Start With the Paintable Area
For a rectangular room, begin by estimating the total wall area. The common wall area formula is:
Wall Area = 2 x (Room Length + Room Width) x Wall Height
Doors and windows can be subtracted from that area when they will not be painted. If you are painting a ceiling, calculate the ceiling area separately:
Ceiling Area = Room Length x Room Width
Use Paint Coverage and Coats
Paint coverage tells you how much area one litre or gallon can usually cover. After you know the paintable area, use this formula:
Paint Needed = Paintable Area / Coverage per Coat x Number of Coats
Coverage can vary by paint product, surface texture, primer, application method and surface condition. The coverage stated by the manufacturer is the best value to use.
Add a Practical Allowance
A small waste allowance can help account for cutting in, roller loading, touch-ups and measurement differences. The Interior Tailor calculator includes a waste allowance field so you can see how it changes the estimate.
Use the Paint Calculator
For a quicker estimate, enter your room measurements, doors, windows, coats and coverage into the Interior Tailor paint quantity calculator.
After estimating paint quantity, you can also preview wall colors or explore custom paint colors.