How to Calculate How Much Paint You Need
Calculating paint quantity is straightforward once you understand the three factors that determine it: the surface area to be painted, the number of coats required, and the coverage rate of your chosen paint. Getting this right means avoiding extra trips to the hardware store — or ending up with too much leftover paint.
Step 1: Measure Your Wall Area
For a rectangular room, calculate the perimeter by adding the length and width of the room, then multiplying by two:
Perimeter = 2 × (length + width)
Then multiply the perimeter by the wall height to get the gross wall area:
Gross wall area = perimeter × wall height
For example, a 12 × 10 ft room with 8 ft ceilings has a perimeter of 44 ft and a gross wall area of 352 sq ft.
Step 2: Subtract Openings
Doors and windows don't need paint (typically), so subtract their area:
- Standard interior door: 3 × 7 ft = 21 sq ft
- Standard window: 3 × 5 ft = 15 sq ft
For a room with 1 door and 2 windows: 21 + 30 = 51 sq ft deducted. Paintable area = 352 − 51 = 301 sq ft.
Step 3: Apply Number of Coats
Most walls need 2 coats for even coverage, especially when painting over a different color. Multiply the paintable area by the number of coats:
Total paintable area = paintable area × coats
With 2 coats: 301 × 2 = 602 sq ft.
Step 4: Divide by Coverage Rate
Paint coverage varies by product. A typical interior wall paint covers about 350 sq ft per gallon. Check the paint can label for the manufacturer's specific coverage rate:
Gallons needed = total paintable area ÷ coverage rate
With 350 sq ft/gal: 602 ÷ 350 ≈ 1.72 gallons.
Step 5: Add Waste Allowance
Add 10% for roller absorption, spills, and touch-ups:
Recommended = gallons needed × 1.10
1.72 × 1.10 ≈ 1.89 gallons. Round up to 2 gallons.
Common Mistakes
- Forgetting the ceiling: Ceiling paint is calculated separately (length × width). Use the Paint Calculator for wall paint.
- Not accounting for texture: Textured walls absorb more paint — increase waste to 15%.
- Ignoring primer: If painting over bare drywall, plan for 1 coat of primer (200-300 sq ft/gal) before your 2 coats of paint.
When to Use the Paint Calculator
Use the Paint Calculator for a quick, accurate estimate. For exterior walls, use the Exterior Paint Calculator. For dedicated interior wall painting, the Wall Paint Calculator shows the perimeter-first workflow.