A Rule of Thumb

A rule of thumb is a rule or principle that you follow which is not based on exact calculations or theory, but rather on experience or practice.

It’s rather like a guideline or an unwritten rule. A rule that is wise to follow perhaps.

In German we talk about a Faustregel. 

An example:
As a rule of thumb, if you can put a whole finger under the brim of a hat, it’s too big! 😉

Can you think of any rules of thumb?