there’s a nice online calculator that does the job for you. and if you like it you can download a desktop version.

i’ve been coding css with px as measuring unit. when i started web development most resources suggested using px. to ensure static look across all browsers.

nowadays you need to cater for a variety of browsers, screen resolutions, devices etc. static / fix sizes doesn’t look good in all.

relative measuring unit such as em and % is the preferred way. to allow your site to scale accordingly with devices.

the article also gives a better explanation and contain links to other resources.