Measuring a room before buying paint is easier when you work in sections. Start with one wall at a time. Measure the width and height, then multiply those numbers to get the wall area. Repeat this for each wall and add the results together.
Next, subtract large openings such as doors and windows. A standard door is often estimated around 21 square feet, while a standard window is often estimated around 15 square feet. These numbers are planning estimates, so measure your own openings when accuracy matters.
Multiply the paintable area by the number of coats. Most repainting projects need two coats for a smooth finish. New drywall, strong colour changes, porous plaster, or damaged walls may need primer or an additional coat.
Finally, add a small waste allowance. Ten percent is a common planning allowance for roller loss, cutting-in, touch-ups, and uneven surfaces. Use the paint calculator to turn your measurements into a quick litre or gallon estimate.