The calculator
Four modes, one engine. Pick the direction of your calculation — results update with full show-your-work steps.
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What you can calculate here
DNA copy number calculator (ng/µL → copies)
Mode 1 above is the core DNA copy number calculator — a copies-per-µL calculator (often searched as 'copies per ul calculator'): enter concentration in ng/µL (or ng/ul), template length, and template type to get copies per µL (copies per ul) and total copies — the standard DNA copy number formula with Avogadro's number, fully worked.
Plasmid copy number calculator (ng → fmol / pmol)
Mode 4 is a dedicated plasmid copy number calculator: mass in ng plus plasmid length in bp gives femtomoles, picomoles, total copies, molecular weight, and nM concentration — everything a ligation or transfection protocol asks for.
ng to copy number — and back
Convert ng to copy number in Mode 1, or flip the direction in Mode 2: enter the copies you need and get the exact ng to pipette. Both directions use the same documented formula, so forward and reverse always agree.
RNA copy number calculator
Select the RNA template type (340 Da/nt) in Mode 1 for an RNA copy number calculator — e.g. 40 ng/µL of a 1500 nt transcript = 4.72 × 1010 copies/µL. ssDNA (330 Da/nt) works the same way.
NEB copy number calculator — same math, more modes
Searching for the NEB copy number calculator or Thermo Fisher DNA copy number and dilution calculator? They compute the same Avogadro formula (NEB fixes 660 Da/bp — available here as a one-click option). This page adds a reverse solver, a full qPCR dilution planner with serial intermediates, and documented constants, all client-side.
Worked examples (hand-checked)
Example 1 — ng/µL to copies/µL
A 5000 bp dsDNA plasmid prep measures 50 ng/µL (NanoDrop), eluted in 100 µL.
MW = 5000 × 650 = 3,250,000 g/mol.
copies/µL = 50 × 6.02214076×1023 / (5000 × 650 × 109) = 9.2648 × 109 copies/µL
Total = 9.2648×109 × 100 = 9.2648 × 1011 copies (5000 ng = 5 µg).
Example 2 — reverse: how much DNA is 106 copies?
For a qPCR standard curve you need 1,000,000 copies of a 3000 bp plasmid, diluted into 20 µL.
ng = 106 × 3000 × 650 × 109 / 6.02214076×1023 = 0.003238 ng
Working concentration = 0.003238 / 20 = 1.619 × 10−4 ng/µL (= 0.162 pg/µL). Use Mode 3 to plan the serial dilution from your stock.
Example 3 — qPCR dilution plan
Stock is 5×108 copies/µL; each reaction needs 2×106 copies in 5 µL, for 8 reactions (+10% overage).
Working mix: 8 × 1.1 × 5 = 44 µL at 2×106/5 = 4×105 copies/µL.
Pipetting 0.8 µL stock directly is below reliable range, so: 0.8 µL stock + 99.2 µL diluent → 4×106/µL, then 4 µL of that + 36 µL diluent → 44 µL working mix; pipette 5 µL per reaction.
Methodology — the exact formula
length (bp) × MW per bp (Da) × 109
Avogadro's number (6.02214076×1023, exact since the 2019 SI revision) converts moles to molecules. The denominator is the molar mass of one template molecule: length × average mass per base pair, with 109 converting grams to nanograms. Divide, and you get the number of molecules ("copies") in that mass.
Assumed mass per base (documented, not hidden)
| Template | Assumed mass | Notes |
|---|---|---|
| dsDNA | 650 Da/bp | Standard literature average |
| dsDNA (NEB) | 660 Da/bp | NEB/Thermo convention — results differ ≈1.5% |
| ssDNA | 330 Da/nt | Average nucleotide residue |
| RNA | 340 Da/nt | Average ribonucleotide residue |
These are averages: a GC-rich fragment is slightly heavier per base than an AT-rich one. The true molar mass of a known sequence can differ by a few percent — negligible next to pipetting and quantification error, but disclosed here because honest tools document their constants. A custom Da-per-base field is provided for published values.
