Protein concentration calculator.
Turn an A280 reading into a protein concentration. Enter the absorbance, your dilution and the extinction coefficient, and the calculator returns the concentration in mg/mL.
Absorbance
LiveConcentration = A280 times dilution, divided by the 0.1% extinction coefficient. A generic protein is about 1.0; look up your protein for accuracy.
Concentration
8 mg/mL
of protein in the sample
Assumes a 1 cm path length. Accuracy depends on the extinction coefficient. Calculations run in your browser; nothing you enter is stored.
How it works
Absorbance to concentration
Proteins absorb ultraviolet light at 280 nm, mostly from tryptophan and tyrosine. Under the Beer-Lambert law, that absorbance is proportional to concentration. Dividing the A280 by the protein’s 0.1% extinction coefficient, and multiplying by the dilution, gives the concentration in mg/mL.
For a generic protein the coefficient is about 1.0, so an A280 of 0.8 on a 10-fold diluted sample is about 8 mg/mL. Specific proteins have their own coefficients.
Reference
Extinction coefficients.
The 0.1% extinction coefficient (A280 of a 1 mg/mL solution) for common proteins.
| Protein | Coefficient |
|---|---|
| Generic protein | 1.0 |
| BSA | 0.66 |
| IgG antibody | 1.40 |
| Lysozyme | 2.64 |
The full guide
The complete guide to A280 protein quant.
How A280 gives concentration, why the coefficient matters, and where it can mislead.
How A280 measures protein
Tryptophan and tyrosine residues absorb at 280 nm, so most proteins have a measurable A280. Under the Beer-Lambert law, absorbance equals concentration times path length times the extinction coefficient, so you can rearrange to get concentration from a reading.
Using the 0.1% coefficient, which is the A280 of a 1 mg/mL solution in a 1 cm cell, gives the concentration directly in mg/mL.
Choosing the extinction coefficient
The coefficient depends on the protein’s aromatic amino acid content, so it varies. A generic value of 1.0 is a rough default; BSA is about 0.66 and a typical IgG antibody about 1.4. For accuracy, use the value for your specific protein, which can be computed from its sequence.
Using the wrong coefficient scales the result up or down, so it is the main source of error.
Dilution and path length
Dilute the sample so the A280 reads below about 1.0 for accuracy, then multiply by the dilution factor. The standard path length is 1 cm; small-volume spectrophotometers correct for their shorter path automatically.
Subtracting a reading at 320 nm can correct for light scattering from aggregates or turbidity.
Where it can mislead
A280 counts anything that absorbs at 280 nm, so nucleic acid contamination or buffer components can inflate the result. Proteins with few aromatic residues read low. For tricky samples, a colorimetric assay like Bradford or BCA may be better.
Always blank against your buffer and check the A260/A280 ratio if nucleic acid contamination is a concern.
The formula
Light to
milligrams.
Concentration is A280 times dilution, divided by the 0.1% extinction coefficient (about 1.0 generic).
DNA concentration ›# Protein concentration
conc_mg_per_mL = A280 × dilution / epsilon(0.1%)
# worked example
0.8 × 10 / 1.0 = 8 mg/mLQuestions
Protein questions.
How do I calculate protein concentration from A280?
+
Divide the A280 reading by the 0.1% extinction coefficient and multiply by the dilution. With a generic coefficient of 1.0, an A280 of 0.8 diluted 10-fold is about 8 mg/mL.
What extinction coefficient should I use?
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It depends on the protein. A generic value is 1.0, BSA is about 0.66 and a typical IgG antibody about 1.4. Use your protein’s own value for accuracy, ideally from its sequence.
Why dilute before reading?
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For accuracy the A280 should be below about 1.0. Diluting brings concentrated samples into range; you then multiply back by the dilution factor.
When is A280 unreliable?
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When the sample has nucleic acid contamination, few aromatic residues, or turbidity. A Bradford or BCA assay can be more reliable in those cases.
Is this protein concentration calculator free?
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Yes. It is completely free with no sign-up, and every calculation runs locally in your browser, so nothing you enter is stored or sent anywhere.
About the developer
Jean Borg
Jean builds and maintains every calculator on freecalculators.pro from Malta, with a focus on tools that are fast, free and show their working. The protein concentration calculator uses the A280 Beer-Lambert method and is for lab estimates.