Measure your walls, count the doors and windows, and get an estimate of how much paint to buy. Ceilings and trim are not included — measure those separately if you are painting them.
Formula
Paint = (Wall length × Height − Doors − Windows) × Coats ÷ Coverage
A standard door is taken as about 1.9 m² (21 ft²) and a window as about 1.5 m² (16 ft²).
Worked example
A room with 48 ft of walls, 8 ft high, one door and two windows has about 384 − 21 − 32 ≈ 331 ft² to paint. Two coats at 350 ft² per gallon need about 1.9 gallons, so buy two gallon cans.
Frequently asked questions
How many coats of paint do I need?
Two coats is standard for an even finish. A single coat may do when repainting the same colour; dark-to-light changes can need a primer and two or three coats.
How much does a gallon of paint cover?
Usually 350–400 ft² (about 9–10 m² per litre) per coat on smooth walls. Rough or porous surfaces take more.
Last updated · Calculations checked by automated tests· Editorial policy