Calculators
ng to pmol
Convert a mass of DNA or RNA into picomoles, femtomoles and copy number, with the molecular weight derived from sequence length. For ligation, transformation and qPCR standards.
Molecular weight uses average base masses and is approximate below roughly one hundred bases. Exact base-composition weights for oligos will come with the primer tool.
Formula
n (mol) = m (g) ÷ MW (g/mol)
- m
- mass of the sample
- MW
- molecular weight, derived from the length
- dsDNA
- MW ≈ n × 607.4 + 157.9
- ssDNA
- MW ≈ n × 303.7 + 79.0
- RNA
- MW ≈ n × 320.5 + 159.0
Worked example
How many picomoles is 500 ng of a 5000 bp fragment?
- m = 500 ng = 5 × 10⁻⁷ g
- n = 5000 bp, double-stranded
- Molecular weight: 5000 × 607.4 + 157.9 ≈ 3,037,158 g/mol
- Moles: 5 × 10⁻⁷ ÷ 3,037,158 ≈ 1.65 × 10⁻¹³ mol
- To picomoles: multiply by 10¹²
- Copy number: multiply the moles by Avogadro's number, 6.022 × 10²³
Answer 0.165 pmol and roughly 9.9 × 10¹⁰ copies
Common mistakes
- Using the double-stranded formula for an oligo. Oligonucleotides are single-stranded and weigh about half as much per base, so the wrong choice puts the answer out by a factor of two.
- Trusting average base weights for very short sequences. Below roughly one hundred bases the actual base composition matters and an exact calculation should be used.
- Entering the whole plasmid length instead of the insert length, or the reverse, when setting up a ligation ratio.
- Forgetting that a NanoDrop reading includes RNA and free nucleotides; the mass you enter should belong to the fragment you care about.
Frequently asked
How do I set a vector to insert ratio?
Convert each mass to picomoles separately, then set the molar ratio. Sticky-end ligations commonly use one part vector to three parts insert by moles.
What is copy number useful for?
It is what you need to build an absolute standard curve for qPCR. Knowing the copies in each dilution lets you estimate target copies in an unknown sample.
Does this hold for a circular plasmid?
Yes. Molecular weight depends on the number of base pairs, not on the shape of the molecule, so a circular plasmid and a linear fragment of the same length weigh the same.