A mythical number is a number used and accepted as deriving from scientific investigation and/or careful selection, but whose origin is unknown and whose basis is unsubstantiated.

Famous examples are the recommendation to chew exactly 27 times, or that if food touches the ground for less than 3 seconds it is still sanitary to eat. Their origins are akin to those of urban legends and may include (among others):

• misinterpretation of examples
• extrapolation from apparently similar fields
• especially successful pranks
• guess-estimates of public officials
• deliberate misinformation