zweieinhalb
two and a half
Refers to the number 2.5; an amount consisting of two whole units and one half unit.
Ich warte schon seit zweieinhalb Stunden auf dich.
I've already been waiting for you for two and a half hours.

two and a half
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two and a half
Refers to the number 2.5; an amount consisting of two whole units and one half unit.
Ich warte schon seit zweieinhalb Stunden auf dich.
I've already been waiting for you for two and a half hours.

The word follows the pattern '[number] + ein + halb'. This pattern is also used for other numbers, for example 'dreieinhalb' (3.5) or 'viereinhalb' (4.5).
Die Besprechung dauerte nur eineinhalb Stunden.
The meeting lasted only one and a half hours.
When 'zweieinhalb' (two and a half) stands before a noun, it is not declined. It always keeps this one form, regardless of the noun's gender, case, or number.
Er hat zweieinhalb Liter Milch gekauft.
He bought two and a half liters of milk.
'Zweieinhalb' (two and a half) is often used with units of measurement such as hours, years, kilos, or meters to indicate an exact amount of 2.5.
Das Baby ist jetzt zweieinhalb Monate alt.
The baby is now two and a half months old.
one and a half
'anderthalb' and 'zweieinhalb' are both numerals with a half unit. That is exactly why they can be confused. But the difference is clear: 'anderthalb' means 1.5, so one and a half. 'zweieinhalb' means 2.5, so two and a half. With time, money, weight, or amounts, this difference matters a lot. Remember: 'anderthalb' is less than two, while 'zweieinhalb' is more than two.

Für die Fahrt zur Küste brauchen wir zweieinhalb Stunden.
We need two and a half hours for the trip to the coast.
Am Bahnhof wartet sie anderthalb Stunden auf ihren Freund.
At the train station, she waits one and a half hours for her friend.