Work out how much paint you need for a room from its dimensions, number of coats and paint coverage. Subtracts doors and windows.
Wall area is the room perimeter times the height, minus about 2 m² for each door or window. Coverage is usually printed on the tin (about 10 to 12 m² per litre per coat is typical). Buy a little extra for touch-ups.
Multiply the room perimeter by the wall height for the wall area, subtract the doors and windows, then multiply by the number of coats and divide by the paint's coverage. This tool does it for you from the room dimensions.
Most wall paints cover about 10 to 12 square metres per litre per coat, and the exact figure is on the tin. Rough or porous surfaces and strong colour changes need more, so round up.
Yes. Buy a little more than the estimate for touch-ups and a second look at patchy areas, and keep a small amount for future repairs so the colour matches.