A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination.

It is often claimed that the term's etymological origin lies in a law that limited the maximum thickness of a stick with which it was permissible for a man to beat his wife. British common law before the reign of Charles II permitted a man to give his wife "moderate correction," but no "rule of thumb" (whether called by this name or not) has ever been the law in England. Such "moderate correction" specifically excluded beatings, allowing the husband only to confine a wife to the household.

Another possible origin of the phrase comes from measurement, in particular in agricultural fields. The plants need a fairly precise depth to seed properly, whether planted from seed or being replanted, but the depth can sometimes be estimated using the thumb. That is, a rule "(measurement) of thumb". According to Gary Martin, "The origin of the phrase remains unknown. It is likely that it refers to one of the numerous ways that thumbs have been used to estimate things - judging the alignment or distance of an object by holding the thumb in one's eye-line, the temperature of brews of beer, measurement of an inch from the joint to the nail to the tip, or across the thumb, etc. The phrase joins the whole nine yards as one that probably derives from some form of measurement but which is unlikely ever to be definitively pinned down."

Another theory is that the coarseness of ground flour produced by grist mills in Old England was assessed by the miller's rubbing it between his thumb and forefinger.