Rule of thumb
A “rule of thumb” means a general principle built by personal experience, not a scientific calculation. It is not a 100% accurate rule, but considered as a pretty good estimate (example: A good rule of thumb in Boston is to check the weather each hour, as it can change so quickly and often). There are several hypotheses on the origin of the expression, “rule of thumb.” One of them is that the phrase came from construction workers–they used the width of their thumbs to approximately calculate an inch when they are doing works (Clarck, 2013). Try it with your thumb today!