Article — DNA Copy Number Calculator
DNA copy number calculator: convert ng/µL to copies/µL
DNA copy number is calculated from concentration and template length: copies/µL = (ng/µL × 6.022 × 10²³) / (length × 650 × 10⁹) for double-stranded DNA. A 100 ng/µL solution of a 1000 bp template contains 9.13 × 10¹⁰ copies/µL. This DNA copy number calculator uses Avogadro's number and the 650 Da-per-base-pair conversion to convert mass to molecule count for qPCR standards, ddPCR, and library preparation.
Copy number is the per-molecule unit of nucleic acid concentration. While ng/µL tells you how much DNA is there by mass, copies/µL tells you how many template molecules a reaction will see. The two units are linked by the molecule's molecular weight — a 100 bp amplicon at 1 ng/µL has 10x the copy concentration of a 1000 bp amplicon at the same mass.
What is DNA copy number?
DNA copy number is the absolute count of identical DNA template molecules in a given volume. In a qPCR reaction containing 1000 copies of an amplicon, the first cycle theoretically produces 2000 copies, the second produces 4000, and so on. After 30 efficient cycles, the original 1000 copies become roughly 10¹² copies — and that exponential amplification only works because copy number, not mass, drives the kinetics.
The basic copy-number unit converts mass to molecules through three constants: Avogadro's number (6.022 × 10²³ molecules per mole), the average molecular weight per base pair (650 Da for dsDNA, 330 Da per nucleotide for ssDNA, 340 Da for RNA), and the conversion from ng to g (10⁻⁹). The formula is simply mass × molecules per mole ÷ molecular weight = total molecules, then divided by volume.
A single human cell contains about 6.4 picograms of genomic DNA. That works out to roughly 300 copies of any single-copy gene per nanogram of human genomic DNA. The seemingly tiny mass per cell hides an enormous information density — the DNA in one cell, stretched end to end, would be about 2 meters long.
The DNA copy number formula
The full DNA copy number formula combines Avogadro, the molecular weight, and unit conversions: N = (C × 10⁻⁹ × NA) / (L × MW). Where N is copies per microliter, C is concentration in ng/µL, NA is 6.022 × 10²³, L is template length in bp (or nt for ssDNA/RNA), and MW is the per-base molecular weight.
Avogadro NA 6.022 × 10²³dsDNA MW 650 Da/bpssDNA MW 330 Da/ntRNA MW 340 Da/ntdsDNA shortcut N ≈ ng × 9.13e11 / L_bpThe shortcut formula for dsDNA collapses the constants: copies/µL ≈ (ng/µL × 9.13 × 10¹¹) ÷ template length in bp. For mental math, remember that 1 ng of a 1 kb fragment contains about 9 × 10⁸ (a bit under a billion) copies. Scale linearly with concentration and inversely with length.
DNA copy number for qPCR standards
Real-time quantitative PCR (qPCR) measures the fluorescence crossing threshold (Ct or Cq) and converts it to copy number using a standard curve. The standard curve is a serial 10-fold dilution of a known template, typically spanning 10² to 10⁷ copies per reaction. Plotting Ct against log10(copies) should give a straight line with slope between -3.1 and -3.6, corresponding to 90 to 110 percent PCR efficiency.
Sample copy number falls out by interpolation: read the sample Ct, find the matching log(copies) on the curve, take 10 to the power. The accuracy of absolute quantification by qPCR depends on three things: well-prepared standards (this is where the DNA copy number calculator earns its keep), tight technical replicates, and a clean standard curve with R² > 0.99.
Below 10 copies per reaction, statistical sampling becomes the dominant error source. A reaction expected to contain 5 copies actually contains 5 only on average — some reactions get 2, others get 9, by Poisson distribution. Replicate scatter widens dramatically below 10 copies. Either use higher concentrations or switch to digital PCR.
DNA copy number in digital PCR
Digital droplet PCR (ddPCR) avoids standard curves entirely by partitioning a sample into 20,000 nanoliter-scale droplets and running PCR independently in each. Droplets that contain at least one target template molecule fluoresce; droplets without target stay dark. Poisson statistics convert the fraction of positive droplets back to copy number per microliter — no calibrator required.
ddPCR's main advantages over qPCR: absolute quantification without a standard curve, robust to PCR inhibition, single-copy sensitivity, and ±5 percent precision in the working range. Drawbacks: more expensive instrument (~$80k), narrower dynamic range (5 logs vs 8 for qPCR), and longer run time. ddPCR has become the gold standard for liquid biopsy, viral load monitoring, and rare mutation detection.
Copy Number Variation (CNV)
Beyond the laboratory unit, DNA copy number is a biological feature with clinical importance. Copy Number Variation (CNV) refers to the natural variation in copy number of specific DNA segments between individuals. The human genome contains thousands of polymorphic CNV regions, some of which underlie disease susceptibility, drug response, and immune diversity.
Examples: HER2 amplification (8 to 50 copies versus normal 2) is a key breast cancer biomarker and Herceptin response predictor. CYP2D6 gene copy number varies from 0 (poor drug metabolizers) to 13 (ultra-rapid metabolizers), affecting dosing of codeine, tamoxifen, and antidepressants. The 16p11.2 microduplication and microdeletion are well-characterized genomic disorders. Detection methods include FISH, MLPA, array-CGH, qPCR, ddPCR, and increasingly next-generation sequencing.
Building a qPCR standard curve
A reliable qPCR standard curve starts from a quantified stock of clean template — linearized plasmid, synthetic gBlock, or precisely measured PCR amplicon. Use this DNA copy number calculator to convert the stock concentration (ng/µL) to copies/µL, then serially dilute 10-fold in nuclease-free water with carrier (10 µg/mL yeast tRNA prevents adsorption to tube walls).
Aliquot the highest-concentration standard immediately and freeze at -80°C in low-binding tubes. Standards lose 5 to 10 percent activity per freeze-thaw cycle. A working dilution series can be used for 2 to 4 months from a single freeze. Re-verify with a new dilution series from the frozen master stock if Ct values drift.
Run each dilution in triplicate. Plot Ct on the Y axis, log10(copies per reaction) on the X axis. Slope -3.32 = 100 percent PCR efficiency (each cycle doubles the product). Slope -3.1 to -3.6 is acceptable (90 to 110 percent). Outside that range, redesign the primers or troubleshoot the chemistry. R² must be > 0.99 for the curve to be usable.
DNA copy number accuracy and limits
The DNA copy number calculation is mathematically exact given clean inputs. Error in practice comes from two sources: the concentration measurement and the template length assumption. NanoDrop concentration measurement is accurate within ±10 percent on clean samples; Qubit fluorometry tightens that to ±5 percent. Template length is exact for synthetic constructs and PCR amplicons but uncertain for fragmented genomic DNA or partial cDNA.
For applications where copy number precision matters absolutely — gene therapy product release, rare variant detection, viral load quantitation — ddPCR is now the reference. For routine standard curve construction, ng/µL × calculator-derived copies/µL is excellent. For relative quantification (comparing one sample to another), the ΔΔCt method on qPCR avoids absolute copy number entirely and only needs reliable internal controls.
- Main formula = copies/µL = (ng × NA) / (length × MW × 1e9)
- Avogadro's number = 6.022 × 10²³ mol⁻¹
- dsDNA MW = 650 Da per base pair
- 1 ng of 1 kb dsDNA = 9.13 × 10⁸ copies
- qPCR standard range = 10² to 10⁷ copies/reaction
- Slope target = -3.32 (100% efficiency)
- Acceptable slope = -3.1 to -3.6 (90–110%)
- Standard curve R² = > 0.99 required