DNA Concentration Calculator

Calculate DNA concentration from your spectrophotometer absorbance readings. Enter your A260 value, dilution factor, and nucleic acid type (dsDNA, ssDNA, or RNA) to get concentration in ng/µL. Optional purity ratios (A260/A280 and A260/A230) help assess sample quality. You can also convert between mass concentration (ng/µL) and molar concentration (nM) using fragment length.

Select the type of nucleic acid being measured.

µg/mL per A260

Only used when Custom Factor is selected above.

Absorbance reading at 260 nm from your spectrophotometer.

If sample was diluted before measurement, enter the dilution factor here.

cm

Standard cuvettes are 1 cm. Adjust if using a different pathlength.

Used to calculate A260/A280 purity ratio. Pure DNA ≈ 1.8.

Used to calculate A260/A230 purity ratio. Pure DNA ≈ 2.0–2.2.

µL

Enter to calculate total DNA yield in your sample.

bp

Enter fragment length in base pairs to convert ng/µL to nM.

Results

DNA Concentration

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Concentration (µg/mL)

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A260/A280 Purity Ratio

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A260/A230 Purity Ratio

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Total DNA Yield

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Molar Concentration

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Frequently Asked Questions

Why is absorbance measured at 260 nm for DNA?

Nucleic acids — DNA, ssDNA, and RNA — contain aromatic bases (adenine, guanine, cytosine, thymine/uracil) that strongly absorb UV light at approximately 260 nm. This wavelength corresponds to the peak absorbance of these bases, making A260 the standard measurement for quantifying nucleic acid concentration.

What are the standard conversion factors for DNA and RNA?

The widely accepted approximations are: 1 A260 unit = 50 µg/mL for double-stranded DNA (dsDNA), 33 µg/mL for single-stranded DNA (ssDNA), and 40 µg/mL for RNA. These factors assume a 1 cm pathlength and are built into most spectrophotometer software.

What does the A260/A280 ratio tell me about my sample?

The A260/A280 ratio is used to assess protein contamination in nucleic acid samples. A ratio of ~1.8 is generally accepted as pure DNA, while ~2.0 indicates pure RNA. Ratios significantly lower than these values suggest protein or other UV-absorbing contaminants that could interfere with downstream applications.

What does the A260/A230 ratio indicate?

The A260/A230 ratio is a secondary measure of nucleic acid purity, with expected values typically between 2.0 and 2.2 for pure samples. A low ratio may indicate contamination by organic compounds such as EDTA, carbohydrates, phenol, or chaotropic salts commonly used in extraction protocols.

How do I account for a dilution factor in my calculation?

If your sample was diluted before measurement (e.g., 1:10 dilution), enter the dilution factor in the corresponding field. The calculator multiplies the calculated concentration by this factor to report the true concentration of the original undiluted sample.

How is DNA concentration converted from ng/µL to nM?

To convert mass concentration (ng/µL) to molar concentration (nM), you need the fragment length in base pairs. The formula is: nM = (ng/µL × 10⁶) / (fragment length in bp × 660). Here, 660 g/mol is the average molecular weight per base pair of dsDNA. Enter your fragment length in the Molar Conversion section to see this result.

What is the pathlength and why does it matter?

The pathlength is the distance the light travels through the sample in the cuvette. Standard spectrophotometer cuvettes have a 1 cm pathlength. The Beer-Lambert Law accounts for this: A = ε × c × l, where l is the pathlength. If you use a microvolume instrument or a non-standard cuvette, adjust the pathlength value accordingly so the concentration is calculated correctly.

What should I do if my absorbance reading is above 1.0?

Absorbance readings above 1.0–1.5 may exceed the linear detection range of your spectrophotometer, leading to inaccurate results due to signal saturation. If this happens, dilute your sample (e.g., 1:10 or 1:100), re-measure, and enter the correct dilution factor into the calculator to obtain a reliable concentration estimate.

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