How to Estimate Paint for a Room
Measure paintable area, subtract doors and windows, and convert coverage per gallon into a shopping list.
4 min read
Step by step
- Wall area = room perimeter × wall height.
- Subtract about 21 sq ft per door and 15 sq ft per window.
- Divide by coverage per gallon (usually 350–400 sq ft) and multiply by coats.
- Example: a 12×14 ft room with 8 ft ceilings needs about 2 gallons for two coats.
Key takeaway: Buy for two coats plus a little extra for touch-ups.
Measure the paintable area
Wall area is the room's perimeter × wall height. A 12 ft × 14 ft room with 8 ft ceilings has a perimeter of 52 ft, so 416 sq ft of wall. Subtract about 21 sq ft per door and 15 sq ft per window — a typical room loses 50–80 sq ft this way.
Add the ceiling separately if you are painting it: length × width, 168 sq ft in this example.
Coverage and coats
One gallon of interior paint typically covers 350–400 sq ft per coat. Two coats are the default for color changes; drastic changes (dark over light) may need a primer plus two coats.
Our example room at 350 sq ft of paintable wall needs about 1 gallon per coat, so 2 gallons for two coats — buy a little extra for touch-ups.
Costing the job
Paint is only part of the budget: add primer, tape, rollers, trays and drop cloths, typically 15–25% on top of the paint itself. Quality paint at $40–$60 per gallon often covers better than budget paint, sometimes saving a whole coat.
The calculator multiplies area, coats and price per gallon so you can compare brands on true project cost, not shelf price.