Paint Per Square Foot: How Much Paint Do You Need? (Interior & Exterior)

Most paint covers about 350 square feet per gallon on a smooth surface, so you divide your wall area by 350 and multiply by the number of coats. Rough surfaces and exteriors often need more, so check the label on your paint can.

Skip the math with our paint calculator.

The Paint Per Square Foot Formula

1. Wall area = wall height × wall width (add all walls)
2. Subtract openings: doors and windows
3. Gallons needed = (paintable area × number of coats) ÷ coverage per gallon

Step 1: Measure the Walls

Measure the width and height of each wall in feet and multiply. For a room that is 12 ft × 10 ft with 9 ft ceilings:

• Two walls of 12 × 9 = 108 sq ft each, so 216 sq ft
• Two walls of 10 × 9 = 90 sq ft each, so 180 sq ft
• Total wall area = 396 sq ft

Step 2: Subtract Doors and Windows

As a rough guide, a standard door is about 20 sq ft and an average window about 15 sq ft. With one door and two windows:

• 20 + 15 + 15 = 50 sq ft
• Paintable area = 396 − 50 = 346 sq ft

Step 3: Calculate the Gallons

For two coats and 350 sq ft per gallon:

• 346 × 2 = 692 sq ft of coverage needed
• 692 ÷ 350 = 1.98 gallons

Buy 2 gallons, and add a little extra for touch-ups.

Interior Paint Per Square Foot

Interior walls are usually smooth, so the 350 sq ft per gallon estimate is a good starting point. Dark colors, new drywall and a color change from dark to light can need a primer or a third coat. Don't forget the ceiling if you are painting it too.

Exterior Paint Per Square Foot

Exterior surfaces like brick, stucco or rough wood soak up more paint, so coverage can be closer to 250 to 300 sq ft per gallon. Measure each side of the house, subtract doors, windows and garage doors, and leave extra for trim.

Example: Exterior Wall of 1,200 sq ft

If the paintable area is 1,200 sq ft, two coats at 300 sq ft per gallon:

• 1,200 × 2 = 2,400 sq ft
• 2,400 ÷ 300 = 8 gallons

Estimating Cost

Multiply the gallons you need by the price per gallon. For 8 gallons at 35 per gallon, the paint costs 280. Labor, primer, tape and brushes are extra.

Tips to Avoid Running Short

• Always buy all the paint for one room from the same batch so the color matches.
• Add 10% extra for waste and touch-ups.
• Use primer on new or stained walls.
• Check the coverage number printed on your paint can, since brands differ.

FAQ

How many square feet does 1 gallon of paint cover?

About 350 sq ft on smooth walls, less on rough surfaces.

How much paint do I need for a 12 × 10 room?

For 9 ft ceilings and two coats, about 2 gallons, minus doors and windows.

Do I need two coats?

Usually yes. One coat is enough only when you repaint with a similar color.

How do I calculate paint for exterior walls?

Measure each wall, subtract openings, and use a coverage of 250 to 300 sq ft per gallon.

You can also use our paint calculator to get the answer instantly.