PrintSizer

PrintSizer is a darkroom printing calculator that lets you calculate the change in print exposure time when changing the size of the print you are making.

It works by being calibrated to the specific characteristics of your enlarger, and then helping you determine necessary time adjustments when your print size changes.

To make an enlarger profile more accurate, you can add a set of two test
exposures to its profile. These should be height/time combos made on that
same enlarger, with the same lens and aperture, which produce identical
light-gray prints. Ideally, these two prints should be made at approximately
the smallest and largest sizes you intend to print.

If you have an enlarger meter, then you can skip the test printing process
and simply use two readings from that meter to build your profile.
It should also be possible to use an incident light meter, but your enlarger
lamp may be too dim to produce an accurate result via that method.

Source code and documentation:
https://github.com/dkonigsberg/printsizer

Reporting issues and feature requests:
https://github.com/dkonigsberg/printsizer/issues