Standard Curve Calculator

How to use a standard curve

1. Enter each standard's concentration and its reading (absorbance, fluorescence or Ct), one pair per line.
2. Enter the readings of your unknown samples, and their dilution factor if you diluted them.
3. The line's equation and R² appear, with the concentration of each unknown in the table.

Frequently asked questions

How is the concentration worked out?

The standards are fitted with a straight line, reading = slope × concentration + intercept, by least squares. Each unknown is then (reading − intercept) ÷ slope, multiplied by its dilution factor.

What R² is good enough?

Above about 0.99 is usual for a protein or DNA assay. A lower value suggests a pipetting error, an outlier, or standards beyond the range where the assay is linear.

Why does it flag readings outside the standards?

The line is only known between the lowest and highest standards. Readings beyond them are extrapolated and less reliable; dilute strong samples and measure again.

Should the line go through zero?

Usually not: a blank often gives a small reading of its own. Choose Through the origin only when the blank has already been subtracted and you know the response is zero at zero concentration.