How paint calculator works
This calculator estimates paint for the walls of a rectangular room. Enter the room length, width and ceiling height plus the number of coats; optionally subtract the area of doors and windows you will not paint.
Wall area is the perimeter times the height: 2 times (length + width) times height. Paintable area subtracts openings. Total coverage needed is paintable area times coats. Paint needed is total area divided by your paint's coverage per gallon or liter, rounded up to a whole number of cans since paint is sold whole.
Coverage defaults to a typical 350 sq ft per gallon (imperial) or 10 sq m per liter (metric) for one coat of interior wall paint. Premium or primer-in-one paints often cover more; dark colors over light may need an extra coat. Always buy a little extra for touch-ups.
Frequently asked questions
How much paint do I need for a 12x12 room with 8 ft ceilings?
Wall area = 2 times (12 + 12) times 8 = 384 sq ft. At 350 sq ft per gallon with one coat that is about 1.1 gallons, so buy 2 gallons for one coat or 3-4 for two coats. Enter your exact dimensions and coats here for a precise number.
Should I subtract doors and windows?
Yes, to avoid overbuying. A standard door is about 21 sq ft and an average window about 15 sq ft. Enter the total openings area to subtract and the calculator reduces the paintable surface accordingly. Many people simply round up a can instead.
Does this include the ceiling or trim?
No, this calculator covers walls only. Ceilings and trim are painted differently and usually need a separate product. Run the calculator again treating the ceiling as a wall (its length times width) if you want a rough allowance.
Why round up to whole cans?
Paint is sold in whole gallons or liters and you cannot buy a partial can. Rounding up also leaves a small surplus for touch-ups, which is almost always worth the small extra cost versus running out mid-wall.
