Formula for the DNA/RNA Copy Number

The core formula used by this DNA copy number calculator:

Where:

• C_DNA — DNA concentration in ng/µL
• N_A — Avogadro's constant: 6.022 × 10²³
• l — Length of the template in base pairs (bp) or nucleotides (nt)
• 10⁹ — Conversion factor for nanograms
• w_bp — Average weight of a base or base pair in daltons (Da):
  • dsDNA: 660 Da per base pair
  • ssDNA: 330 Da per nucleotide
  • ssRNA: 340 Da per nucleotide

How to Use the DNA Copy Number Calculator — Example

1. Calculate the Copy Number

Suppose you measured a stock sample DNA concentration (C_DNA) of 150 ng/µL. The total template length (l) is 4,700,000 bp (e.g., a bacterial genome). Since it is double-stranded DNA, we use w_bp = 660 Da.

Plug into the formula:

2. Calculate Stock Dilution

Suppose you need a total pipette volume of 10 µL with a final concentration of 2,000,000 copies/µL. Divide the desired concentration by the stock concentration and multiply by the desired total volume:

So you pipette 0.69 µL of stock and add 9.31 µL of nuclease-free water.

Formula for the Gene Copy Number per PCR Cycle

Each PCR cycle amplifies your target DNA. The estimated copy number after n cycles with amplification efficiency E (0 = 0%, 1 = 100%) is:

For a typical PCR with 95% efficiency (E = 0.95) and 40 cycles, starting from 3 × 10⁷ copies/µL:

This is why PCR is so powerful — even a few starting copies can produce billions of amplicons. Enable the PCR Amplification section in the calculator to compute this automatically.

Improving PCR Amplification

Several factors influence PCR amplification efficiency and the final copy number:

• Template quality — Degraded or inhibitor-containing DNA reduces efficiency.
• Primer design — Primers with Tm of 55–65 °C and low self-complementarity improve specificity.
• Annealing temperature — Too low → non-specific bands; too high → reduced yield.
• Mg²⁺ concentration — Typically 1.5–2.5 mM; affects polymerase activity.
• dNTP balance — Usually 200 µM each; imbalances cause errors.
• Cycle number — More cycles = more copies, but also more errors and non-specific products.
• Hot-start polymerase — Reduces non-specific amplification before denaturation.

Measurement Systems

The calculator supports two common concentration notation systems:

• Metric system: concentration in ng/µL (nanograms per microliter) — standard in most molecular biology labs worldwide.
• US / American system: concentration in ng/mL (nanograms per milliliter) — used in some clinical and American lab protocols. Note: 1 ng/µL = 1000 ng/mL.

Both systems produce the same result — the calculator automatically converts between them.

FAQs

What does copy number mean in genetics?

Copy number refers to the number of copies of a particular DNA sequence in a cell or solution. In PCR, it is the number of target molecules per unit volume (typically copies/µL).

Why use Avogadro's number?

Avogadro's number (6.022 × 10²³) converts mass to the number of molecules. Since 1 mole of any substance contains exactly N_A molecules, dividing the number of nanograms by the molar mass (daltons) and multiplying by N_A gives the number of molecules.

Does strand type matter?

Yes. The average molecular weight per base differs between double-stranded DNA (660 Da/bp), single-stranded DNA (330 Da/nt), and single-stranded RNA (340 Da/nt). Using the wrong value can introduce up to a 2× error in your copy number estimate.

What is a good DNA concentration for PCR?

For most PCR assays, 10–100 ng of genomic DNA template per 25–50 µL reaction is recommended. For plasmid DNA, much less is needed — typically 1–10 ng. The ideal copy number input is usually 10³–10⁶ copies per reaction.

How accurate is this calculator?

The calculator uses the standard bioinformatics formula with the accepted average base weights (660 Da for dsDNA, 330 Da for ssDNA, 340 Da for ssRNA). In practice, the exact weight depends on the GC content and specific sequence, but the error is typically < 1% for random sequences.