ssDNA (oligo) concentration converter
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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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
- Is pmol/µl the same as µM?
- 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.
- How do I resuspend a lyophilised primer to 100 µM?
- 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.
- Does this apply to RNA oligos?
- 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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