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DNA and RNA Concentration Calculator (A260)

dsDNA, ssDNA and RNAPurity ratios explainedRuns in your browser

This calculator converts an A260 reading into nucleic acid concentration: concentration (µg/mL, equal to ng/µL) = A260 x conversion factor x dilution factor ÷ path length in cm. The factors are 50 for double-stranded DNA, 33 for single-stranded DNA and 40 for RNA. Add A280 and A230 readings to check purity, and a fragment length for dsDNA molarity.

A260 reading
Dilution factor
1 if you measured the sample undiluted.
Path length
Microvolume spectrophotometers usually report values normalized to 1 cm.
A280 reading (optional)
A230 reading (optional)
Sample volume (optional)
Fragment length for molarity (optional)

Runs in your browser. Nothing you type is saved or sent.

Result
Enter values
Enter the A260 reading and choose the sample type.

concentration (µg/mL) = A260 × conversion factor × dilution factor ÷ path length (cm)

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How do you calculate DNA concentration from A260?

One A260 unit, measured through a 1 cm path, corresponds to about 50 µg/mL of double-stranded DNA, 33 µg/mL of single-stranded DNA and 40 µg/mL of RNA (Thermo Fisher NanoDrop documentation). Multiply the reading by the factor and by any dilution, and divide by the path length if it is not 1 cm.

Worked example: dsDNA diluted 10-fold reads A260 = 0.412 in a 1 cm cuvette. Concentration = 0.412 x 50 x 10 = 206 ng/µL. Microvolume instruments usually report readings already normalized to a 1 cm path.

What is a good A260/280 ratio?

About 1.8 is generally accepted as pure for DNA and about 2.0 for RNA; A260/230 is commonly 2.0 to 2.2. Lower ratios point to contamination, such as protein or phenol for 260/280 and salts or other reagents for 260/230 (Thermo Fisher Technical Note 52646). These are rules of thumb: a good ratio does not prove the sample is intact, and the ratios shift with pH.

How do you convert ng/µL to nM?

For double-stranded DNA, molecular weight is about (length in bp x 617.96) + 36.04 g/mol, and nM = concentration (ng/µL) ÷ molecular weight x 10⁶. Enter the fragment length and the calculator does it. For oligonucleotides, use the sequence-specific extinction coefficient rather than a fixed factor.

Frequently asked questions

Why is the conversion factor for dsDNA 50?

Because a 50 µg/mL solution of double-stranded DNA gives an absorbance of about 1.0 at 260 nm in a 1 cm path. RNA (40) and single-stranded DNA (33) absorb more per µg.

What is a good A260/280 ratio for RNA?

About 2.0. For DNA, about 1.8. Lower values suggest protein or phenol contamination.

What does a low A260/230 mean?

Possible carry-over of salts, guanidine or carbohydrates from extraction. Values of about 2.0 to 2.2 are typical for clean samples.

Is ng/µL the same as µg/mL?

Yes. 1 ng/µL equals 1 µg/mL.

What absorbance range is reliable?

Every instrument has a specified linear range. Very low readings are noisy and very high ones lose linearity, so dilute or concentrate the sample to stay within your instrument's range.

Is anything I enter saved or sent?

No. The calculator runs in your browser. Nothing you type is saved, sent to BioBridgeX or added to the page address, and analytics never see your numbers.

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Sources

Check every result before use. Formulas assume ideal conditions; follow your validated lab procedures and the reagent supplier's instructions. Facts on this page last checked 27 October 2026.

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