BCA & Bradford Protein Assay Calculator

Accurate protein quantification is the foundation on which almost every downstream cellular and molecular experiment is built from normalizing loading amounts on a western blot to standardizing input material for an ELISA, a kinase assay, or a proteomics workflow. The BCA / Bradford Protein Assay Calculator from BioQuant Tools automates the two steps that are most often done by hand on a calculator or in a half-finished spreadsheet: fitting a standard curve to your absorbance readings and back-calculating unknown sample concentrations, corrected for dilution. Unlike a simple slope-intercept spreadsheet formula, this tool automatically compares a linear fit against a quadratic fit, reports R² for both, and flags any sample that falls outside the validated range of your standard curve — a step that is frequently skipped in manual analysis but is exactly where quantification errors creep in. For a complete cell-based workflow, pair this calculator with our cell seeding calculator and qPCR fold-change calculator to move seamlessly from lysate preparation to gene expression analysis.
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BCA / Bradford Protein Assay Calculator

Build a standard curve, fit linear or quadratic regression with automatic R² comparison, and calculate unknown sample protein concentrations with dilution correction — all in your browser, no data leaves this page.

1 Standard Curve Data

Enter your protein standard concentrations (e.g. BSA, µg/mL) and their measured absorbance values (e.g. 562 nm for BCA, 595 nm for Bradford).

Concentration (µg/mL)Absorbance

2 Curve Fit Model

Regression type
Model used
—
R²
—
Valid range
—

3 Unknown Samples

Enter sample name, measured absorbance, and any dilution factor applied before measurement (use 1 if undiluted).

Sample nameAbsorbanceDilution factor

4 Results

SampleMeasured Conc.Final Conc. (×dilution)
Add standards and samples above to see results.
For research use only. Results depend entirely on the accuracy of your entered standard curve data; always verify that unknown sample absorbance values fall within the linear/valid range of your standard curve. Re-dilute and re-assay out-of-range samples for accurate quantification.

Why This BCA Protein Assay Calculator Matters for Accurate Quantification

Both the bicinchoninic acid (BCA) assay and the Coomassie dye-binding (Bradford) assay convert protein concentration into a colorimetric signal: BCA relies on the reduction of Cu²⁺ to Cu¹⁺ by peptide bonds followed by chelation with bicinchoninic acid, while Bradford relies on a shift in Coomassie Brilliant Blue G-250 absorbance upon binding to basic and aromatic residues. In both cases, the relationship between protein concentration and absorbance is not perfectly linear across the full working range — it tends to plateau at higher concentrations. Treating a curved response as if it were linear systematically underestimates concentrated samples and overestimates dilute ones, which can distort normalization in downstream experiments such as western blotting, enzymatic activity assays, or mass-spectrometry sample prep.
Research-grade tip: Because color development continues slowly at room temperature even after the recommended incubation window, standards and unknown samples should always be read within the same short time frame, on the same plate, using the same pipetting technique — variability in timing is one of the most common hidden sources of BCA assay error.

Core Mathematical Logic Behind the BCA Protein Assay Calculator

1. Blank Correction
The mean absorbance of the zero-concentration standard (blank) is subtracted from every other standard and from every unknown sample reading before any curve fitting takes place. Skipping blank correction on the unknown samples — a mistake we deliberately catch in this tool — shifts every calculated concentration upward by a constant offset. Acorrected = Ameasured − Ablank
2. Standard Curve Regression
The calculator fits both a linear and a quadratic least-squares regression to your blank-corrected standard points, then reports the coefficient of determination (R²) for each: Linear: A = m·C + b Quadratic: A = a·C² + b·C + c
Why quadratic matters: BCA standard curves in particular tend to bend downward above ~1000 µg/mL. A quadratic fit routinely improves R² from the mid-0.99 range to above 0.998 across a 0–2000 µg/mL BSA standard set, which directly improves the accuracy of concentrated sample back-calculation.
3. Back-Calculating Unknown Concentration
For a linear fit, the concentration is solved algebraically. For a quadratic fit, the calculator solves the quadratic equation for C given a measured absorbance, selecting the physically valid (non-negative) root: Linear: C = (A − b) / m Quadratic: C = [−b + √(b² − 4a(c − A))] / 2a
4. Dilution Factor Correction
If a lysate or homogenate was diluted before it was measured (to bring its absorbance within the validated range of the standard curve), the back-calculated concentration must be multiplied by the dilution factor to reflect the concentration of the original, undiluted sample: Cfinal = Cmeasured × Dilution Factor

