If you don't want to always look around for a measuring cup, one Redditor shares a neat trick to make permanent markings on your metal pot. Most of the items needed should be in your house, or you can pick them up for very little.
Check out the Imgur album below to see the steps in action:
The Reddit thread has plenty of useful suggestions by commenters, so be sure to check that out too.
How to add permanent volume markings to a kettle. | Imgur via Reddit