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 include: the claim that we only use 10% of our brains, the recommendation to chew exactly 27 times, and the five second rule, which states that if food touches the ground for less than 5 seconds it is still sanitary to eat.

The origins of such numbers 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