How to convert diopters to focal length?

A lens of focal length f has the “optical power” $\phi$ = $\frac{1}{f}$

As the focal length is given in mm, the optical power is measured in units of $\frac{1}{mm}$ . Converting the optical power to units of $\frac{1}{m}$ results in “diopters”.

A lens of 4 diopters has a optical power of $\frac{4}{m} = \frac{4}{1 m} = \frac{4}{1000mm}$ .
So it has a focal length of $\frac{1000mm}{4} = 250mm$ .