Protein Concentration Calculator

What Is a Protein Concentration Calculator?

A protein concentration calculator converts a spectrophotometric absorbance reading or a colorimetric assay result into a protein mass concentration — expressed in mg/mL, µg/mL, or µg/µL. Three methods are supported by this calculator, each suited to different sample types and laboratory conditions.

A280 (UV absorbance): Proteins absorb ultraviolet light at 280 nm due to the aromatic amino acids tryptophan and tyrosine. The Beer-Lambert law relates this absorbance directly to concentration using a protein-specific extinction coefficient. A280 measurement is fast, requires no reagents, is non-destructive, and is ideal for purified proteins. Its limitation is that nucleic acids also absorb at 280 nm — any DNA or RNA contamination inflates the reading. For samples with nucleic acid contamination, the Warburg-Christian correction formula uses both A280 and A260 readings to subtract the nucleic acid contribution. To quantify the nucleic acid component independently, use our DNA Concentration Calculator for a dedicated A260-based measurement.

BCA Assay: The bicinchoninic acid (BCA) assay is a two-step colorimetric method. Proteins reduce Cu²⁺ to Cu⁺ under alkaline conditions; BCA then chelates Cu⁺ to form a purple complex absorbing at 562 nm. Concentration is read from a BSA standard curve. According to Thermo Fisher's BCA protocol, the assay is highly compatible with ionic and non-ionic detergents (up to 5% SDS) but is significantly inhibited by reducing agents including DTT, 2-mercaptoethanol (2-ME), and TCEP, which reduce Cu²⁺ directly and cause false positive signals.

Bradford Assay: The Bradford assay uses Coomassie Brilliant Blue G-250 dye, which shifts from brown (unbound) to blue (bound to protein) at 595 nm. It is faster than BCA (5 min vs 30–60 min), more sensitive at low concentrations, and fully compatible with reducing agents. However, SDS, Triton X-100, NP-40, and most other detergents interfere with dye binding and give false readings, making Bradford unsuitable for cell lysates prepared in detergent-containing buffers.

This calculator is used by biochemists quantifying purified enzyme preparations before activity assays, protein chemists determining antibody concentrations before conjugation or formulation, cell biologists normalising Western blot loading using total protein concentration, and pharmaceutical scientists measuring recombinant protein yields from fermentation runs.

How to Use the Protein Concentration Calculator

1. Choose your mode: A280 or Standard Curve (BCA / Bradford). Use A280 if you have a purified protein and know its extinction coefficient, or if you have both A280 and A260 readings and want to apply the Warburg-Christian nucleic acid correction. Use Standard Curve if you have run a BCA or Bradford assay with a BSA standard curve and have absorbance readings for both standards and your sample.
2. For A280 mode — select your protein from the dropdown. Pre-loaded extinction coefficients are available for BSA (0.667), IgG (1.40), lysozyme (2.65), haemoglobin (0.83), and GFP (0.92). If your protein is not listed, select Custom and enter its A0.1% value — the absorbance of a 1 mg/mL solution at 1 cm path length. This value can be found in the literature or calculated from the protein's amino acid sequence using tools such as ExPASy ProtParam.
3. Enter your A280 reading and dilution factor. If you measured the sample undiluted on a NanoDrop, enter 1 for the dilution factor. If you diluted the sample before measuring in a cuvette, enter the dilution factor (e.g. 10 if you added 1 part sample to 9 parts buffer). For precise protein sample dilutions, our Cell Dilution Calculator calculates exact stock and diluent volumes using C₁V₁ = C₂V₂.
4. For Warburg-Christian — enter both A280 and A260. This correction is recommended when A260 > 0.6 × A280, which signals significant nucleic acid contamination. The formula (1.55 × A280 − 0.76 × A260) subtracts the nucleic acid contribution to give a corrected protein concentration.
5. For Standard Curve mode — enter slope and intercept from your BSA calibration. Plot your BSA standard absorbance values against their known concentrations in µg/mL, fit a straight line, and read off the slope (m) and intercept (b). Enter your sample absorbance and any dilution factor applied to your sample before the assay.
6. Read results. The calculator outputs concentration in mg/mL, µg/mL, and µg/µL. The method comparison table below the result shows which methods are compatible with detergents, reducing agents, and nucleic acids — a quick reference for choosing the right assay for your sample type.

Formula and Methodology

A280 Using Extinction Coefficient

The Beer-Lambert law for protein concentration using A280:

c (mg/mL) = (A280 / ε) × dilution factor

Where ε is the extinction coefficient expressed as A0.1% (absorbance of a 0.1% = 1 mg/mL solution at 1 cm path length). Note: some references express ε as the molar extinction coefficient (ε_molar in M⁻¹ cm⁻¹). To convert: ε_A0.1% = ε_molar / (10 × MW in Da).

Warburg-Christian Formula (Nucleic Acid Correction)

c (mg/mL) = (1.55 × A280 − 0.76 × A260) × dilution factor

The constants 1.55 and 0.76 were derived empirically by Warburg and Christian (1942) from the absorption properties of pure protein and pure nucleic acid at 260 and 280 nm. This method provides a reasonable estimate for crude extracts but loses accuracy when nucleic acid concentrations are very high (A260/A280 > 2.5).

Standard Curve (BCA / Bradford)

From linear regression of the standard curve (Absorbance = m × concentration + b):

c (µg/mL) = (A_sample − b) / m × dilution factor

Where A_sample is the absorbance of the unknown, m is the slope, and b is the y-intercept.

