How do I estimate how much paint is needed?

[Michael Dorey]

The reason I pose this question is don't want to buy more than I need.

I recently ordered paint for my business and had a Sherman William's rep come measure and give me an estimate of what I needed. It was over by 10 gallons of paint.

Good thing is it's a neutral tan color so I can use it in other places but I know the wife will want to use different colors in some rooms.

So how do I estimate the amount needed?