Last updated: June 19, 2026
DNA Copy Number Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
The DNA Copy Number Calculator converts a DNA or RNA mass into the number of template molecules using Avogadro's number (6.022 × 10²³). Copies = (mass × Nₐ) ÷ (length × MW per base), where MW per residue is 660 g/mol for double-stranded DNA, 330 g/mol for single-stranded DNA, and 340 g/mol for RNA. Typical references: 1 ng of a 100 bp dsDNA amplicon ≈ 9.1 × 10⁹ copies; 100 ng of a 3,000 bp plasmid ≈ 3.0 × 10¹⁰ copies. Optional volume input returns copies/µL — the unit qPCR and ddPCR consume directly.
Copies equal mass in grams times Avogadro's number divided by length times molecular weight per base. For dsDNA use 660 grams per mole per base pair. For example, 1 nanogram of a 100 base-pair amplicon contains about 9.1 billion copies; 100 nanograms of a 3,000 base-pair plasmid contains about 30 billion copies.
Key Takeaways
- copies = (mass × Avogadro) ÷ (length × MW per base) — Nₐ = 6.022 × 10²³
- MW per residue: 660 g/mol per bp (dsDNA), 330 per nt (ssDNA), 340 per nt (RNA)
- 1 ng of a 100 bp dsDNA amplicon ≈ 9.1 × 10⁹ copies — useful qPCR sanity check
- 100 ng of a 3,000 bp plasmid ≈ 3.0 × 10¹⁰ copies (≈ 50 amol)
- Use Qubit (not NanoDrop) to mass-quantify qPCR / ddPCR standards — accuracy matters
- For ligation, work in copies/molar ratios — 3:1 insert:vector for sticky ends, 5:1 for blunt
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
copies = (mass × Nₐ) ÷ (length × MW)
Where:
- N=Number of template copies(copies)
- m=DNA / RNA mass(g)
- N_A=Avogadro's number (6.022 × 10²³)(molecules / mol)
- L=Template length(bp (dsDNA) or nt (ssDNA / RNA))
- MW_{base}=Average MW per base (dsDNA = 660; ssDNA = 330; RNA = 340)(g / mol)
Worked Examples
qPCR standard curve — 1 ng of a 100 bp amplicon
A typical qPCR standard. Convert a known mass into starting copy number for a serial dilution.
- 1MW(template) = 100 bp × 660 g/mol = 66,000 g/mol
- 2copies = (1 × 10⁻⁹ g × 6.022 × 10²³) ÷ 66,000 g/mol
- 3 = 9.124 × 10⁹ copies in 1 ng
- 4Serial-dilute 1:10 seven times to span 10⁹ → 10² copies/reaction
Plasmid prep — 100 ng of a 3,000 bp plasmid
Convert miniprep yield into molecule count for ligation reactions or transfection.
- 1MW = 3,000 × 660 = 1.98 × 10⁶ g/mol
- 2copies = 100 ng × 6.022 × 10¹⁴ ÷ 1.98 × 10⁶
- 3 ≈ 3.04 × 10¹⁰ copies
- 4In 50 µL → 6.08 × 10⁸ copies / µL
ssDNA oligo — 100 ng of a 20-nt primer
Short single-stranded oligo. Use 330 g/mol per nt (half of dsDNA per residue).
- 1MW = 20 × 330 = 6,600 g/mol
- 2copies = 100 × 6.022 × 10¹⁴ ÷ 6,600
- 3 ≈ 9.124 × 10¹² copies (≈ 15 pmol)
In-vitro transcribed RNA — 1 µg of a 1,000 nt mRNA
RNA standard for RT-qPCR — convert mass to copies of full-length transcript.
- 1MW = 1,000 × 340 = 3.4 × 10⁵ g/mol
- 2copies = 1,000 × 6.022 × 10¹⁴ ÷ 3.4 × 10⁵
- 3 ≈ 1.77 × 10¹² copies
- 4In 100 µL → 1.77 × 10¹⁰ copies / µL
Introduction
The DNA Copy Number Calculator converts a DNA or RNA mass into the number of template molecules in the sample, using Avogadro's number (6.022 × 10²³ molecules/mol) and the average molecular weight per base. This is the conversion you need to build a qPCR standard curve, set up a digital-droplet PCR (ddPCR) standard, calculate insert:vector molar ratios for ligation, or work out how much template to load into NGS library prep. It supports double-stranded DNA (660 g/mol per bp), single-stranded DNA / oligos (330 g/mol per nt) and RNA (340 g/mol per nt). For sample mass, pair it with the DNA Concentration Calculator; for PCR-related work, see the Annealing Temperature Calculator and qPCR Efficiency Calculator.
The Formula — Mass to Molecules in One Step
Avogadro's number is the bridge between mass-based laboratory measurements (ng / µg on a NanoDrop or Qubit) and the molecule-based counts that qPCR, ddPCR and ligation reactions actually 'see'. The full identity is just unit conversion:
Moles = mass (g) ÷ molecular weight (g/mol)
Copies = moles × Nₐ, where Nₐ = 6.022 × 10²³ molecules/mol
MW(template) = length × MW_per_base — 660 for dsDNA bp, 330 for ssDNA nt, 340 for RNA nt
Combining: copies = (mass × Nₐ) ÷ (length × MW_per_base)
With mass in ng (the lab default), the convenient form is copies = (ng × 6.022 × 10¹⁴) ÷ (length × MW_per_base)
The 660 g/mol value for dsDNA already accounts for both strands of the helix. Don't double-count by multiplying length by 2.
