Expressions with time and money: should you add an “s” or not?

This is one of those small, annoying grammar points that even very advanced learners of English can get wrong. When you have a phrase that contains an amount, such as a price in pounds, it can be difficult to know if you should add an “s” or not (pound / pounds).

Look at these examples:

a. I bought a seventy pound jacket.

b. I bought a jacket for seventy pounds.

A typical mistake that a lot of learners make is to say: “I bought a *seventy pounds jacket”.

So, what’s the difference?

In sentence a the phrase seventy pound is like an adjective. You could replace seventy pound with the word expensive:

I bought an expensive jacket.

So, if the phrase could be replaced by an adjective you DON’T add an -s.

In sentence b the phrase seventy pounds is like a noun. You could replace this with another phrase containing a noun:

I bought a jacket for a lot of money.

If the phrase is like a noun, you DO add the -s.

Here are some more examples:

A forty dollar bag.
Hannah spent forty dollars on a bag.

A three hundred euro ticket.
My ticket cost three hundred euros.

We also use this pattern to talk about time.

Look at these examples:
A two-minute video.
This video lasts two minutes.

A four hour flight.
The flight took four hours.

A three day course.
The course is three days long.

Note that we often use a hypen (-) with the adjective-like phrase (without the -s):

a seventy-pound jacket
a three-day course