A standard curve is a quantitative research tool, a method of plotting assay data that is used to determine the concentration of a substance, particularly proteins and DNA.

The assay is first performed with various known concentrations of a substance similar to that being measured. For example a standard curve for protein concentration is often created using known concentrations of bovine serum albumin . The assay procedure may measure absorbance, optical density, luminescence, fluorescence, radioactivity, or something else. The assay for protein is called the Bradford assay; it is a colourimetric assay. The reagent coomassie brilliant blue turns blue when it binds to arginine and aromatic amino acids present in protein. The intensity of the colour is best measured at 595 nm with a spectrophotometer. In the case of the Bradford assay the greater the absorbance, the higher the protein concentration.

This data is used to make the standard curve, plotting concentration on the X axis, and assay measurement on the Y axis. The same assay is then performed with samples of unknown concentration. To analyze the data, one locates the measurement on the Y-axis that corresponds to the assay measurement of the unknown substance and follows a line to intersect the standard curve. The corresponding value on the X-axis is the concentration of substance in the unknown sample.