DNA Copy Number Calculator
Convert DNA mass to copy number or copy number back to mass using fragment length and a clear molecular weight assumption.
Calculate DNA Mass, Copies, and Concentration
Assumption used in this calculator
How to use this calculator
Most competitor tools either stop at pmol or hide the copy-number math. This page keeps the workflow explicit.
Choose DNA type
Select dsDNA for plasmids, PCR products, and linear double-stranded fragments. Select ssDNA or ssRNA for single-stranded templates.
Enter length
Enter the fragment length in bp for dsDNA or nt for ssDNA/ssRNA.
Enter mass or copies
Choose the direction of conversion, then enter either DNA mass or copy number.
Add volume if needed
Optionally enter a volume to estimate copies/µL and ng/µL for your working stock.
Formula and variables
The calculator keeps the math explicit. For mass-to-copies, start with DNA concentration in ng/µL. For copies-to-mass, reverse the same relationship using the fragment length and the strand-specific weight.
C is concentration in ng/µL, L is fragment length, W is the average weight per bp or nt, and 10⁹ converts nanograms to grams.
Use the same length and weight assumption, then multiply by 10⁹ at the end to report the answer in ng.
Use the measured stock concentration in ng/µL when you want copies per microliter.
Use the full molecule length. For plasmids, that means backbone plus insert, not only the insert.
Use 660 g/mol per bp for dsDNA, 330 g/mol per nt for ssDNA, or 340 g/mol per nt for ssRNA unless you need sequence-specific molecular weight.
6.022×10²³ molecules/mol connects molecular weight to molecule count.
Assumptions and limits
This calculator intentionally uses a simple wet-lab approximation so the logic stays easy to audit.
- dsDNA is approximated as 660 g/mol per base pair.
- ssDNA is approximated as 330 g/mol per nucleotide.
- ssRNA is approximated as 340 g/mol per nucleotide.
- Exact molecular weights vary with base composition, end chemistry, and modifications.
Worked examples
The examples below mirror common lab questions: first calculate stock copies/µL, then reverse the formula when you already know the target number of molecules.
Example 1: 150 ng/µL of a 4.7 Mb dsDNA template
Measured stock concentration = 150 ng/µL, full template length = 4,700,000 bp, molecule type = dsDNA, so W = 660 g/mol per bp.
The stock contains about 2.91×10⁷ copies/µL.
This is the number you usually need before planning a standard curve, spike-in level, or absolute-quantification control.
Example 2: How much mass is 1×10⁹ copies of a 500 bp dsDNA fragment?
Target copies = 1×10⁹, fragment length = 500 bp, molecule type = dsDNA, so the same 660 g/mol per bp assumption applies.
You need about 0.548 ng of DNA.
The answer is tiny on purpose. For low-copy standards, you usually create them by dilution from a higher-concentration stock rather than trying to weigh the DNA directly.
After copy number: prepare the working dilution
Once you know your stock copies/µL, the next wet-lab question is usually how much stock and water to pipette. That step uses the same logic as the dilution calculator.
Use copy-number concentration as the concentration unit. Ctarget is the desired copies/µL, Vfinal is the final reaction or aliquot volume, and Cstock is your measured stock copies/µL.
Example 3: Make 10 µL at 2.0×10⁶ copies/µL
Use the stock from Example 1, so Cstock = 2.91×10⁷ copies/µL. Target concentration = 2.0×10⁶ copies/µL. Final volume = 10 µL.
Pipette 0.687 µL of stock and 9.313 µL of water.
If the stock volume becomes too small to pipette accurately, first make an intermediate dilution and then prepare the final working solution.
Practical reminders before you trust the result
For plasmids, use the full plasmid length. Using only the insert length will overestimate copy number.
Do not switch between dsDNA, ssDNA, and ssRNA assumptions across the same workflow. Your standard curve and reported values should use one convention.
If you need exact oligo molecular weight, account for base composition, modifications, and terminal chemistry in a dedicated sequence-aware tool.
Use the dilution calculator to prepare a working concentration once you know the target stock amount.
Open the Solution Dilution CalculatorCheck GC content when you also need sequence-level context for primer design or amplification behavior.
Open the GC Content CalculatorFAQ
Choose dsDNA for plasmids, PCR products, and other double-stranded fragments. Choose ssDNA for single-stranded templates, probes, or oligos.
Different tools use different molecular weight conventions. Some use a simple 660 g/mol per bp approximation, while others include base composition or terminal corrections.
No. Volume is only needed if you want concentration output such as copies per µL or ng per µL.
Use it as a fast estimate. For exact oligo calculations, include the actual sequence, modifications, and end chemistry in a dedicated molecular weight tool.
Lab notes
Keep the same DNA type assumption across your full standard curve so your dilution series remains internally consistent.
Use the full plasmid length, not only the insert length, when converting plasmid mass to copy number.
If a collaborator gives copies and you need mass, convert back with the same fragment length they used.
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This page was rebuilt after reviewing Omni Calculator's DNA copy number page, then reorganized around the user workflow used on our dilution page: formula first, real substituted numbers second, and dilution follow-up last.