Honest limitations
- Supercoiled vs linear: topology does not change the molecule count — one plasmid is one copy either way. It does change qPCR amplification efficiency, so a supercoiled standard can bias your standard curve; linearize standards for qPCR.
- 650 Da/bp is a rule of thumb, not a measurement of your fragment. For exact work with a known sequence, compute the true molecular weight from base composition.
- Copies ≠ amplifiable copies. Degraded, nicked, or inhibitor-bound molecules count in the math but may not amplify. qPCR "copy numbers" are functional estimates.
- Low-copy pipetting is Poisson-limited: below ~100 copies/µL, sampling noise dominates — aliquoting "10 copies" is a statistical statement, not a guarantee.
- Garbage in, garbage out: the copy number is only as good as your concentration input. A NanoDrop overestimates dirty preps; for standards, prefer Qubit dsDNA-specific quantification.
Where this tool starts (and our other tool ends)
This calculator starts from a concentration (ng/µL) and converts it to copy number (molecules). If you only have a spectrophotometer reading, first convert absorbance → concentration with our DNA Concentration Calculator (A260 → ng/µL, purity ratios, dilution factor), then paste the ng/µL here. Absorbance → concentration → copy number: a clean two-step pipeline with no overlap.
Frequently asked questions
The copy-number formula is: copies = (mass in ng × 6.02214076×10²³) / (length in bp × mass per bp in Daltons × 10⁹). The constant 6.02214076×10²³ is Avogadro's number — molecules per mole. The denominator is the molar mass of one template molecule. Example: 50 ng of a 5000 bp dsDNA plasmid → 50 × 6.02214076×10²³ / (5000 × 650 × 10⁹) = 9.2648×10⁹ copies. The calculator above applies this formula and shows each step.
To calculate DNA copy number from ng/ul, multiply your concentration by Avogadro's number and divide by the template's molar mass: copies/µL = (ng/µL × 6.02214076×10²³) / (length in bp × 650 × 10⁹) for dsDNA. A 50 ng/µL prep of a 5000 bp plasmid gives 9.2648×10⁹ copies/µL. Enter your NanoDrop or Qubit ng/µL reading in Mode 1 above — it also reports total copies for your elution volume.
650 Da/bp is the long-standing literature average for one dsDNA base pair (sodium salt). NEB and Thermo use 660 Da/bp in their copy-number tools — a convention difference of about 1.5%, smaller than typical pipetting and quantification error. This calculator defaults to 650 but offers the 660 NEB convention as a one-click option, plus ssDNA (330 Da/nt), RNA (340 Da/nt), and a custom field, so you can match whichever reference your lab protocol cites.
Rearrange the formula: ng = (copies × length × Da per base × 10⁹) / 6.02214076×10²³. One million copies of a 3000 bp plasmid = 10⁶ × 3000 × 650 × 10⁹ / 6.02214076×10²³ = 0.003238 ng. Mode 2 (reverse solver) does this instantly and also tells you the ng/µL if you dilute into a chosen volume — the usual first step before planning a standard curve.
It depends on length, because longer molecules weigh more per copy. For dsDNA: 1 ng of a 1000 bp fragment ≈ 9.26×10⁸ copies; of a 5000 bp plasmid ≈ 1.85×10⁸ copies; of a 3-billion-bp human genome ≈ 308 copies. Rule of thumb: copies per ng ≈ 9.26×10¹¹ / length in bp (dsDNA, 650 Da/bp). Mode 1 computes the exact figure for any length.
Yes, with the right mass per base: use ~330 Da per nucleotide for ssDNA and ~340 Da per nucleotide for RNA instead of 650 Da/bp for dsDNA, and enter the length in nucleotides. Example: 40 ng/µL of a 1500 nt RNA transcript → 40 × 6.02214076×10²³ / (1500 × 340 × 10⁹) = 4.72×10¹⁰ copies/µL. Select the ssDNA or RNA template type in Mode 1.
