This trick is something I learned recently that has changed the measurement game for me! If you're a typical person, you PROBABLY don't always have measuring tape on you (if you do, kudos). But sometimes you have to unexpectedly measure stuff... it just happens.
So here you go:
Measure the length between your thumb and pinky with your fingers stretched out as far as they can go. Remember the measurement, and there you go! A ruler that is always with you.