Linear Interpolation

Linear interpolation lets you estimate the value of a function between two known points, assuming it changes along a straight line. For points $(x_1,y_1)$ and $(x_2,y_2)$:

$y=y_1+(y_2-y_1)\cdot\frac{x-x_1}{x_2-x_1}$

It is an everyday tool for physicists and chemists — for reading values from tables (e.g. density, vapour pressure) at an intermediate value that is not listed. If the target $x$ lies outside the interval $[x_1,x_2]$, we speak of extrapolation (which is less reliable).