Room Paint Quantity Calculator

Paint quantity calculation ensures you buy the right number of cans for a complete job without wasteful excess or frustrating mid-project shortages. The calculation accounts for wall area, ceiling area, number of coats, and paint coverage rate.

Wall area formula: Wall Area = Perimeter × Wall Height − Door and Window Area

Where:

• Perimeter = 2 × (Room Length + Room Width)
• Door area ≈ 20 sq ft each (a 3 ft × 6.7 ft opening)
• Window area ≈ 15 sq ft each (3 ft × 5 ft)

Set the window count to zero if you plan to paint the wall behind blinds or heavy curtains rather than cutting in around the frame. The deduction only makes sense for what you genuinely will not roll over.

Ceiling area: Ceiling Area = Room Length × Room Width

Total paint needed: Gallons = (Total Area × Number of Coats) ÷ Coverage per Gallon

Where:

• Coverage per Gallon is typically 350 sq ft on smooth surfaces and 250 to 300 on textured or porous ones. Cans usually claim 400, which is a lab figure. Use the lower end for a dark-to-light change.
• Number of Coats is 1 for a same-color touch-up, 2 as standard, and 3 for a dramatic color change or a surface you skipped priming.

What each variable means:

• Walls vs. ceiling are usually different sheens, flat or matte on the ceiling and eggshell or satin on the walls, which is why this calculator prices them as two separate purchases rather than one total
• Primer vs. paint: over bare drywall, prime once and paint twice. Over existing paint in a similar color, two coats of paint alone is usually enough
• Round up: running out mid-wall leaves a visible lap line once the paint dries, and the trip back to the store is exactly long enough for the wet edge to set

Reference: paint sheen guide:

• Flat/Matte: Ceilings and low-traffic areas; hides imperfections, not washable
• Eggshell: Living rooms, bedrooms; slightly washable
• Satin: Kitchens, hallways; more washable, light sheen
• Semi-gloss: Bathrooms, trim, doors; highly washable, noticeable sheen
• High-gloss: Cabinets, furniture; extremely durable and washable

Worked example: Room: 12 ft × 14 ft, 9 ft ceiling. 1 door, 2 windows. 2 coats on walls, 1 coat on ceiling, 350 sq ft per gallon.

• Perimeter = 2 × (12 + 14) = 52 ft
• Gross wall = 52 × 9 = 468 sq ft
• Deductions = 20 (door) + 30 (two windows) = 50 sq ft
• Net wall = 468 − 50 = 418 sq ft
• Ceiling area = 12 × 14 = 168 sq ft
• Wall paint = (418 × 2) ÷ 350 = 836 ÷ 350 = 2.39 gallons, so buy 2 gallons plus 2 quarts
• Ceiling paint = (168 × 1) ÷ 350 = 0.48 gallons, so buy 2 quarts

Two gallons and two quarts rather than three gallons is the part worth pausing on. It covers the 2.39 you need with a little spare, and it leaves half a gallon of wall paint un-bought. On a premium paint that is real money, and the half gallon you did not buy is also a half gallon that does not skin over in the garage two summers from now.

A note on units: this calculator works in feet and US gallons, because the gallon and quart can sizes are what the arithmetic above is for. If you measure in meters and buy in liters, the quick room paint calculator handles that directly.