The Quick Answer

Who's is short for who is or who has. (This is a 100% rule - it has no other uses.)

Whose is a bit more complicated. It sits before a noun to state (or ask) to whom it belongs.

For example:

  • A king whose crown is too big.
  • Whose crown is this?

Who's and Whose

The terms whose and who's sound identical, but they perform very different roles in English.

Whose

Whose is the possessive form of who. It means belonging to whom. Whose usually sits before a noun.

  • Conscience is a mother-in-law whose visit never ends.

Whose is before the noun visit
Whose in this example is a relative pronoun

  • Whose bike was expensive?

Whose is before the noun bike
Whose in this example is an interrogative pronoun

  • Carl knows the girl whose phone was stolen.

Whose is before the noun phone
Whose in this example is a relative pronoun

Who's

Who's is a contraction of either who is or who has. It has no other uses.

  • Who's coming to fix the bed?  -  who is
  • Who's eaten the last muffin?   -  who has
  • I met the inspector who's delivering tomorrow's briefing.   -  who is