Visit a website with printable paper rulers. Download a ruler and open the image in any program that allows you to view and print pictures.
Turn off auto-size and auto-formatting options. Some computers will re-size images so that they appear smaller, and some printers will also try to rescale pictures when you print them.
Print the paper ruler. If you have turned off rescaling options, your paper ruler should provide an accurate measure. Print more than one copy, as paper rulers may not be durable enough to withstand repeated use.
Cut out your ruler along the edges so that you can use it. You can also glue it on to a piece of cardboard to make it more sturdy.
Download a ruler app for your smartphone or tablet computer. If you don't want want to carry a paper ruler around with you, you can use an app such as Ruler, Free Ruler, or Ruler App. These apps allow you to switch between measurements and adjust the height. If you're using a tablet, you can even place the object you're measuring directly on the screen to find an accurate measurement.
Find an object that you know the exact measurements of. This can work as a temporary ruler if you don't have access to a computer. Although you will not be able to get an exact measurement, you should be able to tell approximately how long something is by using a standard measure.