kilometers an hour vs kilometers per hour

Hi all,

What is right: “50 kilometers an hour” or “50 kilometers per hour”?

In Murphy’s book (English Grammar in Use, 3rd Edition ex 72.4.5) the correct answer is:

What’s the usual speed limit in towns in your country? 50 kilometers an hour.

But why can’t we say: 50 kilometers per hour?

Many thanks

Both possible. The second is more formal.


Foreigner, many thanks for the answer.

You are welcome! :slight_smile: