Assess the size of the area that has to be painted. Get help if needed. Measure the width and height of walls with a measuring tape and multiply these together to calculate the total square footage.
Don’t worry too much about subtracting the space used by windows and doors, as the extra paint will come in handy.
Some extra paint will be required for touch ups. While painting a small area, you’ll want to have a small amount left for touch-ups. Painting a whole home? Keeping a whole extra tin might be a wise move.
Matte, satin or gloss? The type of paint will make a difference and so will the manufacturer. High quality paint may cover a smaller area but you’ll need fewer coats and the finish will be better and easier to achieve.