Calculate Paint Needed
Enter your room dimensions below. The calculator subtracts door and window areas automatically.
- Most interior latex paint covers 350–400 sq ft per gallon per coat.
- Buy 10% extra to account for touch-ups and waste.
- Darker colors and bare drywall may need 3 coats.
- One gallon of primer usually covers the same area as paint.
© 2025 GardenMath. Estimates are for planning only. Check product labels before purchase. This page does not store personal data.
How Much Paint Do I Need?
The formula for calculating paint is straightforward: measure the total wall area, subtract openings like doors and windows, then divide by the paint's coverage rate. Multiply by the number of coats you plan to apply.
Wall area = 2 × height × (length + width)
For example, a 12 × 10 foot room with 8-foot ceilings has 352 square feet of wall area before subtracting doors and windows. With two doors (21 sq ft each) and two windows (15 sq ft each), you're down to about 280 sq ft of paintable surface. At 350 sq ft per gallon with two coats, you'd need roughly 1.6 gallons — so buy 2 gallons to be safe.
What Coverage Rate Should I Use?
Coverage rates vary by paint type and surface condition. As a general guide, 350 sq ft per gallon is a safe default for most interior latex paints on a previously-painted surface. Use a lower number (280–300) if you're painting over a darker color or bare drywall, and a higher number (400+) if you're applying a light color over light.
Do I Need Primer?
Primer is recommended when painting over bare drywall, making a drastic color change (especially light over dark), painting over stains or water damage, or switching paint sheens. Calculate primer separately using the same formula — primer typically covers 200–300 sq ft per gallon.