anderthalb
one and a half
A numeral that describes the amount 'one and a half' (1.5).
Der Film dauert anderthalb Stunden.
The film lasts one and a half hours.

one and a half
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one and a half
A numeral that describes the amount 'one and a half' (1.5).
Der Film dauert anderthalb Stunden.
The film lasts one and a half hours.

The word 'anderthalb' (one and a half) comes from an older form of the language. 'Ander' used to mean 'the second.' So 'anderthalb' literally means 'the second half,' meaning: one whole and half of the second.
Für das Rezept brauche ich anderthalb Liter Milch.
For the recipe, I need one and a half liters of milk.
'Anderthalb' (one and a half) is a numeral that is not declined; it always keeps this form. The following noun is in the plural if it is countable.
Wir haben anderthalb Stunden auf den Bus gewartet.
We waited one and a half hours for the bus.
'Anderthalb' (one and a half) is very often used with units of time, weight, or measurement.
Er isst jeden Morgen anderthalb Brötchen zum Frühstück.
He eats one and a half rolls for breakfast every morning.
half
'halb' and 'anderthalb' can both describe amounts, but they do not mean the same thing. 'halb' means only one half, so 0.5 of something. 'anderthalb' means one plus one half, so 1.5. The risk of confusion comes from the fact that 'halb' is part of 'anderthalb'. The important point is: 'ein halber Liter' (half a liter) is less than one liter, but 'anderthalb Liter' (one and a half liters) is more than one liter.

Wegen des Staus hat die Fahrt anderthalb Stunden gedauert.
Because of the traffic jam, the trip took one and a half hours.
Sie hat für den Kurs nur eine halbe Stunde Zeit gehabt.
She only had half an hour for the course.