Oligo and DNA/RNA Conversion Cheat Sheet
A bench reference for the conversions you reach for when quantifying nucleic acids: turning an A260 reading into a concentration, checking purity with A260/A280 and A260/A230 ratios, converting between ng and pmol, estimating copy number, and resuspending a freshly synthesised oligo to a target molarity. Exact figures depend on the sequence — see the note below.
Reading A260 / quantifying by UV
A spectrophotometer reads absorbance at 260 nm. One A260 unit (1 cm path length) corresponds to a different concentration depending on the molecule, because single- and double-stranded nucleic acids absorb differently.
| Nucleic acid | Concentration per A260 unit |
|---|---|
| dsDNA | ≈ 50 µg/mL per A260 |
| ssDNA (incl. oligos) | ≈ 33 µg/mL per A260 |
| ssRNA | ≈ 40 µg/mL per A260 |
The single-stranded oligo factor is sequence-dependent — the true value comes from nearest-neighbour extinction coefficients, so 33 µg/mL per A260 is a rule of thumb. Verify for your sequence when precision matters.
Purity ratios (A260/A280, A260/A230)
Absorbance ratios flag contaminants. A260/A280 catches protein and phenol; A260/A230 catches salts, solvents and carbohydrate carryover. They are diagnostic, not exact — values vary by source and instrument.
| Ratio | Expected (pure) | Low value suggests |
|---|---|---|
| A260/A280 | ~1.8 = pure DNA; ~2.0 = pure RNA | lower suggests protein / phenol contamination |
| A260/A230 | ~2.0–2.2 expected | lower suggests guanidine, phenol, EDTA or carbohydrate carryover |
ng ↔ pmol and copy number
When you only know the length, use these average molecular weights — the same values SeqBench's DNA Molarity tool uses.
| Nucleic acid | Average molecular weight |
|---|---|
| dsDNA | ≈ 650 g/mol per base pair |
| ssDNA | ≈ 330 g/mol per nucleotide |
| ssRNA | ≈ 340 g/mol per nucleotide |
The core conversions:
- pmol = (mass in ng × 1000) / molar mass (g/mol)
- molar mass of dsDNA ≈ length(bp) × 650 → pmol dsDNA ≈ ng × 1000 / (bp × 650)
- copies = moles × Avogadro (6.022 × 10²³); moles = (ng × 1e-9) / molar mass(g/mol)
- concentration: 1 pmol/µL = 1 µM; multiply by 1000 for nM
Worked example. 1 µg (1000 ng) of a 1 kb dsDNA fragment: 1000 × 1000 / (1000 × 650) ≈ 1.54 pmol.
Resuspending an oligo to a target concentration
Synthesised oligos ship dry with the yield given in nmol on the spec sheet. Because nmol/µL equals mM, you can hit a round stock molarity just by choosing the right water volume.
| Target stock | Water to add | Example |
|---|---|---|
| 100 µM | add (nmol × 10) µL water | 20 nmol → 200 µL |
| 1 mM (1000 µM) | add (nmol × 1) µL water | 20 nmol → 20 µL |
The logic: nmol/µL = mM. Dissolving an oligo's nmol amount in (nmol × 10) µL gives 0.1 mM = 100 µM; in (nmol × 1) µL it gives 1 mM = 1000 µM.
These average-mass shortcuts are close enough for routine work, but exact values need the actual sequence (nearest-neighbour). The DNA Molarity tool does the conversion precisely from your sequence.
Frequently asked questions
- How do I convert A260 to concentration?
- Multiply the A260 reading (at 1 cm path length) by the conversion factor for your molecule: about 50 µg/mL per A260 unit for dsDNA, 33 µg/mL for single-stranded DNA and oligos, and 40 µg/mL for ssRNA. So an A260 of 0.5 for dsDNA is roughly 25 µg/mL. The single-stranded factors are rules of thumb and are sequence-dependent.
- What does an A260/A280 of 1.8 mean?
- An A260/A280 ratio of about 1.8 indicates pure DNA; about 2.0 indicates pure RNA. A noticeably lower ratio suggests protein or phenol contamination, since those absorb strongly at 280 nm.
- How do I convert ng to pmol of DNA?
- Use pmol = (mass in ng × 1000) / molar mass in g/mol. For dsDNA the molar mass is about length(bp) × 650, so pmol ≈ ng × 1000 / (bp × 650). For example, 1 µg (1000 ng) of a 1 kb dsDNA fragment is about 1000 × 1000 / (1000 × 650) ≈ 1.54 pmol.
- How much water do I add to resuspend an oligo to 100 µM?
- Add (nmol × 10) µL of water — the nmol figure is on the synthesis spec sheet. For example, a 20 nmol oligo needs 200 µL to reach 100 µM. For a 1 mM (1000 µM) stock, add (nmol × 1) µL instead, so 20 nmol needs 20 µL.
- How do I calculate copy number?
- Convert mass to moles, then multiply by Avogadro's number: moles = (ng × 1e-9) / molar mass(g/mol), and copies = moles × 6.022 × 10²³. Knowing the fragment length lets you estimate molar mass as bp × 650 for dsDNA.
See also
Related tools and references
Use these related pages when this table raises a practical calculation or workflow question.