Nucleic acids · Quantification
A260 DNA and RNA concentration calculator.
Convert A260 to concentration for dsDNA, ssDNA or RNA, with total yield and an A260/A280 purity check.
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How to convert A260 to concentration
Nucleic acids absorb at 260 nm, and for a 1 cm path an absorbance of 1.0 corresponds to about 50 ng/uL of double-stranded DNA, 40 ng/uL of RNA, and 33 to 37 ng/uL of single-stranded DNA (Thermo's NanoDrop guide uses 33 and its molecular-weight table lists 37). The calculation is concentration = (A260 / path length) x factor x dilution factor, and ng/uL is the same as ug/mL.
Worked example. A260 of 0.5 on dsDNA, undiluted: 0.5 x 50 = 25 ng/uL. In 50 uL that is 25 x 50 / 1000 = 1.25 ug.
Purity. Pure DNA reads an A260/A280 of about 1.8 and pure RNA about 2.0; clearly lower values suggest protein or phenol. The ratio depends on the pH and ionic strength of the buffer (Thermo notes acidic buffers read 0.2 to 0.3 low and basic buffers 0.2 to 0.3 high), only checks for some contaminants, and says nothing about whether the sample is intact. Short oligonucleotides need an oligo-specific factor; the general ssDNA 33 is not reliable for them. For molar amounts, use the DNA copy number calculator.
Frequently asked questions
What is the A260 conversion factor for DNA?
About 50 ng/uL per 1.0 A260 for double-stranded DNA, 40 for RNA, and 33 to 37 for single-stranded DNA.
What should the A260/A280 ratio be?
About 1.8 for pure DNA and about 2.0 for pure RNA. Much lower values point to protein or phenol contamination.
Why is a reading above 1.0 not reliable?
Absorbance becomes non-linear at high values. Dilute the sample, read again and enter the dilution factor.
Why does my buffer change the A260/A280 ratio?
The ratio depends on pH and ionic strength. Blank with the same buffer you dissolved the sample in, and compare ratios only between samples in the same buffer.
Is ng/uL the same as ug/mL?
Yes. 1 ng/uL = 1 ug/mL = 1000 ng/mL.
Sources
Checked
- Thermo Fisher, NanoDrop Nucleic Acid Technical Guide (factors 50, 33, 40 ng-cm/uL; A260/A280 about 1.8 for DNA and 2.0 for RNA; pH effect on the ratio)
- Thermo Fisher, DNA and RNA molecular weights and conversions (1 A260 dsDNA = 50, ssRNA = 40, ssDNA = 37 ug/mL)
- Thermo Fisher, Accurate Oligonucleotide Quantification with NanoDrop Spectrophotometers (TN52607: the general 33 factor is not reliable for oligos)
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Research use and scope
This is arithmetic for research-bench planning, not clinical, diagnostic or safety advice. Conversion factors are standard approximations; strain, instrument, reagent and protocol differences matter. Check results against your own protocol, kit manual and lab supervisor, and follow your institution's biosafety rules.