I am looking for a lens with a specific diopter. How can I tell which magnifier is right for me?

A diopter is a measurement of power and has units of inverse meters.

Power (diopters) = 1/EFL where EFL = effective focal length of a lens with units of meters

For a thin lens element, the power is calculated below. Using the magnifying power (MP) of a magnifier or comparator you can calculate the power in diopters.

For the smaller magnifiers:

Power (diopters) = MP/0.25 where EFL is in units of meters

For the larger magnifiers:

Power (diopters) = (MP-1)/0.25 where EFL is in units of meters

Note that this should not be confused with prism diopters. This is also a measure of power, typically used with wedge prisms, that is defined as a 1cm deviation of an input beam at a distance of 1m from the prism.