Protein Concentration Calculator
A₂₈₀ to mg/mL, and when that stops working.
Work out Protein Concentration. A₂₈₀ to mg/mL, and when that stops working. Says where the method stops working.
From the sequence — ProtParam computes it. BSA is 43,824.
Optional — converts molarity to mg/mL.
1 cm for a standard cuvette. Microvolume instruments use much less.
Protein concentration
0.0008 mg/mL
11.409 µM at 1 cm path
Beer-Lambert applied at 280 nm, where absorbance comes almost entirely from tryptophan and tyrosine with a small contribution from disulfide bonds. The extinction coefficient follows directly from the sequence and should be computed for your specific protein rather than assumed. Keep readings between about 0.1 and 1.0 absorbance units. Below that the signal is close to the noise; above it the detector response bends and the reading understates the concentration. Dilute rather than trusting a reading of 2.
How the Protein Concentration Calculator works
Enter your A₂₈₀ reading and the protein's extinction coefficient for concentration in both molar and mass terms. A protein with few tryptophans absorbs weakly, and the page flags that case rather than returning a confident number from a weak signal.
Also known as: a280 protein calculator · extinction coefficient protein calculator · nanodrop protein concentration · beer lambert for protein
Frequently asked questions
How do I calculate protein concentration from A280?
Beer-Lambert: concentration = A₂₈₀ ÷ (extinction coefficient × path length). Multiply by molar mass for mg/mL.
Where does the extinction coefficient come from?
The sequence. Absorbance at 280 nm comes almost entirely from tryptophan and tyrosine, with a small disulfide contribution, so it can be computed exactly — ProtParam and similar tools do this from a sequence.
When does A280 stop being reliable?
When the protein has few or no tryptophans, giving a very low extinction coefficient. The signal is then small and errors are amplified. A colorimetric assay such as Bradford or BCA is more dependable in that case.
What absorbance range should I aim for?
Between about 0.1 and 1.0. Below that the signal approaches the instrument noise; above it the detector response bends and the reading understates the true concentration. Dilute rather than trusting a reading of 2.
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