Measure distances between pixels on your screen.
Once you have started the application some parameters will be asked. You should just leave "auto" checked and click confirm.
Now all you have to do is measure. You can double click to delete all triangles. You can precisely move the cursor by 1 pixel with the arrow keys and click with the enter key.
You can press P to convert the program into an overlay, so you can use your computer as normal with the triangles on top. To go back to the ruler you have to click on it on the task bar (or with alt+tab), then press P again.
To exit the program you have to press alt+F4 while it is not an overlay.
If you own more than one monitor, it is advised that you click on "confirm" when the window is in the monitor where you want to measure.
You can change monitors using win+shift+arrow keys, however, it will likely not work properly if you change between monitors of different resolution. It will also give bad measurements (in cm and inches) if they are a different model.
If you need precision in the amount of cm or inches, make sure that all measurements match up with a physical ruler, if all measurements are good, you can leave it in auto. If they are not correct, you will have to uncheck auto and enter the screen parameters manually yourself.