I’ve been told off a couple of times by different English native speakers for saying “an hour and a half”. They insisted I should say “one and a half hours”. Were they right?
After being told off for, apparently, not saying 1.5 h properly, I carried out some research and here’s the result. Were you to search the Internet you’d find three different options we’re going to study:
a) an/one hour and a half
b) one and a half hour
c) one and a half hours
Let’s start with b and c. They differ in the number of the word hour, which is singular in b and plural in c. Since the singular is used to refer to just to one person or thing, and 1.5 is more than 1, the plural needs to be used. Therefore, b is not correct and c is.
Apparently, this is not that obvious and the ongoing debate on whether it is hour or hours has led some native speakers to think that one and a half hours is the only correct way to say it. Poor souls, they couldn’t be more wrong. There’s nothing ungrammatical about an/one hour and a half. Both versions a and c are included as examples in the Oxford Advanced Learner’s Dictionary. Moreover, option a is almost sixfold more popular than option c in the .uk domain according to Google.
To sum up, the proper ways to say 1.5 h are:
an/one hour and a half
one and a half hours
Or you can always keep it simple and go for 90 minutes.