Measure wall area, account for doors and windows, and estimate the paint for every coat.
Buying too little paint can interrupt a project, while extra unopened cans add cost and may be difficult to match later. A few room measurements and the coverage printed on the paint can give you a useful estimate before you shop.
Measure the room length, width, and wall height. Add the wall lengths to find the perimeter, then multiply by the height. Measure large doors and windows and subtract their area if you want a closer estimate.
For example, a 4 m by 3 m room with 2.5 m walls has 35 m² of wall area before subtracting openings. A door and window totaling about 3 m² leave roughly 32 m² to paint.
Check the coverage rate on the exact paint you plan to use; it varies by product and surface. Multiply the paintable area by the number of coats, then divide by the coverage per litre or gallon. If 32 m² needs two coats and the label says 10 m² per litre, the estimate is 6.4 litres.
Add a small allowance for touch-ups and paint lost in the tray or roller. Textured, porous, or unprimed walls can use more paint, while a primer may reduce how much finish paint is needed. Round up to the next can size and follow the manufacturer’s instructions for coats and drying time.