Worked Example — A280

A researcher purifies human IgG (extinction coefficient A0.1% = 1.40) and measures A280 = 0.84 on a NanoDrop (undiluted, DF = 1).

c = (0.84 / 1.40) × 1 = 0.60 mg/mL = 600 µg/mL = 0.60 µg/µL

To prepare a 2 mg/mL working stock from this 0.60 mg/mL stock is impossible — the stock is less concentrated than the target. To dilute to 0.2 mg/mL: use C₁V₁ = C₂V₂ → V₁ = (0.2 × 1 mL) / 0.60 = 333 µL stock into 667 µL buffer.

Real-World Applications

Normalising Western blot loading

A cell biologist at a UK research institute is running a Western blot to compare protein expression across six HeLa cell treatment groups. Equal loading (equal total protein per lane) requires knowing the total protein concentration of each lysate. Lysates were prepared in RIPA buffer containing SDS and NP-40, which rules out Bradford. The researcher uses the BCA assay with a BSA standard curve (0–2,000 µg/mL), reads absorbance at 562 nm, and uses this calculator to determine concentrations ranging from 2.1 to 3.8 mg/mL across the six lysates. All samples are diluted to 2 mg/mL before loading — ensuring equal protein per lane and valid comparison of band intensities across the blot.

Quality control of a monoclonal antibody preparation

A process scientist at a contract biomanufacturing organisation is releasing a purified monoclonal antibody batch for formulation. The release specification requires concentration between 10 and 12 mg/mL. The antibody was purified by Protein A affinity chromatography and is in PBS with no detergents or reducing agents. A280 measurement on the NanoDrop gives 14.2. Using the IgG extinction coefficient of 1.40: c = 14.2 / 1.40 = 10.14 mg/mL — within specification. The result is recorded in the batch release record alongside the A280 reading and extinction coefficient used.

Quantifying a crude cell extract with nucleic acid contamination

A molecular biology PhD student extracts total soluble protein from E. coli by sonication without a DNase treatment step. The NanoDrop reads A280 = 1.23, A260 = 1.85 — an A260/A280 ratio of 1.50, which clearly indicates significant nucleic acid contamination (pure protein would give 0.55–0.75). The student applies the Warburg-Christian formula: c = (1.55 × 1.23 − 0.76 × 1.85) = 1.906 − 1.406 = 0.500 mg/mL. Without the correction, A280 alone would give 1.23 / 1.0 = 1.23 mg/mL — a 2.5-fold overestimate that would cause severe under-loading in subsequent enzyme activity assays.

Enzyme activity normalisation by BCA

A food biotechnology researcher is measuring amylase activity from a fungal fermentation broth. The broth contains DTT in the extraction buffer — ruling out BCA. The researcher switches to Bradford assay, constructs a fresh BSA standard curve (0–1,500 µg/mL) in the same DTT-containing buffer, fits a linear regression (slope = 0.00095, intercept = 0.018), and reads the broth sample at A595 = 0.38. Using this calculator: c = (0.38 − 0.018) / 0.00095 × 1 = 381 µg/mL. Enzyme activity is then expressed as units per mg protein, allowing comparison across different fermentation conditions regardless of total volume or dilution.

Common Mistakes and Troubleshooting

Using BCA when the sample contains reducing agents

Problem: BCA assay measures the product of Cu²⁺ being reduced to Cu⁺ by protein peptide bonds. Any reducing agent in the sample — DTT, 2-mercaptoethanol (2-ME), TCEP, dithionite, or ascorbic acid — also reduces Cu²⁺ directly, producing a false blue colour independent of protein. The result is a significantly overestimated protein concentration. This is the most commonly reported BCA assay failure in lab forums and ResearchGate questions. Fix: Use the Bradford assay (fully compatible with DTT and 2-ME), or remove reducing agents by diluting the sample at least 1:20 (if the concentrations are low enough to tolerate the dilution), or use the Reducing Agent Compatible BCA protocol from Thermo Fisher, which includes a pre-treatment step.

Using Bradford when the sample contains detergents

Problem: Bradford reagent (Coomassie G-250) must bind to protein to produce the colour shift from brown to blue. SDS, Triton X-100, NP-40, Tween-20, and most other detergents compete with the protein for the dye, interfering with binding and giving unreliable, typically overestimated readings. RIPA buffer, which contains both SDS and NP-40, is completely incompatible with the standard Bradford assay. Fix: Use BCA for samples in detergent-containing buffers. Alternatively, dilute the sample sufficiently to bring the detergent concentration below its critical micellar concentration, but verify that the resulting protein concentration is still within the assay's detection range.

Preparing the BSA standard curve in a different buffer than the sample

Problem: The components of a lysis or storage buffer (salts, glycerol, imidazole, guanidine) can shift the standard curve baseline, alter colour development kinetics in BCA/Bradford, and introduce a systematic error between the standard and sample signals. A BSA standard prepared in water but measured alongside samples in 500 mM NaCl will produce a calibration curve that does not represent the sample matrix, resulting in over- or under-estimation of concentration. Fix: Prepare all BSA standards in the same buffer at the same concentration as the sample buffer. If the sample buffer is unknown or complex, dilute samples 1:10 or 1:20 into a simple buffer and adjust the standard curve accordingly.

Using A280 for impure protein samples containing nucleic acids

Problem: DNA and RNA absorb strongly at 280 nm as well as 260 nm. If a cell lysate or crude extract is measured at A280 without correcting for nucleic acid content, the protein concentration will be significantly overestimated. This is particularly problematic for bacterial cell lysates prepared by sonication without DNase treatment. Fix: Check the A260/A280 ratio of your sample. If it is above 0.8 (pure protein is approximately 0.55–0.75), nucleic acid contamination is significant. Either use the Warburg-Christian correction (both A280 and A260 are required), treat the sample with DNase/RNase before measurement, or use BCA/Bradford instead of A280.