Demi vs. moitié
While both demi and moitié translate to "half" in English, they function differently grammatically. Demi is primarily used as an adjective referring to measurements or arithmetic units, whereas moitié acts as a noun representing a specific portion or 50% of a physical whole.
Demi
A1This word functions as an adjective indicating a mathematical quantity of 0.5. It is typically used with time, age, weights, and measurements. When placed before a noun, demi is connected by a hyphen and does not change form. When placed after a noun, it agrees in gender.
Nous avons attendu une demi-heure sous la pluie.
(We waited for half an hour in the rain.)
Mon fils a trois ans et demi.
(My son is three and a half years old.)
Veuillez me donner un demi-kilo de pain.
(Please give me half a kilo of bread.)
C'est ma demi-sœur du côté de ma mère.
(She is my half-sister on my mother's side.)
Le conducteur a fait demi-tour en voyant le trafic.
(The driver made a U-turn upon seeing the traffic.)
Moitié
A2This word is a noun that refers to one of two equal parts of a whole entity. It is frequently followed by the preposition de (of) to define what is being divided. It is also used in the expression à moitié, which functions as an adverb meaning partially or halfway.
Il a généreusement offert la moitié de son sandwich.
(He generously offered half of his sandwich.)
La moitié des étudiants ont réussi l'examen.
(Half of the students passed the exam.)
Coupez l'orange en deux moitiés.
(Cut the orange into two halves.)
Ton verre est encore à moitié plein.
(Your glass is still half full.)
Je n'ai pas cru la moitié de ce qu'il a dit.
(I didn't believe half of what he said.)
Summary
Use demi when treating half as a unit of measurement, time, or as a prefix (like half-brother). Use moitié when talking about a physical section, a split share, or a percentage of a specifically defined group or object.