Which regression does this use?

It fits an ordinary least squares straight line, absorbance equals slope times concentration plus intercept, through your standard points. This linear model is the standard approach for the working range of both BCA and Bradford assays.

How are unknown concentrations calculated?

Once the line is fitted, each unknown concentration is found by rearranging the equation: concentration equals absorbance minus the intercept, divided by the slope. The tool flags any result that falls below the curve as out of range.

What does R-squared tell me?

R-squared measures how well the line fits your standards, where 1.0 is a perfect fit. A good protein standard curve usually reaches 0.99 or better. A lower value suggests pipetting error, a bad blank, or absorbances outside the linear range.

What if a sample reads above my top standard?

Do not extrapolate beyond the highest standard, because both assays curve at high absorbance. Dilute the sample, re-read it within the standard range, and multiply the result back by the dilution factor.

Does BCA or Bradford change the maths?

The fitting maths is identical. The chemistries differ in their useful range and linearity, so you choose the standards and read window appropriate to the kit, but the standard curve is fitted and inverted the same way for both.

BCA / Bradford Protein Assay Standard Curve Calculator

Both the BCA and Bradford assays measure protein indirectly through a colour change read as absorbance. To turn an absorbance into a concentration you build a standard curve from known amounts of a reference protein, usually bovine serum albumin, then read your unknowns off that line. This tool does the regression for you.

How it works

The standards are fitted with ordinary least squares to a straight line relating absorbance to concentration:

absorbance    = m x concentration + b
concentration = (absorbance - b) / m

The slope m and intercept b come from minimising the squared vertical distances between the points and the line. The tool also reports the coefficient of determination, R^2, so you can judge whether the fit is good enough to trust. Each unknown absorbance is then inverted through the equation to give its concentration in the same units you used for the standards.

Tips and notes

Always include a blank (zero concentration) as one standard so the intercept reflects your reagent background. Keep unknown readings inside the span of your standards; a value above the top standard sits in the non-linear region and must be diluted and re-read rather than extrapolated. Aim for an R-squared of at least 0.99, and if it falls short, suspect a pipetting error, a contaminated blank, or absorbances drifting out of the linear range of the assay.