Copy Number Calculator: Calculate Gene Copies

This calculator estimates the number of copies of a DNA target in a sample from its measured mass and fragment length. Enter the DNA mass in nanograms and the target length in base pairs to get an estimated copy count and amount in moles. It assumes double-stranded DNA and uses an average molecular weight per base pair.

Formula

Copy number is the mass of DNA divided by the molar mass of one DNA molecule, then multiplied by Avogadro’s constant:

Copies = [mass (g) ÷ molecular weight (g/mol)] × 6.02214076 × 10²³ molecules/mol

For double-stranded DNA in this calculator, molecular weight is estimated as length in bp × 617.96 + 36.04 g/mol. The 617.96 g/mol per base pair convention represents the average anhydrous free-acid form of dsDNA, not the sodium salt. Mass entered in nanograms is converted to grams by multiplying by 10⁻⁹. The output called copy number is an estimated number of DNA molecules; the other output is the amount in moles.

Here, m is DNA mass in ng, bp is the length of one DNA molecule in base pairs, and 6.02214076 × 10²³ mol⁻¹ is the exact Avogadro constant. The formula treats each molecule as the same length and assumes the mass measurement is DNA from the target being counted.

Worked Example

Using the calculator’s default values, the sample contains 10 ng of a 1,000 bp double-stranded DNA fragment.

1. Calculate the estimated molecular weight: 1,000 bp × 617.96 g/mol per bp + 36.04 g/mol = 617,996.04 g/mol.
2. Convert the mass to grams: 10 ng × 10⁻⁹ g/ng = 1.0000 × 10⁻⁸ g.
3. Calculate moles: (1.0000 × 10⁻⁸ g) ÷ (617,996.04 g/mol) = 1.6181 × 10⁻¹⁴ mol.
4. Calculate copies: (1.6181 × 10⁻¹⁴ mol) × (6.02214076 × 10²³ copies/mol) ≈ 9,746,000 copies.

The calculator displays the copy number rounded to a whole copy and the amount to six decimal places. The unrounded amount is approximately 1.6181 × 10⁻¹⁴ mol.

Assumptions

• The DNA is double-stranded, and its length is known in base pairs.
• The entered mass is entirely the DNA fragment being counted, with no carrier, contaminant, or unrelated DNA included.
• The average dsDNA free-acid molecular-weight convention is used; sequence-specific base composition and salt form are not modeled.
• Each counted molecule is intact and has the entered length. This is a mass-based estimate, not a direct measurement of target copies.

What You’ll Need

• A DNA mass measurement in nanograms. If the measurement is in another unit, convert it before entering it.
• The length of the DNA molecule or amplicon in base pairs, not the number of bases in a single strand unless that is the intended molecule length.
• A clear definition of what counts as one copy: for example, one plasmid molecule or one amplicon molecule.
• For a biological sample, a reliable DNA quantification method and information about the target’s structure, purity, and assay design.

Step-by-Step

1. Define the target. Decide whether you are counting whole plasmid molecules, a linear DNA fragment, or a PCR product. Enter the length of that complete molecule. For a plasmid, include the entire plasmid length, not only the insert, if the mass represents intact plasmid.
2. Obtain the DNA mass. Use a measurement appropriate to the sample and record the amount in ng. If the sample contains mixed DNA, its total mass cannot automatically be treated as target DNA mass. A target-specific assay or a justified purity fraction may be needed.
3. Enter the mass and length. Use the calculator inputs. The defaults are only an example: 10 ng and 1,000 bp. Replace both with your sample values, and check that the length is in bp rather than kilobases.
4. Read the estimated copy number. The first output is the calculated number of molecules, rounded to a whole copy. Treat it as an estimate because input measurement error and the average molecular-weight model affect the result.
5. Use the mole output if needed. The second output is the amount of DNA in mol. It can be useful for stoichiometric comparisons, but it is not a concentration. To calculate mol/L, you also need the final sample volume in liters and must divide the amount by that volume.
6. Check the scope of the calculation. For a target sequence within a longer DNA molecule, counting whole molecules is not necessarily the same as counting target copies. If each molecule contains one target, the counts coincide; if it contains multiple target repeats, multiply by the number of target sites per molecule.

