Quadratic Regression Calculator

This tool fits a parabola to a set of data points by least squares. Paste a list of x values and a matching list of y values and it returns a, b and c for y = ax^2 + bx + c, along with R squared, the residual standard error, the largest residual and the vertex of the fitted curve. A second action reads a value off the curve at any x and says whether that point sits inside the data or beyond it. Three different x values are the minimum, and with exactly three the curve passes through them exactly, so R squared of 1 there means interpolation rather than a good fit.