DNA Concentration Calculator
Use this DNA concentration calculator to turn a spectrophotometer A260 reading into a concentration in ng/µL. Enter the A260, choose dsDNA, ssDNA or RNA, and set the dilution factor — it returns the concentration.
Enter the A260 reading, the dilution factor, and the nucleic acid type to get the concentration in ng/µL.
How to calculate DNA concentration from A260
Nucleic acids absorb ultraviolet light at 260 nm, and the absorbance is proportional to concentration. Each type has a standard factor for an absorbance of 1.0:
The factor is 50 for double-stranded DNA, 33 for single-stranded DNA and 40 for RNA. The dilution factor accounts for any dilution before reading (use 1 if you measured the neat sample). A clean sample has an A260/A280 ratio near 1.8 for DNA.
Worked example
A double-stranded DNA sample reads A260 = 0.8 at a 10× dilution: concentration = 0.8 × 50 × 10 = 400 ng/µL.
What your A260/A280 ratio means
Most spectrophotometers report A260/A280 alongside concentration — it's a fast purity check worth reading before you trust the number:
| A260/A280 ratio | Interpretation |
|---|---|
| ~1.8 | Pure DNA |
| ~2.0 | Pure RNA |
| < 1.8 | Possible protein or phenol contamination |
| > 2.0 (expecting DNA) | Possible RNA contamination |
| < 1.6 | Significant contamination — consider re-purifying |
Related tools
To resuspend a dried oligo or primer to a target µM, use the resuspension calculator. To dilute your stock down to a working concentration, use the home dilution calculator.
Frequently asked questions
›How do I calculate DNA concentration from A260?
Multiply the A260 reading by the extinction factor for your nucleic acid and by the dilution factor: ng/µL = A260 × factor × dilution. For double-stranded DNA the factor is 50, so an A260 of 0.8 measured at a 10× dilution gives 0.8 × 50 × 10 = 400 ng/µL.
›Why is the factor 50 for dsDNA but 33 or 40 for others?
An absorbance of 1.0 at 260 nm corresponds to about 50 ng/µL of double-stranded DNA, 33 ng/µL of single-stranded DNA, and 40 ng/µL of RNA. The difference comes from how strongly each nucleic acid absorbs UV light, so pick the type that matches your sample.
›How do I convert ng/µL of DNA to nM?
Divide the mass concentration by the molar mass of the fragment. For dsDNA, molar mass ≈ length (bp) × 650 g/mol, so nM = (ng/µL × 10⁶) ÷ (length × 650). You need the fragment length; the ng/µL result here is the mass concentration to start from.
›Is this DNA concentration calculator free?
Yes. It runs entirely in your browser, needs no sign-up, and never sends your numbers to a server.
›What A260/A280 ratio means my DNA is pure?
Around 1.8 is the accepted benchmark for pure DNA. Lower values usually point to protein or phenol carryover from extraction; a ratio above 2.0 (when you expected DNA) can indicate RNA contamination.
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