Moles = mass (g) / molar mass (g/mol), with plasmid molar mass = length in bp × 650. So 100 ng of a 5000 bp plasmid = 10⁻⁷ g / 3.25×10⁶ g/mol = 30.77 fmol = 0.0308 pmol = 1.85×10¹⁰ copies. In 50 µL that is 0.615 nM (fmol/µL equals nM). Mode 4 reports fmol, pmol, copies, molecular weight, and nM in one step.
First convert your stock to copies/µL (Mode 1), decide copies per reaction and the µL added per reaction (Mode 2 reverses this), then let Mode 3 plan the pipetting: it computes the working mix via C₁V₁ = C₂V₂ and inserts serial intermediate dilutions whenever the direct pipetting volume would fall below 0.5 µL. Example: 5×10⁸ copies/µL stock → 2×10⁶ copies/reaction in 5 µL × 8 reactions gives 0.8 µL stock + 99.2 µL diluent → 4×10⁶/µL, then 4 µL + 36 µL → 44 µL working mix at 4×10⁵/µL.
They are two halves of one pipeline. The DNA concentration calculator turns a spectrophotometer reading into a concentration (absorbance A260 → ng/µL, with purity ratios and dilution factor). This copy-number calculator starts where that one ends: it turns a concentration (ng/µL) into a molecule count (copies) using Avogadro's number and template length. If you only have an A260 value, run the concentration tool first, then paste the ng/µL here.
Searching for 'copy number calculator neb' lands on NEB's tool: a single thin input (mass + length → copies, fixed 660 Da/bp) and Thermo's DNA Copy Number & Dilution Calculator covers the basic conversion plus a simple dilution. This calculator adds: four modes (forward, reverse solver, full dilution planner with serial intermediates, plasmid fmol/pmol), dsDNA/ssDNA/RNA/custom templates with documented constants, show-your-work steps for every result, three hand-checked worked examples, honest limitations (supercoiled vs linear, Poisson noise), and 100% client-side computation with no data leaving your browser.
The molecule count is identical — one plasmid molecule is one copy regardless of topology. What differs is behavior in the assay: supercoiled plasmids amplify with different efficiency in qPCR, which can bias a standard curve made from supercoiled standards. Best practice is to linearize plasmid standards (restriction digest) before quantifying and diluting them. The calculator's math is topology-independent; your qPCR is not.
Three usual suspects beyond arithmetic: (1) the input concentration — NanoDrop overestimates impure preps, so quantify standards with a dsDNA-specific dye (Qubit); (2) amplifiable vs total copies — nicked or degraded molecules count in the math but not in the reaction; (3) pipetting at low copy numbers is Poisson-limited — diluting to '10 copies/µL' means some aliquots contain 6 and some 14. Validate the curve empirically (efficiency 90–110%, R² ≥ 0.99) rather than trusting the calculation alone.
To answer 'what is copy number' in molecular biology: it is the number of DNA molecules in your tube. No — same phrase, different field. Here 'copy number' means the number of DNA molecules in your tube, computed from mass via Avogadro's number. In genomics, 'copy-number variation/variants (CNV)' means segments of the genome that are deleted or duplicated between individuals. This tool does the molecular-biology calculation, not genomic CNV analysis.
Yes. Use Mode 4 or Mode 1 with the ssDNA template type (330 Da/nt) and the oligo length in nucleotides. For example, 100 ng of a 20-mer primer: MW = 20 × 330 = 6600 g/mol → 10⁻⁷/6600 = 15.15 pmol. For the most exact oligo quantification from an A260 reading, a sequence-specific extinction coefficient is better — see our DNA concentration calculator's oligo mode.
Nothing leaves your browser. All four modes run entirely in JavaScript on this page — no server, no analytics on your inputs, no uploads. You can verify this by disconnecting from the internet after the page loads: every calculation keeps working. That is also why the tool works identically offline in the lab.