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CalcBelt

Paint calculator

Enter your room size, doors and windows to see how many gallons of paint and primer to buy, and what a painter might charge.

Units
Room

Floor to ceiling.

About 20 sq ft each

About 15 sq ft each

Paint

Benjamin Moore recommends two for full color.

sq ft per gallon

Check the can. Many wall paints are rated 350 to 400 sq ft.

Paint needed

1.91 gallons

Buy 2 gallons for 2 coats

Wall area
334 sq ftDoors and windows taken off

Cost estimate

per gallon

Paint: $76 for 2 gallons

Typical US price $30 to $50 per gallon (Home Depot, This Old House, October 2026).

per sq ft painted

Hiring a painter: $668 (range $334 to $1,169)

Typical US price $1 to $3.50 per sq ft of wall (HomeGuide, This Old House, Fixr, October 2026).

How it's calculated
  1. Walls: 2 × (12 ft + 12 ft) × 8 ft = 384 sq ft
  2. Minus 1 doors at 20 sq ft and 2 windows at 15 sq ft: 334 sq ft
  3. 334 sq ft × 2 coats ÷ 350 sq ft per gallon = 1.91 gallons

Estimates use manufacturer data sheets and industry guides (sources). Check local codes and your supplier's figures before you order.

How to use the paint calculator

  1. Measure the room's length and width, and the wall height from floor to ceiling.
  2. Count the doors and windows. Each is taken off the wall area.
  3. Tick the ceiling box if you're painting it with the same paint.
  4. Set the number of coats and check the coverage printed on your paint can.
  5. Read the gallons to buy. Tick Prime first for new drywall or bare wood.

How to calculate paint

Wall area = 2 × (length + width) × height − doors − windows
Gallons = area × coats ÷ coverage per gallon

Example: a 12 × 14 ft bedroom with 8 ft walls has 2 × (12 + 14) × 8 = 416 square feet of wall. Take off one door (20 sq ft) and two windows (30 sq ft) and 366 sq ft remain. Two coats at 350 sq ft per gallon need 2.09 gallons: 2 gallons and a quart.

The calculator rounds to whole gallons plus quarts. When you’d need three quarts, it rounds up to another gallon instead.

Coverage by product

The spread rate on the can assumes a smooth, sealed surface and a specific film thickness. These are the published rates for some common lines:

Paint Coverage per gallon, one coat
Sherwin-Williams ProMar 200 (interior) 350 to 400 sq ft
Behr Premium Plus interior 250 to 400 sq ft
Benjamin Moore Regal Select interior 400 to 450 sq ft
Primer (Sherwin-Williams ProMar 200 primer) 400 sq ft

Behr notes that the application method and how porous the surface is change the coverage, so textured or new surfaces use more paint. In metric, 350 sq ft per gallon is about 8.6 m² per liter.

The drywall calculator uses the same room measurements for new walls, and the square footage calculator handles odd-shaped rooms. For the floor, see the flooring calculator.

Questions and answers

How much paint do I need?

About 1 gallon for every 350 sq ft of wall per coat. Take the wall area (the room's perimeter × the wall height), subtract 20 sq ft per door and 15 sq ft per window, multiply by the number of coats and divide by 350. A 10 × 10 ft room with 8 ft walls, one door and one window has 285 sq ft of wall: two coats need 1.6 gallons, so buy 2.

How much does a gallon of paint cover?

About 350 square feet per coat, within a range of roughly 250 to 450 depending on the paint and the surface. Sherwin-Williams rates ProMar 200 at 350 to 400 sq ft per gallon, Behr gives 250 to 400 and Benjamin Moore Regal Select 400 to 450. The calculator uses 350; type the figure on your can for a closer answer.

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

With 8 ft walls, one door and two windows, the walls are 334 square feet. Two coats at 350 sq ft per gallon need 1.9 gallons, so buy 2 gallons. Painting the ceiling too adds 144 square feet per coat, for about 2.7 gallons in total, so buy 3.

How many coats of paint do I need?

Two. Benjamin Moore recommends two coats for full color and durability, and Sherwin-Williams specifies a primer coat plus two finish coats on new drywall. Benjamin Moore says previously painted surfaces can be finished with 1 or 2 coats.

Do I need primer?

Usually on new drywall. Sherwin-Williams specifies one coat of primer on new drywall before two finish coats. Some paints, such as Benjamin Moore Regal Select, are rated self-priming on drywall, so check the label of the paint you buy.

How much do doors and windows take off?

Behr's guide counts an average door as 20 square feet and an average window as 15 square feet. The calculator subtracts those amounts.

How much does it cost to paint a room?

Mid-range interior paint costs about $30 to $50 a gallon. A painter typically charges $1 to $3.50 per square foot of wall, or about $300 to $1,000 for an average room's walls (HomeGuide, This Old House, Fixr).

How much does a quart of paint cover?

About 88 square feet per coat. A quart is a quarter of a gallon, so at 350 sq ft per gallon it covers about 88 sq ft, and 62 to 112 sq ft across the makers' range of 250 to 450 sq ft per gallon.

Sources

Figures checked October 2026.