DNA Concentration Calculator

Calculate DNA, RNA, or oligonucleotide concentration from spectrophotometer absorbance readings using the Beer-Lambert Law. Assess sample purity with the A260/A280 ratio.

Spectrophotometer Input

Enter your UV absorbance readings and sample parameters below.

Absorbance at 260 nm (A₂₆₀)

Absorbance at 280 nm (A₂₈₀) for purity

Nucleic Acid Type

Custom Conversion Factor μg/mL per OD

Path Length (cm)

Dilution Factor

Sample Volume (μL)

Results

Concentration

μg/mL

Concentration

ng/μL

A₂₆₀/A₂₈₀ Ratio

purity indicator

Total Yield

μg

OD₂₆₀

absorbance units

✅

Purity Assessment

Enter values and click Calculate to see purity assessment.

What is DNA Concentration?

DNA concentration refers to the amount of DNA present in a solution, typically measured in micrograms per milliliter (μg/mL) or nanograms per microliter (ng/μL). Accurately determining DNA concentration is essential for many molecular biology techniques, including:

PCR (Polymerase Chain Reaction)
DNA sequencing
Cloning and ligation
Gel electrophoresis
Southern blotting
Transfection experiments
Genotyping

How UV Spectrophotometry Works

The most common method for measuring DNA concentration uses UV spectrophotometry based on the Beer-Lambert Law. Nucleic acids absorb ultraviolet light with maximum absorption at 260 nm wavelength.

Why 260 nm?

The nitrogenous bases in DNA and RNA (adenine, guanine, cytosine, thymine/uracil) have conjugated double bonds that absorb UV light. The peak absorption wavelength is 260 nm due to the electronic transitions in these aromatic ring structures.

Beer-Lambert Law

The Beer-Lambert Law states that the absorbance of a solution is directly proportional to the concentration of the absorbing species and the path length of the light through the solution:

A = ε × c × l

Where:
A = absorbance (dimensionless)
ε = molar extinction coefficient (L/mol·cm)
c = concentration (mol/L)
l = path length (cm)

For practical DNA measurements, this is simplified to:

Concentration = A₂₆₀ × Conversion Factor × Dilution Factor / Path Length

Conversion Factors (Extinction Coefficients)

Different types of nucleic acids absorb UV light differently:

Double-stranded DNA (dsDNA)

Conversion factor: 50 μg/mL per OD₂₆₀ unit
An A₂₆₀ of 1.0 = 50 μg/mL of dsDNA
The stacking of bases in the double helix results in hypochromicity (reduced absorption compared to free nucleotides)

Single-stranded DNA (ssDNA)

Conversion factor: 33 μg/mL per OD₂₆₀ unit
Single-stranded DNA absorbs more UV light than double-stranded DNA (hyperchromicity)

RNA

Conversion factor: 40 μg/mL per OD₂₆₀ unit
RNA has an intermediate absorption due to its partially single-stranded nature with secondary structures

Nucleic Acid Type	Conversion Factor	1 OD₂₆₀ =
dsDNA	50 μg/mL per OD	50 μg/mL
ssDNA	33 μg/mL per OD	33 μg/mL
RNA	40 μg/mL per OD	40 μg/mL

The A₂₆₀/A₂₈₀ Ratio — Assessing Purity

The ratio of absorbance at 260 nm to 280 nm is the standard measure of nucleic acid purity:

For DNA:

~1.8: Pure DNA
< 1.7: Indicates protein contamination (proteins absorb at 280 nm due to aromatic amino acids — tryptophan, tyrosine, phenylalanine)
> 2.0: Indicates RNA contamination

For RNA:

~2.0: Pure RNA
< 1.8: Indicates protein contamination
> 2.1: May indicate sample degradation

What Causes Low A₂₆₀/A₂₈₀ Ratios?

Protein contamination (most common)
Phenol contamination from extraction
Guanidine salts from column purification
Very low nucleic acid concentration (noise in measurement)

What Causes High A₂₆₀/A₂₈₀ Ratios?

RNA contamination in DNA preparations
Nucleotide degradation products
Free nucleotides in solution

Common Pitfalls in DNA Quantification

Dilution errors: Always account for any dilutions made before measurement
Path length: Some instruments use 0.1 cm path length instead of 1 cm — adjust accordingly
Blank errors: Always measure a blank (buffer only) before samples
Saturation: A₂₆₀ values above 2.0 are unreliable — dilute the sample
Contamination: Proteins, phenol, RNA, and salts all affect readings
Low concentration: Very dilute samples (A₂₆₀ < 0.1) have poor signal-to-noise ratio

Recommended DNA Concentrations

DNA concentration requirements depend on the downstream application:

Application	Recommended Concentration
PCR template	1 – 100 ng/μL
Sanger sequencing	10 – 100 ng/μL
Next-gen sequencing	2 – 50 ng/μL (library-dependent)
Cloning	50 – 200 ng/μL
Transfection	100 – 500 ng/μL
Gel electrophoresis	50 – 500 ng/μL

Example Calculation

A spectrophotometer reading shows:

A₂₆₀ = 0.35
A₂₈₀ = 0.19
Sample type: dsDNA
Path length: 1 cm
Dilution factor: 10
Sample volume: 100 μL

Concentration = 0.35 × 50 × 10 / 1 = 175 μg/mL
A₂₆₀/A₂₈₀ = 0.35 / 0.19 = 1.84 (good purity)
Total yield = 175 μg/mL × 0.1 mL = 17.5 μg

Frequently Asked Questions

Q: What is a good A₂₆₀/A₂₈₀ ratio for DNA?

A: Around 1.8 indicates pure DNA. Values between 1.7–1.9 are generally acceptable for most downstream applications.

Q: What conversion factor should I use?

A: Use 50 μg/mL for double-stranded DNA (dsDNA), 33 μg/mL for single-stranded DNA (ssDNA), and 40 μg/mL for RNA.

Q: Why is my A₂₆₀/A₂₈₀ ratio low?

A: Low ratios (< 1.7) typically indicate protein contamination. Re-purify your sample using an additional extraction or clean-up step such as ethanol precipitation or column purification.

Q: What if my A₂₆₀ reading is above 2.0?

A: Readings above 2.0 are unreliable due to the non-linear range of the spectrophotometer. Dilute your sample and re-measure to get an accurate reading.

Q: Is ng/μL the same as μg/mL?

A: Yes, 1 ng/μL = 1 μg/mL. They are numerically equivalent because 1 μg = 1000 ng and 1 mL = 1000 μL, so the factors cancel out.

Q: How much DNA do I need for PCR?

A: Typically 1–100 ng per reaction, depending on the template quality, source, and PCR protocol. For genomic DNA templates, 10–100 ng is common; for plasmid DNA, 1–10 ng is usually sufficient.