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Protein quantificationBeer-Lambert lawA280 calculator

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A280 Protein Concentration Calculator

Calculate protein concentration from absorbance at 280 nm using a molar extinction coefficient, mass extinction coefficient, path length, and optional molecular weight.

Calculator

Enter A280 values

Calculation mode

Measurement note

A280 concentration can be affected by buffer absorbance, nucleic acids, contaminants, aggregates, and light scattering. Blank carefully and interpret results alongside purity checks when accuracy matters.

How A280 protein concentration works

A280 refers to absorbance measured at 280 nm. Proteins absorb at this wavelength mainly because of aromatic residues such as tryptophan and tyrosine, with additional contribution from disulfide bonds. The signal can be useful for estimating purified protein concentration without adding a colorimetric reagent.

Enter the absorbance, path length, and the coefficient type that matches your protein information. Molar coefficients are useful when you need M, µM, or nM values. Mass coefficients are useful when your workflow is based on mg/mL or µg/mL.

Practical interpretation

If molecular weight is provided, the calculator can connect molar and mass concentration units. This is helpful when comparing A280 output with protein assay records, dilution plans, binding assays, or sample loading requirements.

Use the results as an estimate for purified proteins and confirm concentration with an orthogonal method when contaminants, nucleic acids, turbidity, or unusual buffer components may influence the absorbance reading.

How to cite this tool

Citation

If this calculator supports your research, you may cite it as a web-based tool.

Abdulfattah, M. A. A280 Protein Concentration Calculator. Mustafa A Abdulfattah Academic Website. Available at: https://www.mustafaabdulfattah.com/tools/a280-protein-concentration-calculator. Accessed today.

BibTeX
@misc{abdulfattah_a280_calculator_2026,
author = {Abdulfattah, Mustafa A.},
title = {A280 Protein Concentration Calculator},
year = {2026},
url = {https://www.mustafaabdulfattah.com/tools/a280-protein-concentration-calculator},
note = {Accessed today}
}

FAQ

A280 protein concentration questions

How do you calculate protein concentration from A280?↑

Use the Beer-Lambert relationship with the correct extinction coefficient and path length. This calculator handles either molar extinction coefficients or mass extinction coefficients.

What extinction coefficient should I use?↑

Use the extinction coefficient specific to your protein sequence and measurement unit. Molar extinction coefficients are usually reported in M-1 cm-1, while mass extinction coefficients are often reported in mL mg-1 cm-1.

What path length should I use?↑

Use the optical path length of the measurement. Standard cuvettes are often 1 cm. Microvolume instruments such as NanoDrop-style devices may use an automatically adjusted path length, so use the corrected value reported by the instrument when available.

Can A280 measure impure protein?↑

A280 estimates absorbance from aromatic amino acids and disulfide bonds, but nucleic acids, buffer components, aggregates, contaminants, and light scattering can affect the result. Purity should be checked with appropriate complementary methods.

How do I convert µM protein to mg/mL?↑

Multiply concentration in µM by molecular weight in kDa, then divide by 1000. For example, 10 µM of a 50 kDa protein is 0.5 mg/mL. Use the Protein µM to mg/mL Converter. Protein µM to mg/mL Converter

Why is my A280 concentration different from Bradford assay?↑

A280 and Bradford assays respond to different chemical properties. A280 depends strongly on aromatic residues and optical clarity, while Bradford depends on dye binding. Buffer composition, protein sequence, standards, and contaminants can cause differences.