[datr]

I know what you're getting at but to be fair that probably is a slightly trickier question than it seems. You need to factor in how absorbent the walls are (are you using unfinished drywall?), what colour the walls currently are (Is it very different to the paint? You'll probably need multiple coats.), what's the brand of paint you're using (I've seen different brands range between 350 - 450 sqft / gallon).

[DanBC]

The math version of the question would say "This little can of paint can give a single coat to 5 square metres of wall. How many cans would you need to give two coats to a room that is 4 metres by 3 metres by 2.5 metres?"

A number of people can't do this.