ssDNA (oligo) concentration converter
Maintained by the Talindrew team · Last updated
To convert an oligo concentration from µg/ml to pmol/µl, multiply by 1,000 and divide by the oligo length times 330 g/mol per nucleotide: pmol/µl = (µg/ml) × 10³ / (N × 330). A 20-mer at 33 µg/ml is 5 pmol/µl (5 µM).
Result
Assumes an average of 330 g/mol per nucleotide (Promega BioMath convention). For exact oligo work, use the extinction-coefficient calculator instead.
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What it looks like
Formula
pmol/µl = (µg/ml) × 10³ / (N nt × 330)
µg/ml = (pmol/µl) × N nt × 330 / 10³
Source: Inverse of the same relation
Worked example
Given: A 20 nt primer measured at 33 µg/ml
- 1.Molar mass of the oligo: 20 nt × 330 g/mol/nt = 6,600 g/mol.
- 2.Convert: 33 µg/ml × 10³ / 6,600 = 5 pmol/µl.
- 3.pmol/µl is the same number as µM: the primer stock is 5 µM.
33 µg/ml of a 20-mer = 5 pmol/µl = 5 µM
How the calculation flows
Units & constants
| Average nt weight | 330 g/mol (Promega BioMath convention; ssDNA) |
|---|---|
| Handy identity | pmol/µl ≡ µM ≡ nmol/ml |
| Inputs | Concentration (µg/ml or pmol/µl), oligo length (nt) |
| Applies to | Primers, probes, ssDNA oligos |
| More exact method | Sequence-specific extinction coefficient (Cavaluzzi & Borer 2004) |
| Price | Free |
What is a 100 µM primer stock in µg/ml?
For a 20-mer, 100 µM (100 pmol/µl) is 660 µg/ml: 100 × 20 × 330 / 1,000. Primer vendors ship lyophilised oligos quantified in nmol; resuspending an oligo in (nmol × 10) µl of buffer gives the standard 100 µM stock regardless of length, which is why that shortcut works.
Why 330 g/mol instead of 660?
660 g/mol is the weight of a base pair, two nucleotides. A single-stranded molecule has one nucleotide per position, so the average is halved to 330 g/mol. Using the wrong constant is the most common error in oligo math and puts every downstream concentration off by exactly a factor of two.
When is the flat 330 g/mol average not good enough?
The average ignores base composition: an A is 313 Da and a G is 329 Da as monophosphates, so short or extreme-composition oligos deviate by a few percent. For quantifying oligos by absorbance, the accurate route is the sequence-specific extinction coefficient rather than any average weight: that calculator is linked below.
Frequently asked questions
Yes, exactly. 1 pmol/µl = 1 µmol/L = 1 µM. This calculator reports pmol/µl, and you can read it directly as micromolar when setting up PCR: a typical reaction uses primers at 0.2–0.5 µM final.
Multiply the nmol amount on the tube by 10 and add that many microlitres of buffer. 25 nmol resuspended in 250 µl gives 100 µM (100 pmol/µl), because 25 nmol / 250 µl = 0.1 nmol/µl = 100 pmol/µl.
Use 340 g/mol per nucleotide for single-stranded RNA instead of 330, the ribose adds mass. The formula shape is identical; this page's calculator is calibrated for DNA.
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