How to Use This Calculator (Step by Step)
Three required inputs — mass, length, type — and an optional volume to get copies/µL for downstream dilutions.
Measure the mass of your sample (NanoDrop or, preferably, Qubit for dsDNA standards). Enter in nanograms.
Enter the template length: plasmid size in bp for dsDNA; oligo length in nt for ssDNA or RNA.
Pick the nucleic acid type — the calculator switches the MW per residue (660 / 330 / 340).
Optional: enter the sample volume in µL to see copies/µL alongside total copies.
Read the result. For a qPCR standard curve, take the copies value and serial-dilute 1:10 to span ~7 orders of magnitude (10⁸ → 10²).
Average MW per Base / Base-Pair
These are the standard values published in molecular biology references. They assume average base composition — actual MW varies a few percent with GC content but rarely matters for copy-number work.
| Nucleic acid | Average MW per residue | Units | Notes |
|---|---|---|---|
| dsDNA | 660 g/mol per bp | g/mol/bp | Includes both complementary strands (≈ 2 × 330) |
| ssDNA / oligo | 330 g/mol per nt | g/mol/nt | Use for primers, ASOs, ssDNA libraries |
| RNA | 340 g/mol per nt | g/mol/nt | Slightly heavier than ssDNA — uracil + ribose vs thymine + deoxyribose |
For precise oligo work (e.g. mass spectrometry or fluorescent labelling), use the actual molecular weight returned by your synthesis provider rather than these averages.
Building a qPCR Standard Curve
qPCR standard curves rely on knowing the copy number per reaction at each dilution. Get the maths wrong and your absolute quantification is off by orders of magnitude.
Pick a template matching your amplicon — a synthetic gBlock, plasmid carrying the target, or a purified PCR product.
Measure mass with Qubit (more accurate than NanoDrop for dsDNA standards).
Compute copies/µL with this calculator at the stock concentration.
Make a 10-fold serial dilution spanning ~7 logs — e.g. 10⁸, 10⁷, … 10² copies per reaction.
Run the standards in triplicate; calculate slope (-3.32 = 100% efficiency) and R² (≥ 0.98) of Cq vs log(copies).
Plug Cq values of unknown samples into the regression to back out absolute copy number.
For digital PCR (ddPCR), you don't need a standard curve at all — the partitioning gives absolute counts. But you still need this calculator to dilute your template into the partition's dynamic range (~10² – 10⁵ copies / 20 µL reaction).
Insert:Vector Molar Ratios for Ligation
Cloning protocols specify insert:vector ratios in moles (or copies) — not in nanograms — because what matters is the relative number of DNA ends in the reaction. This calculator gives you the conversion in one step.
Typical sticky-end ratio: 3:1 insert:vector (some protocols use 1:1 or 5:1 — test in parallel)
Typical blunt-end ratio: 5:1 insert:vector (lower efficiency, need more insert)
Take your vector mass (say, 50 ng of a 5,000 bp plasmid) → calculate copies
Multiply by the target insert ratio → insert copies needed
Back-calculate insert mass with the same calculator (insert copies × insert length × 660 ÷ Nₐ in g, then × 10⁹ to get ng)
Common Mistakes & Pitfalls
Most copy-number errors come from a small handful of recurring issues:
Wrong nucleic acid type — using 660 for ssDNA oligos overstates MW by 2× → understates copies by half
Double-counting strands for dsDNA — 660 already includes both; don't multiply by 2
Confusing nt with bp for ssDNA / RNA — a 100 nt ssDNA is half the mass per molecule of a 100 bp dsDNA fragment
Trusting NanoDrop for sub-ng quantification — use Qubit (sensitive to 10 pg/µL) for qPCR standards
Forgetting volume — copies/reaction is what qPCR/ddPCR actually consumes; the volume input gives you copies/µL directly
Ignoring GC bias — average MW is a few % off for very GC-rich or AT-rich amplicons; usually below experimental noise
Handy Reference Points
Useful checkpoints to sanity-check your numbers. Copy counts scale linearly with mass and inversely with length.
