How to calculate paint for a wall
Start with the wall’s length multiplied by its height. That gives its square footage. Subtract the area of windows and doors, then multiply by the number of coats. Divide that total by the coverage listed on your paint can.
For example, a 20-foot by 8-foot wall has 160 square feet. At two coats and 400 square feet of coverage per gallon, it needs 0.8 gallons before subtracting openings. In practice, buying one gallon gives you a little room for touch-ups.
For a whole room, calculate each wall separately and add the results. Ceiling paint and trim paint are usually purchased separately.
How much extra paint should you buy?
Consider a small buffer if walls are new drywall, dark-to-light color changes, highly textured, or if you will need paint for future repairs. Leftover paint is useful only when it is stored correctly and clearly labeled.