Paint Calculator: How Much Paint Do I Need?
Use this free paint calculator to find out exactly how much paint you need for your room. Enter your room dimensions, subtract doors and windows, choose your number of coats, and get an answer in gallons — so you buy the right amount the first time instead of making a second trip to the store.
Standard ceilings are 8 ft.
Each door subtracts ~21 sq ft.
Each window subtracts ~15 sq ft.
Please enter your room length, width, and ceiling height.
How This Calculator Works
This calculator uses the standard formula professional painters use:
Paintable area = (2 × (length + width) × ceiling height) − (doors × 21 sq ft) − (windows × 15 sq ft)
Gallons needed = (paintable area × number of coats) ÷ 375 sq ft per gallon
One gallon of quality interior paint covers 350–400 square feet on a smooth, previously painted surface — we use 375 as a realistic midpoint. Textured walls, porous surfaces, and bare drywall absorb more paint, which is one reason we recommend rounding up. Primer covers less than paint (roughly 300 sq ft per gallon), so the primer estimate is calculated separately.
Two coats is the standard recommendation for a uniform finish. Going from a dark color to a light one may take three.
Frequently Asked Questions
How much paint do I need for a 12x12 room?
A 12x12 room with 8-foot ceilings has about 384 square feet of wall area before subtracting openings. With one door and one window subtracted (~348 sq ft) and two coats of paint, you'll need about 1.9 gallons - buy 2 gallons.
How many square feet does a gallon of paint cover?
Most interior paints cover 350 to 400 square feet per gallon per coat on a smooth, sealed surface. Coverage drops on textured, porous, or unprimed surfaces. Check the manufacturer's coverage number on the can - it's printed on every gallon.
Do I really need two coats of paint?
Almost always, yes. One coat rarely produces a uniform color and sheen, even with paint-and-primer-in-one products. Two coats is the industry standard; a drastic color change (dark to light) may need three.
Should I buy extra paint?
Yes - round up to the next full gallon and keep the extra for touch-ups. Paint from a later batch may not match perfectly, so having leftover paint from the original batch is worth it.
When do I need primer?
Use primer on bare drywall, patched or repaired areas, stained surfaces, glossy surfaces you've scuff-sanded, and when making a drastic color change. For repainting a similar color on a wall in good condition, quality paint alone is usually fine.