Quick answer
For a rectangular room, multiply the room perimeter by wall height to get gross wall area. Subtract meaningful doors and windows if desired, then multiply by the number of coats.
Step by step
- Find perimeter. Add all wall lengths around the room.
- Multiply by height. This gives gross wall area.
- Subtract large openings. Doors and windows can be removed for a tighter estimate.
- Apply coats and coverage. Use the actual paint coverage rate.
Formula or rule of thumb
Example
A 12 × 10 ft room has a 44 ft perimeter. At 8 ft high, gross wall area is 352 ft² before openings.
Common mistakes
Do not use floor square footage as wall area. Wall area depends on perimeter and height, so two rooms with the same floor area can need different amounts of paint.
Planning note: product yields, installation methods, site conditions and local requirements can change the final quantity. Use the exact manufacturer or supplier information before purchasing.
FAQ
Should I subtract every window?
You can subtract meaningful openings, but very small openings may be offset by cutting-in and touch-up needs.
Do I include closets?
Include closet walls when they will receive the same paint.
What about ceilings?
Estimate ceilings separately because their area and paint product may differ.