DNA Concentration Calculator
Sample
Most accurate between 0.1 and 1.0
e.g. 10 if 2 µL went into 18 µL
Cuvette 1 cm; most micro-volume readers report 1 cm-normalised values
Concentration
25.0 ng/µL
= 0.025 µg/µL · 1.25 µg total in 50 µL
A260 × 50 × dilution 1 ÷ path 1 cm
Purity
- A260/A280 = 1.85
- In the expected range for pure dsDNA (1.8–2).
- A260/A230 = 2.17
- In the expected range (about 2.0–2.2).
Calculate the concentration of double- or single-stranded DNA, RNA or an oligo from its absorbance at 260 nm, with dilution and path length, the total amount in your sample, and purity ratios A260/A280 and A260/A230.
Formula
Worked Example
An A260 of 1.0 corresponds to 50 ng/µL of double-stranded DNA, so 0.50 gives 25 ng/µL, and 50 µL holds 1.25 µg. Both purity ratios are in the expected ranges for clean DNA.
Frequently Asked Questions
What A260/A280 ratio means pure DNA?
About 1.8 for DNA and 2.0 for RNA. A lower ratio suggests protein or phenol contamination, which absorbs near 280 nm. The ratio also shifts with the pH of the solution, so measure in a buffer such as TE.
What does a low A260/A230 ratio mean?
Contamination with something that absorbs at 230 nm: guanidine salts from column kits, EDTA, carbohydrates or phenol. Pure nucleic acids usually give 2.0–2.2.
Why is the conversion factor different for RNA and oligos?
Single strands absorb more light per microgram than double strands, so 1.0 absorbance unit means 50 ng/µL of dsDNA, 40 of RNA and about 33 of ssDNA. For short oligos the factor depends on the sequence, so use the extinction coefficient from the supplier.