1. an approximate method or guideline for doing something
it's not exact, it's just a rule of thumb
A 'rule of thumb' is a practical or approximate way of doing or measuring something or a method or rule about what to do in a particular situation or condition that is not exact but based on practical experience. A rule of thumb is general advice to follow. This is a useful expression (noun) to use when you are giving instructions and is more likely to be used in casual contexts. Similar in meaning to 'an approximation'.