Put A Ruler On Your Computer Screen

5 Ways to Display a Ruler on Your Computer Screen. Remember rulers? I used to have a plastic one in my notebook. While I never got into the ones that did advanced math calculations (let's just say I was math challenged), I used them for drawing straight lines and as stress relievers ( I chewed on them when the nuns were around). These days, there is never a ruler around when you need one; so, it is not surprising that some enterprising app developers figured out a way to put on a computer screen. Here are some options. Now, go measure something.