Hello LewRees!
Measure the distance around the room and add the length of each wall.
Then multiply that number times the height of the walls; most are either eight-feet or nine-feet.
EXAMPLE:
Suppose you have a room that has two walls that are fifteen-feet long and two walls that are twelve-feet long.
15+15+12+12 = 54-feet
And, suppose the walls are eight-feet tall.
Multiply the combined length of the walls (54) times the height of the wall (8), and you get square feet of wall to paint (432).
OR 54 ft x 8 ft = 432 sq ft
If you subtract the non-wall space of windows and doors, you have approximately 400-square feet of wall to paint.
Each gallon of paint will provide one coat over approximately 400-square feet of wall.
So, you will need approximately two gallons of paint to apply two coats ... most paint jobs require two coats.
NOTE:
Allow about four-hour dry time between coats.