| Template | Mass | Length | ≈ Copies |
|---|---|---|---|
| qPCR amplicon | 1 ng | 100 bp | 9.1 × 10⁹ |
| Standard plasmid | 1 ng | 3,000 bp | 3.0 × 10⁸ |
| Plasmid prep | 100 ng | 3,000 bp | 3.0 × 10¹⁰ |
| Human genome | 3.3 pg | 3.3 × 10⁹ bp | 1 copy (haploid) |
| NGS library | 1 ng | 500 bp | 1.8 × 10⁹ |
| ssDNA primer | 100 ng | 20 nt | 9.1 × 10¹² |
Glossary
Quick definitions for the terms used in this calculator:
| Term | Definition |
|---|---|
| Copy number | Number of individual template molecules in a sample — what qPCR/ddPCR count. |
| Avogadro's number | 6.02214076 × 10²³ — the exact number of entities in one mole (SI definition since 2019). |
| Molecular weight (MW) | Mass of one mole in g/mol. For nucleic acids, length × average MW per residue. |
| bp | Base pair — one rung of the dsDNA ladder (two paired nucleotides). |
| nt | Nucleotide — one residue of a single-stranded DNA or RNA. |
| amol | Attomole — 10⁻¹⁸ mol; the convenient molar unit for qPCR / ddPCR copy ranges. |
| Standard curve | Serial dilution of a known template across log scales used to convert qPCR Cq → copy number. |
| ddPCR | Digital droplet PCR — partitions a reaction into ~20,000 droplets and counts positives for absolute quantification. |
Quick Reference Card
DNA Copy Number — Quick Reference
Quick reference • DNA Copy Number Calculator
copies = (ng × 6.022 × 10¹⁴) ÷ (length × MW per base) — MW = 660 dsDNA, 330 ssDNA, 340 RNAValid range: Useful 1 fg → 100 µg of nucleic acid; any length ≥ 1 bp / nt
Common Values
⚠ Watch Out
- •Use Qubit (not NanoDrop) for the mass measurement on qPCR/ddPCR standards
- •Don't double-count strands for dsDNA — 660 already covers both
- •Check that you entered nt for ssDNA/RNA and bp for dsDNA — easy to mix up
- •GC bias on average MW is usually < 5 % — only matters for very skewed sequences
Pro Tips
- →Build a 10-fold serial-dilution standard curve spanning 7 logs (10⁸ → 10²) for qPCR
- →For ligation, work in copies — 3:1 insert:vector for sticky ends, 5:1 for blunt
- →For ddPCR, dilute so 10–80 % of partitions are positive (≈ 10³ – 10⁵ copies / 20 µL)
- →Sanity check: 1 ng of a 1,000 bp dsDNA ≈ 9 × 10⁸ copies — same number, just shifted by length
FAQs
How do I calculate DNA copy number from a concentration?
Multiply your concentration (ng/µL) by the volume to get total ng of DNA, then divide by the molecular weight of the template (length in bp × 660 for dsDNA), and multiply by Avogadro's number (6.022 × 10²³). For example, 100 ng of a 3,000 bp plasmid ≈ 3.04 × 10¹⁰ copies.
Why is the molecular weight per bp 660 for dsDNA?
660 g/mol is the average mass of one base pair of double-stranded DNA, summed across both complementary strands and averaged over the four possible base-pair compositions (A-T and G-C). For single-stranded DNA, use 330 g/mol per nt (half of 660). For RNA, use ~340 g/mol per nt — slightly heavier due to the extra hydroxyl on ribose and the swap of thymine for uracil.
What's the difference between bp and nt?
'bp' (base pair) is for double-stranded nucleic acids — one bp = two paired nucleotides. 'nt' (nucleotide) is for single-stranded DNA, RNA or oligos — one nt = one residue. A 100 bp dsDNA fragment has the same mass per molecule as a 200 nt ssDNA strand.
How do I make a qPCR standard curve?
Quantify a pure template (synthetic gBlock or plasmid carrying your target) with Qubit. Use this calculator to get copies/µL at the stock concentration. Serial-dilute 1:10 across 7 logs (e.g. 10⁸ → 10² copies/reaction), run in triplicate, then fit Cq vs log(copies) — slope of -3.32 means 100% PCR efficiency, R² ≥ 0.98 is acceptable.
How many copies are in one human genome?
The haploid human genome is ~3.3 billion bp = ~3.3 pg of DNA. So one diploid cell contains ~6.6 pg of DNA in two copies of the genome. 1 ng of human gDNA contains about 150 haploid genome equivalents — useful for sizing input into multiplex qPCR or whole-genome library preps.
Should I use NanoDrop or Qubit to measure mass for copy-number work?
For qPCR / ddPCR standards, use Qubit. NanoDrop reads all UV-absorbing species (RNA, free nucleotides, degraded DNA) and can overestimate by 2× for impure samples — that propagates straight into your copy number. Qubit fluorometric quantification is dsDNA-specific and accurate down to ~10 pg/µL.
What insert:vector ratio should I use for ligation?
Conventional sticky-end ligations use a 3:1 insert:vector molar ratio; blunt-end ligations use 5:1. Pick a vector mass (e.g. 50 ng), compute copies with this calculator, multiply by the target ratio, then back-calculate the required insert mass for the same number of copies. The molar ratio matters because what counts is the number of DNA ends in the reaction.
Why does my qPCR standard curve give a slope different from -3.32?
-3.32 corresponds to 100% PCR efficiency (one doubling per cycle). Slopes between -3.6 and -3.1 (efficiency 90-110%) are usually acceptable. Outside that range, suspect: (a) inhibitors in the template, (b) wrong copy-number maths at the stock dilution, (c) pipetting errors in the serial dilution, or (d) sub-optimal primer design. Re-check your stock concentration measurement and the calculator inputs first.