Enter your room size, doors, windows and number of coats to see how many gallons of paint to buy.
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How this calculator works
The calculator multiplies the room’s perimeter by the wall height to get total wall area, then subtracts about 21 square feet for each standard door (3 × 7 ft) and 15 square feet for each average window (3 × 5 ft).
It divides the paintable area by your paint’s coverage, which most interior paints list as 350 to 400 square feet per gallon, and multiplies by the number of coats. The “what to buy” figure rounds up to whole quarts so you are not short at the end.
Tips
- Check the coverage on your paint can and enter it; premium paints and primers vary.
- Use two coats for a color change, and add primer when going from dark to light or painting new drywall.
- Buy a little extra for touch-ups and keep it labeled with the room name.
- Textured or porous walls can need 10 to 20% more paint.
Frequently asked questions
How much paint does a 12 × 12 room need?
With 8-foot ceilings, one door, two windows and two coats, a 12 × 12 room needs about 1.9 gallons, so buy 2 gallons.
Do I need to subtract doors and windows?
Yes for an accurate estimate, especially in rooms with many windows. Leaving them in simply gives you a little extra paint.
Should I include the ceiling?
Only if you are painting it. Ceiling paint is usually a different product, so tick the box to see the combined amount, then buy separately.
Sources
- Paint manufacturers’ published coverage rates (typically 350 to 400 sq ft per gallon for interior wall paint)
- Typical US interior door (about 3 × 7 ft) and window (about 3 × 5 ft) sizes
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