Paint Calculator
Gallons needed by coat, with doors and windows subtracted and a live diagram of the painted wall.
Your estimate
You'll need 3 gallons for 2 coats — about $126.
384 sq ft of wall · 845 sq ft painted across all coats · 350 sq ft per gallon
Measurements
Paint details
350 is typical
For a whole room, add up all four walls and enter the total as the wall width.
Subtract doors and windows
No openings yet. Add each door or window and its area comes out of the total.
Waste & cost
10% is standard
The painted surface drawn to scale with your doors and windows removed from the total.
The breakdown
- Gallons to buy
- 3
- Exact gallons needed
- 2.41 gal
- Wall area
- 384 sq ft
- Openings subtracted
- −0 sq ft
- Net paintable area
- 384 sq ft
- Waste overage (10%)
- 38.4 sq ft
- Coats
- 2
- Total painted surface
- 844.8 sq ft
- Estimated cost
- $126
Where to buy
Hand-picked search links for the materials on your list. As an Amazon Associate, ProjectMath earns from qualifying purchases.
Prices and availability are set by the retailer and change often. Always confirm the exact product size matches what this calculator assumed.
How this is calculated
Paint is coverage divided by spread rate, multiplied by coats. Openings come out of the area, and primer or a colour change is simply another coat.
- paintable area = wall area − openings area
- coverage needed = paintable area × coats
- gallons = ceil(coverage needed ÷ coverage per gallon)
- cost = gallons × price per gallon
Most interior latex covers 350 to 400 sq ft per gallon on a smooth, previously painted surface; textured or bare drywall drinks noticeably more.
- 1Measure the surfaceMeasure the width and height (or length) of the area in feet, inches, or meters and enter them in the calculator.
- 2Subtract openingsAdd each door, window, or fixture by size and count so their area is removed from the total.
- 3Set coats and coverageMost interior paint covers around 350 square feet per gallon. Two coats is standard over a similar color, three over a big color change.
- 4Read the shopping listThe calculator rounds up to whole units to buy and multiplies by your price per unit for an estimated total. Copy or print the plan and take it to the store.
Common questions
How much does a gallon of paint cover?
About 350 to 400 square feet on smooth, primed drywall. Textured, porous, or unprimed surfaces can drop that to 250 square feet.
How many coats of paint do I need?
Two coats for almost every repaint. Use three when going from dark to light, or prime first to save a finish coat.
Do I subtract doors and windows from paint area?
Yes for anything larger than about 15 square feet. Small openings roughly cancel out against the extra paint used cutting in around them.
How much paint do I need for a 12x12 room?
A 12 by 12 room with 8 foot ceilings has about 384 square feet of wall. Two coats needs roughly 2.5 gallons before subtracting a door and a window.