Add the lengths of each wall and multiply perimeter by total height of walls. Subtract the windows and door (Standard door - 20 square feet/Standard window - 15 square feet)
Most paint manufacturers claim 400 square feet per gallon. To be safe, estimate 300 square feet per gallon. In doubt? Round up, not down.
If you're not radically changing color, one coat should do the job. If you are radically changing color, especially to a darker color, then use 2-3 coats.
What type of paint should I use?
For Wood: Use satin on wood for a low luster finish.
For Ceilings: Flat
For Walls: Matte or flat
Matte and flat can be touched up without seeing the touch up due to oxidization.
Paints like egg shells can't be touched up. They require a coat to be applied to the entire surface and also require an additional coat to get their finish right.
Gloss also requires perfect walls: the shinier the paint, the more imperfections you see.