BCA vs. Bradford: Choosing the Right Assay

PropertyBCA AssayBradford Assay
Detection wavelength562 nm595 nm
Typical working range20 – 2000 µg/mL125 – 1500 µg/mL
Detergent toleranceHigh (compatible with most ionic and non-ionic detergents)Low (SDS and Triton interfere strongly)
Reducing agent toleranceLow (DTT, β-mercaptoethanol interfere unless a compatibility reagent is used)Moderate
Assay time30–60 min (or 2 min at 60 °C)~5 min
Curve shapeOften mildly non-linear at high concentration — quadratic fit recommendedGenerally more linear across its narrower range

Worked Example

The example below uses a realistic 562 nm BCA data set across the standard BSA working range, followed by three unknown lysate/homogenate samples.
Standard (µg/mL)Raw Absorbance
0 (blank)0.058
250.108
1250.171
2500.268
5000.412
7500.552
10000.689
15000.939
20001.176
The calculator fits a quadratic curve (R² = 0.9989) with equation A = −0.00000007·C² + 0.000678·C + 0.025761. For three unknown samples read at 0.398, 0.365, and 0.210 (with dilution factors of 1×, 2×, and 5× respectively), the tool returns:
  • Sample A: 487.4 µg/mL (undiluted)
  • Sample B: 433.8 µg/mL measured → 867.5 µg/mL final (after ×2 dilution correction)
  • Sample C: 189.9 µg/mL measured → 949.5 µg/mL final (after ×5 dilution correction)

Step-by-Step Guide to Using the BCA Protein Assay Calculator

  1. Enter standard curve data: Input the known concentration and raw absorbance of each protein standard, including the zero-concentration blank.
  2. Enable blank subtraction: Leave the blank-correction option checked so both standards and unknown samples are corrected consistently.
  3. Choose a fit model: Use "Auto" to let the tool select whichever of linear or quadratic regression yields the higher R², or force a specific model if your protocol requires it.
  4. Enter unknown samples: Input each sample's raw absorbance and the dilution factor applied before reading (use 1 for undiluted samples).
  5. Review the output: Check R², the valid concentration range, and whether any sample is flagged as out-of-range — re-dilute and re-read any flagged sample rather than trusting an extrapolated value.

Frequently Asked Questions

1. Why does my BCA standard curve fit better with a quadratic than a linear model?

The Cu¹⁺-BCA chelation reaction approaches saturation at higher protein concentrations, producing a slight downward curvature in the absorbance response. A quadratic (or four-parameter) fit captures this curvature and is the approach recommended in most commercial BCA kit protocols when concentrations extend toward the upper end of the working range.

2. Should I blank-correct my unknown samples the same way I blank-correct my standards?

Yes. Since the regression equation is built from blank-corrected standard absorbance values, feeding it an uncorrected (raw) sample absorbance introduces a constant positive offset into every calculated concentration — the offset equals the blank's own absorbance value.

3. What should I do if a sample falls outside the standard curve's valid range?

Never trust an extrapolated value from outside your tested concentration range — regression models, especially quadratic ones, can behave unpredictably beyond the data used to fit them. Dilute the sample further (for high concentrations) or concentrate it (for low concentrations) and re-read it, then apply the correct dilution factor during back-calculation.

4. Can I use this BCA protein assay calculator for Bradford, Pierce 660 nm, or Lowry assay data?

Yes. The underlying regression and back-calculation logic is assay-agnostic — it works on any colorimetric protein assay where absorbance is plotted against a known standard concentration. Simply use the absorbance values from your chosen assay and wavelength.

5. Which interferents should I watch for before trusting my results?

Reducing agents (DTT, β-mercaptoethanol, TCEP) and strong acids or bases interfere with the standard BCA formulation, while ionic detergents such as SDS strongly interfere with the Bradford assay. If your lysis buffer contains any of these, either use a reducing-agent-compatible BCA kit or dilute the sample enough that the interfering component falls below its threshold concentration.

6. Why does the calculator report R² for two different models instead of just giving me one number?

Reporting both R² values makes the underlying data-quality decision transparent rather than hidden inside a black box. A large gap between the linear and quadratic R² is itself informative — it tells you how much curvature is present in your particular assay run, which can vary with reagent lot, incubation temperature, and plate reader calibration.

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