I was stuck today. I couldn’t find a ruler. Thankfully it’s very easy to print an accurate ruler on a piece of paper that you can use to measure things.
Step 1: Download a pdf with a ruler from here:
Step 2: Print it full-size.
You can print the PDF straight from your web browser, but make sure you select 100% under the ‘Scale’ option so that the ruler comes out the right size.