Paint guide
Paint Calculator Formula
Paint is calculated by finding the paintable wall area, multiplying by the number of coats, and dividing by the paint coverage rate.
Quick answer:
Wall area equals room perimeter multiplied by wall height. Subtract doors and windows, multiply by coats, then divide by paint coverage.
Use the Paint Calculator →Calculate Room Perimeter
Perimeter = (Length + Width) × 2
Calculate Wall Area
Wall area = Perimeter × Wall height
Subtract Doors and Windows
Paintable area = Wall area − Openings
Calculate Paint Needed
Paint needed = Paintable area × Coats ÷ Coverage
Coverage is usually listed on the paint can. Many interior paints cover around 300–400 square feet per gallon.
Example Calculation
For a 20 ft by 15 ft room with 8 ft walls:
(20 + 15) × 2 = 70 ft perimeter
70 × 8 = 560 sq ft wall area
560 − 51 = 509 sq ft paintable area
509 × 2 = 1,018 sq ft total coat area
1,018 ÷ 350 = 2.9 gallons
Common Paint Calculation Mistakes
- Forgetting to multiply by the number of coats
- Not subtracting doors and windows
- Using the wrong coverage rate
- Not rounding up to the next full gallon or litre
- Forgetting that new drywall may need primer
Should You Round Up?
Yes. Paint is normally bought by the gallon or litre, so round up. Having extra paint is useful for touch-ups, future repairs, and uneven coverage.