Who's or Whose?

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?
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The difference between who's and whose

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

Click on the Two Correct Sentences
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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 determiner.)
  • 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)

A Bit More about Whose

There's a lot going on with whose. As well as being a relative pronoun, whose is also a determiner. You will see it listed among the possessive determiners and, because it can be used to ask a question, also as an interrogative determiner. The table below shows whose listed among the other possessive determiners.
Personal PronounPossessive DeterminerExample
ImyI do not choose that my grave should be dug while I am still alive. (Queen Elizabeth I)
youyourIf you want peace, you don't talk to your friends. You talk to your enemies. (South African cleric Desmond Tutu)
hehisIf a man could have half of his wishes, he would double his troubles. (Founding Father Benjamin Franklin)
sheherShe got her looks from her father. He's a plastic surgeon. (Comedian Groucho Marx)
ititsWorry never robs tomorrow of its sorrow. It only saps today of its joy. (Author Leo Buscaglia)
weourHow we spend our days is how we spend our lives. (Author Annie Dillard)
theytheirMen are like steel. When they lose their temper, they lose their worth. (Martial artist Chuck Norris)
whowhoseThe key is to keep company only with people whose presence calls forth your best. (Greek philosopher Epictetus)
Top Tip

Who's = Who is or Who Has

If you cannot substitute the who's in your sentence with either who is or who has, then it is wrong.
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See Also

adverse or averse? affect or effect? appraise or apprise? avenge or revenge? bare or bear? complement or compliment? dependant or dependent? discreet or discrete? disinterested or uninterested? e.g. or i.e.? envy or jealousy? imply or infer? its or it's? material or materiel? poisonous or venomous? practice or practise? principal or principle? tenant or tenet?
What are nouns? What are contractions? What are interrogative pronouns? What are relative pronouns? 
List of easily confused words