Convert a 280 nm absorbance reading into protein concentration (mg/mL) and molarity using the Beer-Lambert law.
M⁻¹cm⁻¹
Da
cm
×
mL
Quick Facts
Method
Beer-Lambert: c (M) = A280 / (ε × path length)
Mass concentration (mg/mL) = molar concentration × molecular weight (Da).
Results
Calculated
Result
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About this calculator
This Protein Calculator converts a UV absorbance reading at 280 nm (A280) into protein concentration using the Beer-Lambert law. Aromatic residues -- tryptophan, tyrosine, and cystine -- absorb light at 280 nm in proportion to how much protein sits in the light path, so a spectrophotometer reading combined with a protein's extinction coefficient and molecular weight gives its concentration directly, without needing a standard curve.
Mass concentration (mg/mL) = molar concentration × molecular weight (Da)
The extinction coefficient (ε280, in M⁻¹cm⁻¹) and molecular weight are properties of the specific protein -- look them up with a sequence-based tool such as ExPASy ProtParam, or from the product datasheet for a known protein like BSA (the calculator's defaults). If you measured a diluted sample, enter the dilution factor so the result is scaled back up to the original stock concentration. Most spectrophotometers are most accurate when A280 falls between about 0.1 and 1.0, so dilute concentrated samples before reading.
How to use
Enter your A280 reading, the protein's extinction coefficient and molecular weight, the cuvette path length (usually 1 cm), any dilution factor, and your sample volume, then click Calculate to see the protein concentration, molarity, and total protein yield. Click Clear to reset the fields to the example values.
Frequently Asked Questions
What units should I use for this calculation?
Always use consistent units throughout — mixing metric and imperial values will give incorrect results. The calculator expects SI units (meters, kilograms, liters) unless otherwise labeled. If your source data uses different units, convert first using a unit conversion tool.
How accurate are biological calculations?
These calculations use standard formulas from peer-reviewed biology and are accurate for typical conditions. Biological systems have natural variability, so treat results as estimates within a range rather than exact predictions. Environmental conditions, genetic variation, and measurement error all introduce uncertainty.
Where can I verify these results?
Cross-reference with your course textbook, a peer-reviewed journal, or reputable databases like NCBI for molecular biology data. For ecology calculations, the USDA and EPA publish reference data. When in doubt, work through the formula by hand using a scientific calculator.
Why does my result differ from textbook values?
Textbook values often represent idealized conditions — standard temperature and pressure, pure solutions, or average population parameters. Real-world data rarely matches these ideals exactly. Small input differences also amplify through exponential and logarithmic functions, so double-check your inputs first.