1. : a method of procedure or analysis based upon experience and common sense and intended to give generally or approximately correct or effective results
seems to have run the ship by rule of thumb and word of mouth — William McFee
2. : a general principle regarded as roughly correct and helpful but not intended to be scientifically accurate
a good rule of thumb is that smart youngsters prepared to enter college at the age of 16 have not been accelerated too much — L.M.Spencer