DNA Copy Number Calculator

About the DNA Copy Number Calculator

Calculating DNA copy number — the number of individual molecules in a sample — is essential for quantitative PCR (qPCR) standard curves, digital PCR absolute quantification, virus titer estimation, and next-generation sequencing library normalization. While a spectrophotometer tells you mass (ng/µL), most quantitative assays need a known number of molecules.

The conversion uses Avogadro's number and the molecular weight of the DNA: copies/µL = (concentration in g/µL × 6.022 × 10²³) / (length in bp × 660 Da/bp for dsDNA). This fundamental relationship connects the measurable (mass) to the countable (molecules). A 5 kb plasmid at 10 ng/µL contains approximately 1.83 × 10⁹ copies per µL — nearly two billion molecules in a microliter.

This calculator handles the complete workflow: mass-to-copies conversion, serial dilution planning for qPCR standard curves (typically 10-fold dilutions from 10⁸ to 10¹ copies), genome copy calculation for complex templates, and back-calculation from target copy number to required mass. It supports dsDNA, ssDNA, and RNA with appropriate molecular weight factors.

When This Page Helps

Accurate copy number calculation underpins the reliability of qPCR quantification, standard curve preparation, and NGS library pooling. Errors in this step propagate through the entire experiment — an incorrect standard curve turns every subsequent quantification wrong.

How to Use the Inputs

1. Enter DNA concentration (ng/µL) and sequence length (bp)
2. Select nucleic acid type (dsDNA, ssDNA, or RNA)
3. Review the calculated copies per µL
4. Use the serial dilution planner for qPCR standards
5. Set a target copy number to calculate the required mass
6. For genome copies, enter the full genome size
7. Generate a dilution scheme for standard curves

Example Calculation

Tips & Best Practices

• Always linearize plasmid standards — supercoiled DNA amplifies with different efficiency than linear targets
• Use carrier DNA (salmon sperm, tRNA) in dilute standards (<10⁴ copies/µL) to prevent adsorption to tube walls
• Prepare serial dilutions in low-bind tubes with 0.1% Tween-20 or BSA for low-copy standards
• Verify your standard curve: efficiency should be 90-110%, R² > 0.99
• For absolute quantification, consider digital PCR — it counts molecules directly without a standard curve
• Standard stocks should be aliquoted on first thaw — never refreeze and reuse dilute standards

qPCR Standard Curve Best Practices

A reliable standard curve requires: **Known template**: linearized plasmid containing the target amplicon, or synthetic gBlock/IDT gene fragment. **Accurate quantification**: measure by triplicate spectrophotometry or fluorometry. **Serial dilution**: use calibrated pipettes and fresh tips; vortex each dilution 5 seconds before taking the next aliquot. **Range**: cover the expected sample range, plus 1-2 logs above and below. **Replicates**: run each standard in triplicate. **Fresh working dilutions**: prepare from concentrated stock each experiment day.

Digital PCR and Absolute Copy Number

Digital PCR (dPCR) partitions a sample into thousands of individual reactions, each containing zero or a few template molecules. After amplification, positive partitions are counted and Poisson statistics calculate the absolute copy number — no standard curve needed. Platforms: Bio-Rad QX200 (droplet dPCR, ~20,000 partitions), Thermo QuantStudio 3D (chip-based, 20,000 partitions), Stilla Naica (Crystal Digital PCR, 30,000 droplets). dPCR is the gold standard for rare mutation detection, copy number variation analysis, and NGS library quantification.

Genome Size Reference Table

Common genome sizes for copy number calculations: **E. coli**: 4.6 Mb. **S. cerevisiae**: 12.1 Mb. **C. elegans**: 100 Mb. **D. melanogaster**: 180 Mb. **A. thaliana**: 135 Mb. **M. musculus (mouse)**: 2.7 Gb. **H. sapiens**: 3.2 Gb. **Wheat (T. aestivum)**: 17 Gb. For metagenomic samples, the average bacterial genome size (~4.7 Mb) can estimate total bacterial genomes from total DNA mass.

Frequently Asked Questions

• How many copies do I need for a qPCR standard curve?

  A typical qPCR standard curve uses 5-8 points in 10-fold serial dilutions: 10⁸, 10⁷, 10⁶, 10⁵, 10⁴, 10³, 10², 10¹ copies per reaction. Start with at least 10⁸ copies in the top standard. Most assays are linear from 10⁷ down to 10-100 copies, depending on efficiency and background.

• Should I use plasmid or PCR product for qPCR standards?

  Linearized plasmid standards are preferred because: (1) they can be accurately quantified by spectrophotometry, (2) the known copy number is traceable, (3) they're stable long-term when aliquoted and frozen. PCR products work but are harder to quantify accurately and may contain primer dimers that inflate concentration readings.

• Why multiply by 660 for dsDNA?

  The average molecular weight of a nucleotide pair (base pair) in dsDNA is approximately 660 Daltons (g/mol). This accounts for both strands: ~330 Da per nucleotide × 2 strands = 660 Da per bp. This is an average — the actual value depends on base composition (GC content), but 660 is accurate to within 1-2% for most sequences.

• How do I convert genome copies to gene copies?

  For single-copy genes: genome copies = gene copies. For multicopy genes (e.g., rRNA genes, which exist in 5-10+ copies per genome), multiply genome copies by the gene copy number. Bacterial genomes typically have 1-15 rRNA operons. The 16S rRNA gene averages ~4.2 copies per bacterial genome.

• What concentration should my qPCR standard stock be?

  Prepare a high-concentration stock at ~10¹⁰ copies/µL and serially dilute. For a 5 kb plasmid, this requires about 55 ng/µL — easily achievable from a miniprep. Aliquot the stock to avoid freeze-thaw degradation, and prepare fresh working dilutions for each experiment.

• How many copies in 1 ng of human genomic DNA?

  The human genome is ~3.2 × 10⁹ bp (haploid). MW = 3.2 × 10⁹ × 660 = 2.11 × 10¹² g/mol. Copies per ng = (1 × 10⁻⁹ × 6.022 × 10²³) / 2.11 × 10¹² ≈ 285 haploid copies. For diploid: ~143 cell equivalents per ng. This is why genotyping typically uses 10-100 ng per reaction.