About the DNA Concentration Calculator
This DNA concentration calculator turns a UV absorbance reading at 260 nm into a nucleic acid concentration. Choose double-stranded DNA, single-stranded DNA, RNA or a custom extinction factor, enter the A260 value, any dilution you made and the cuvette path length, and it returns the concentration in ng/µL (equivalent to µg/mL) plus the total yield in your sample volume.
Add the A280 and A230 readings to check purity. A 260/280 ratio near 1.8 is generally accepted as pure DNA and about 2.0 as pure RNA; lower values suggest protein or phenol contamination. A 260/230 ratio of roughly 2.0–2.2 indicates little carry-over of guanidine salts, carbohydrates or EDTA.
The method relies on the Beer–Lambert law with the standard conversion factors: an A260 of 1.0 in a 1 cm path corresponds to about 50 µg/mL dsDNA, 33 µg/mL ssDNA and 40 µg/mL RNA. Readings are most reliable between roughly 0.1 and 1.0 absorbance units; dilute concentrated samples and blank against the same buffer. Microvolume instruments already normalise to a 10 mm path, so leave the path length at 1 cm for their readouts.
With the default inputs, the concentration is 210 ng/µL. Change any value above to recalculate instantly.
How to use the dna concentration calculator
- 1Blank the spectrophotometer with the same buffer your sample is in.
- 2Measure A260 (and A280/A230 if available).
- 3Choose the nucleic acid type and enter the absorbance readings.
- 4Enter the dilution factor, path length and total sample volume.
- 5Read the concentration, total yield and purity ratios.
Formula and method
By the Beer–Lambert law, absorbance is proportional to concentration and path length. For nucleic acids at 260 nm the proportionality is usually expressed as a conversion factor: 1.0 A260 unit in a 1 cm cuvette equals about 50 µg/mL of double-stranded DNA, 33 µg/mL of single-stranded DNA or 40 µg/mL of RNA. Multiplying by the dilution factor gives the concentration of the original sample, and dividing by the path length corrects for cuvettes that are not 1 cm.
Because 1 µg/mL equals 1 ng/µL, the result is in both units. Total yield = concentration × volume. The 260/280 and 260/230 ratios compare nucleic-acid absorbance with protein and organic-contaminant absorbance.
- A260
- Absorbance at 260 nm, blank-corrected
- factor
- µg/mL per absorbance unit (50 dsDNA, 33 ssDNA, 40 RNA)
- DF
- Dilution factor of the measured solution
- l
- Path length in cm
Worked examples
Plasmid miniprep diluted 1:10
A260 = 0.42 × 50 µg/mL × 10 = 210 ng/µL. In 50 µL that is 10.5 µg of plasmid. The 260/280 of 0.42 ÷ 0.227 = 1.85 and 260/230 of 2.10 indicate a clean prep.
Total RNA, undiluted on a microvolume reader
For RNA the factor is 40, so 1.25 × 40 = 50 ng/µL, and 30 µL holds 1.5 µg. A 260/280 of 2.05 is typical of pure RNA.
ssDNA in a 0.5 cm cuvette
0.30 × 33 × 20 = 198 µg/mL for a 1 cm path; halving the path doubles it to 396 ng/µL. 100 µL of sample contains 39.6 µg of single-stranded DNA.
Frequently asked questions
How do you calculate DNA concentration from A260?+
Multiply the A260 reading by 50 µg/mL for double-stranded DNA (33 for ssDNA, 40 for RNA), then by any dilution factor. For a 1 cm path the result is the concentration in µg/mL, which is the same as ng/µL.
What is a good 260/280 ratio?+
About 1.8 is generally accepted as pure DNA and about 2.0 as pure RNA. Values noticeably lower suggest protein, phenol or other contaminants that absorb near 280 nm.
What does a low 260/230 ratio mean?+
Pure nucleic acid typically gives 2.0–2.2. Lower values indicate contaminants absorbing at 230 nm such as guanidine salts from spin columns, carbohydrates, EDTA or phenol. An extra wash or ethanol precipitation can help.
Why is the factor 50 for dsDNA but 40 for RNA?+
Single-stranded nucleic acids absorb more UV per microgram than double-stranded DNA because their bases are not stacked in a helix (the hyperchromic effect). So less RNA or ssDNA gives the same absorbance.
What factor should I use for oligonucleotides?+
Short oligos vary with sequence; 33 µg/mL is a rough average for ssDNA, but the most accurate approach is to use the extinction coefficient supplied by the oligo manufacturer and enter the equivalent factor as a custom value.