BCA Assay Calculator

The BCA assay calculator turns raw bicinchoninic acid (BCA) protein assay absorbance readings into a usable protein concentration. You enter the absorbance values measured for a series of bovine serum albumin (BSA) standards of known concentration, plus the replicate absorbance readings for your unknown sample, and the BCA assay calculator fits a standard curve, back-calculates the sample concentration, applies your dilution factor, and reports the statistical quality of both the curve and your replicates.

The BCA assay is one of the most widely used protein quantification methods in molecular biology and biochemistry. It works because protein reduces cupric ions (Cu²⁺) to cuprous ions (Cu⁺) under alkaline conditions, and two molecules of bicinchoninic acid then chelate each Cu⁺ ion to form an intense purple complex that absorbs strongly at 562 nm. The amount of purple color produced is proportional to the amount of protein present, so a calibration curve built from BSA standards lets you read protein concentration directly off an absorbance value.

Rather than eyeballing a curve or trusting a single reading, this calculator performs a least-squares linear regression through all your standards, reports the slope, intercept, and the coefficient of determination (R²), and then propagates your replicate scatter into a concentration uncertainty. That makes it useful for protein sample prep, normalizing protein loads for SDS-PAGE and Western blots, and validating that your standard curve is good enough to trust.

This BCA assay calculator builds the calibration curve using ordinary least-squares linear regression, treating standard concentration as the independent variable x and absorbance as the dependent variable y. The fitted line has the familiar form y = mx + b, where m is the slope and b is the intercept. Once the line is fitted, the unknown sample concentration is found by inverting the equation: the mean absorbance of your replicates minus the intercept, divided by the slope.

The slope and intercept are computed from the closed-form least-squares equations, where n is the number of standards:

• Slope: m = (n · Σxy − Σx · Σy) / (n · Σx² − (Σx)²)
• Intercept: b = (Σy − m · Σx) / n
• Sample concentration: C = (A̅ − b) / m, where A̅ is the mean replicate absorbance
• Original concentration: C₀ = C × dilution factor

The goodness of fit is reported as R² = 1 − (SS_res / SS_tot), where SS_res is the sum of squared residuals between measured and predicted absorbances and SS_tot is the total sum of squares about the mean absorbance. An R² above 0.99 indicates an excellent linear standard curve over the working range.

Beyond the curve fit, the BCA assay calculator analyzes the precision of your sample by working with the replicate absorbance readings you enter. It computes the mean absorbance, the population standard deviation of the replicates, and the coefficient of variation (CV%), which expresses that standard deviation as a percentage of the mean. The CV% is the single most useful precision metric for an assay, because it is dimensionless and directly comparable between samples and plates.

The standard deviation of the replicates is then divided by the curve slope to express measurement scatter in concentration units rather than absorbance units, and that value is multiplied by the dilution factor to give the uncertainty on the reported original concentration. This is the ± value shown next to the headline result.

Low CV% values typically come from consistent pipetting, well-mixed working reagent, and identical incubation timing across wells. High CV% often points to bubbles, evaporation at plate edges, or uneven temperature during the 37°C incubation step.

The BCA assay has two common formats with different working ranges, and choosing the right one is essential for an accurate result from this calculator. The standard protocol is linear from roughly 20 to 2000 µg/mL, while the enhanced or micro protocol extends sensitivity down to about 0.5 to 20 µg/mL for dilute samples. Readings should fall comfortably within the bracket of your standards; samples above the top standard should be diluted and re-read, and the dilution factor entered into the calculator so the original concentration is recovered.

The default standard curve in this calculator uses eight BSA points from 0 to 2000 µg/mL, which mirrors the classic Pierce BCA standard set. Because absorbance climbs steadily with concentration over this range, the regression typically returns an R² well above 0.99. If your highest standards begin to plateau or curve, the relationship is no longer linear, R² will drop, and you should either trim the upper standards or use a polynomial fit outside this tool.

Keep in mind that the BCA reaction does not reach a true endpoint; color continues to develop slowly over time. Reading all wells at a consistent time after incubation, and always including a blank (0 µg/mL standard), keeps the intercept meaningful and the protein quantification reproducible. The blank reading becomes the y-intercept of the fitted line and represents the background color of the working reagent itself.

Researchers choosing a protein assay often weigh the BCA assay against the Bradford and Lowry methods. Each relies on a different chemistry and has distinct compatibilities, so understanding the trade-offs helps you decide which calculator and protocol to use.

The BCA assay's main strength is its tolerance of detergents such as SDS and Triton X-100, which makes it the default choice for membrane protein and lysate quantification. Its main weakness is sensitivity to reducing agents like DTT and beta-mercaptoethanol, which themselves reduce Cu²⁺ and inflate the apparent protein concentration. When those reagents are unavoidable, the Bradford assay is often the safer alternative because it is far less sensitive to reducing agents, though it cannot tolerate detergents.

Using the BCA assay calculator takes only a few inputs. First, enter your BSA standard concentrations and their measured absorbance values at 562 nm. The defaults provide a typical eight-point curve from 0 to 2000 µg/mL, but you can edit every concentration and absorbance to match your own plate. Always include a blank standard at 0 µg/mL so the intercept captures the reagent background.

1. Enter standards: type each known BSA concentration and its absorbance.
2. Enter replicates: add the absorbance readings for your unknown sample, ideally in triplicate.
3. Set the dilution factor: if you diluted the sample before reading, enter the factor so the original concentration is recovered.
4. Read the results: the calculator reports sample concentration with uncertainty, mean absorbance, standard deviation, CV%, the curve equation, and R².

Check R² first to confirm the standard curve is linear, then check CV% to confirm your replicates are precise. If both look good, the reported concentration is reliable for downstream applications such as equalizing protein loads, calculating total protein yield, or normalizing enzyme activity.