Your spectrophotometer reports ng/µL. Ligation, library prep and qPCR standards want molar amounts or copies. The conversion depends on length, which is why the same 50 ng/µL means very different things for a primer and a plasmid.
From A260 to ng/µL
At 260 nm, an absorbance of 1.0 (1 cm path) corresponds to about:
| Nucleic acid | 1 A260 unit |
|---|---|
| dsDNA | 50 ng/µL |
| ssDNA | 33 ng/µL |
| RNA | 40 ng/µL |
Short oligos are better quantified with their own extinction coefficient, which depends on sequence. The supplier's sheet gives it.
Purity ratios
- A260/A280: about 1.8 for clean DNA, about 2.0 for clean RNA. Low values suggest protein or phenol.
- A260/A230: usually 2.0-2.2. Low values point to guanidine salts, EDTA or carbohydrate carry-over.
A good concentration with a bad A260/A230 can still fail a ligation. Fluorometric assays (dye-based) are more accurate for low concentrations and for libraries.
Want more options, saved recipes and warnings? Open the full Unit Converter (ng/µL) in BenchMate.
ng/µL to nM
650 g/mol is a common average mass for one base pair of dsDNA (some sources use 660, a 1.5% difference). For ssDNA use about 330 g/mol per base. For RNA use about 340 g/mol per base.
Copy number
This is the number you need for qPCR standard curves and for checking whether a dilution series actually reaches single copies.
Worked examples
| Sample | Conc. | Length | nM | copies/µL |
|---|---|---|---|---|
| PCR product | 50 ng/µL | 500 bp | 153.8 | 9.3 × 10¹⁰ |
| Plasmid | 100 ng/µL | 5000 bp | 30.8 | 1.9 × 10¹⁰ |
| Library | 2 ng/µL | 400 bp | 7.7 | 4.6 × 10⁹ |
For ligation, the usual vector:insert molar ratio is 1:3. With a 3 kb vector at 50 ng, a 1 kb insert needs about 50 ng (50 × 1/3 × 3 = 50 ng). Length does the work.
For library pooling, convert every library to nM using its own average fragment length from the bioanalyzer trace, not a guessed length. A wrong length gives a wrong pool.
BenchMate converts between ng/µL, nM and copies for dsDNA, ssDNA and RNA in one step.
Frequently asked questions
How do I convert ng/µL to nM for DNA?
For dsDNA, multiply ng/µL by 10⁶ and divide by length in bp times 650. A 500 bp product at 50 ng/µL is about 154 nM.
How do I calculate DNA copy number?
Copies/µL = (ng/µL × 6.022 × 10²³) ÷ (length in bp × 650 × 10⁹).
What is a good A260/A280 ratio?
About 1.8 for DNA and about 2.0 for RNA.
What does 1 A260 unit equal?
About 50 ng/µL for dsDNA, 33 ng/µL for ssDNA and 40 ng/µL for RNA with a 1 cm path.
22 core bench techniques, from molarity to sterilisation. Volume 1 of The Biotechnology Techniques Bible.