Common Mistakes

• Entering kilobases as base pairs. A 1 kb fragment is 1,000 bp. Entering 1 instead of 1,000 changes the estimated molecular weight and copy count substantially.
• Mixing up molecule and target counts. A plasmid molecule may contain a target insert, but the target count depends on how many target sites each molecule contains.
• Using total sample mass as target mass. RNA, proteins, salts, unrelated DNA, or other material can make a mass-based estimate too high if they contribute to the measured mass.
• Assuming all DNA has the same length. A mixture of fragment sizes does not have one exact molecular weight. The calculation is most direct for a purified, size-defined fragment.
• Confusing copy number with concentration. Copies describe a count; copies per microliter or mol/L require a defined volume.
• Overinterpreting precision. A whole-number display does not mean the assay is accurate to one molecule. Measurement uncertainty usually dominates rounding.

Tips

• Keep units beside your raw measurements and convert once. This calculator accepts ng and bp only.
• For double-stranded DNA, the commonly quoted 650 or 660 g/mol per bp convention is for the sodium-salt form. This calculator uses 617.96 g/mol per bp plus 36.04 g/mol for the anhydrous free-acid form; the estimated counts differ by about 6% between conventions.
• When comparing samples, use the same molecular-weight convention and ensure the mass refers to the same kind of DNA molecule.
• For quantitative PCR or digital PCR interpretation, distinguish the number of molecules loaded from the number of target copies measured by the assay. Assay efficiency, sampling, and target integrity can affect observed results.
• If bench work is involved, follow your lab method, the safety data sheet (SDS), and your supervisor.

Troubleshooting

• The result is unexpectedly tiny or huge: check whether mass was entered in ng and length in bp. Convert µg to ng by multiplying by 1,000; convert kb to bp by multiplying by 1,000.
• The calculator reports few copies despite a visible DNA band: verify the mass estimate, sample dilution, and whether the measured material is mostly the target fragment. A band’s brightness alone is not a precise mass measurement.
• The calculated count does not match a molecular assay: compare what each method measures. The calculation infers molecules from bulk mass, while an assay may count amplifiable target regions and be affected by degradation, inhibition, or target multiplicity.
• The sample contains several DNA lengths: do not apply one length to the entire mass unless the mixture’s average molecular weight is known. Separate or characterize the components, or report the result as a rough estimate with its limitation.
• You need copies per volume: divide the estimated copy count by the volume containing that mass, using consistent volume units. Include any dilution factor when converting back to the original sample.

Frequently Asked Questions

What does copy number mean?
It is the number of DNA molecules or target sites in a defined sample, depending on what is being counted.

Can I calculate copy number from concentration?
Yes. First multiply concentration by the sample volume to get DNA mass, then enter that mass and the molecule length. Keep the units consistent.

Does this calculator work for single-stranded DNA?
Not exactly. Its molecular-weight formula is for double-stranded DNA. Single-stranded DNA has a different average mass per base and should use an appropriate ssDNA molecular-weight estimate.

Can I use this for RNA?
No. RNA has a different molecular-weight model. Use an RNA-specific calculation based on its length and the relevant average mass per nucleotide.

Why is this only an estimate?
It relies on the accuracy of the mass and length inputs, assumes a uniform DNA species, and uses an average molecular weight rather than sequence-specific composition.

How do I calculate copies per microliter?
Divide the estimated total copies by the sample volume in µL. If the input mass came from a diluted aliquot, apply the dilution factor to report the original concentration.

Related

• Transcribe DNA to mRNA Calculator and Base-Pair Rules
• Transcribe DNA to RNA Calculator: Template to mRNA
• Protein Concentration Calculator: mg/mL to µM From Molecular Weight

in m | DNA mass | ng | 10 | 0
in bp | DNA length | bp | 1000 | 1
out copies | Copy number | copies | (m * 1e-9 / (bp * 617.96 + 36.04)) * 6.02214076e23 | 0
out mol | Amount of DNA | mol | m * 1e-9 / (bp * 617.96 + 36.04) | 6