Save time by not looking for a ruler when you need to measure the size of any small object.
Instead, use iruler (www.iruler.net) to display a digital ruler on your computer screen.
The website detects the size and resolution of your display to guarantee that the ruler is correct. If you have the same difficulty as us, click the ‘select your monitor dimension’ under the ruler, then input your screen size and resolution (you can determine your resolution by right-clicking the desktop and selecting ‘Display settings’). After that, your monitor will have a correctly proportional ruler.
Why not print a ruler instead if you can’t hold the object up to your computer’s monitor? A PDF of a 30cm ruler is available at Onlinelabels (https://uk.onlinelabels.com/docs/rulers/a4-Printable-Ruler.pdf). The PDF contains instructions for printing the ruler, but the most essential thing to remember is that the PDF should not be scaled while printing (set the Scale option in your printer settings to None, Actual Size or similar).