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Matholio

Paint calculator

Find how many gallons and quarts of paint to buy for a room. Enter your walls, doors, windows and coats; the estimate updates as you type.

Updated Checked by Matholio Editorial TeamFree, no sign-up
LengthHeightWidth

How to measure

Measure the length and width of the floor and the height of the walls, then count the doors and windows.

Room size
Doors and windows
Paint

Your paint can lists its coverage. Most interior wall paints list 350 to 400 sq ft per gallon.

Paint needed

2.10 gallons

Buy 2 gallons + 1 quart

Wall area384 sq ft
Doors and windows−50 sq ft
Paintable area334 sq ft
Area to cover (2 coats)668 sq ft
Waste allowance10 %

At 350 sq ft per gallon. Gallons are rounded up to whole cans, with quarts for the remainder.

How much paint do I need?

One gallon of interior paint covers about 350 to 400 square feet per coat on smooth, primed walls. A 12 × 12 ft room with 8 ft walls, one door and two windows has 334 sq ft of paintable wall, so two coats need about 2.10 gallons including 10% extra for touch-ups: buy 2 gallons + 1 quart.

Worked example: a 12 × 12 ft bedroom

Here is the same math the calculator does, for a room with 8 ft walls, one door, two windows, two coats and paint rated at 350 sq ft per gallon.

  1. Wall area: 2 × (12 + 12) × 8 = 384 sq ft.
  2. Subtract openings: 1 door × 20 + 2 windows × 15 = 50 sq ft, which leaves 334 sq ft to paint.
  3. Multiply by coats: 334 × 2 = 668 sq ft to cover.
  4. Divide by coverage: 668 ÷ 350 = 1.91 gallons.
  5. Add 10% for touch-ups: 1.91 × 1.10 = 2.10 gallons.
  6. Round to cans: 2 gallons + 1 quart.

Paint needed by room size

Typical rooms with 8 ft walls, one door, two windows, two coats, 350 sq ft per gallon and 10% extra for touch-ups.

Gallons of wall paint by room size, two coats
Room Paintable wall Gallons needed What to buy
10 × 10 ft 270 sq ft 1.70 2 gallons
10 × 12 ft 302 sq ft 1.90 2 gallons
12 × 12 ft 334 sq ft 2.10 2 gallons + 1 quart
12 × 14 ft 366 sq ft 2.30 2 gallons + 2 quarts
14 × 16 ft 430 sq ft 2.70 3 gallons
16 × 20 ft 526 sq ft 3.31 3 gallons + 2 quarts
20 × 20 ft 590 sq ft 3.71 4 gallons

How the calculator works

Wall area = 2 × (length + width) × wall height

Paintable area = wall area − (doors × 20 sq ft) − (windows × 15 sq ft)

Gallons = paintable area × coats ÷ coverage × (1 + extra %)

A standard interior door is about 3 × 7 ft, so the calculator subtracts 20 sq ft per door. A typical window is counted as 15 sq ft. If you have a large sliding door or picture window, add an extra door or window to match its area.

The result is rounded to the cans stores sell. Whole gallons come first, and the remainder is covered with quarts. When the remainder is three quarts or more, the calculator rounds up to another gallon instead.

How many square feet does a gallon of paint cover?

Sherwin-Williams puts a typical gallon of paint at about 350 to 400 sq ft and a gallon of primer at about 200 to 300 sq ft. Rough, textured and porous surfaces soak up more paint, so plan on less coverage there.

Typical coverage per gallon, one coat
Surface Coverage per gallon Source
Smooth, primed or painted drywall 350–400 sq ft Sherwin-Williams
Textured, porous or new drywall 250–300 sq ft Painter's Solutions
Primer 200–300 sq ft Sherwin-Williams

Tips before you buy

  • Ceilings are not included. A flat ceiling is length × width, so a 12 × 12 ft ceiling is 144 sq ft, about 0.8 gallons for two coats at 350 sq ft per gallon.
  • Covering a dark color with a light one may need a primer coat or a third coat.
  • Buy all the paint for a room at the same time so the color matches from can to can.
  • Keep a labeled quart of leftover paint for touch-ups.

Frequently asked questions

How many square feet does a gallon of paint cover?

About 350 to 400 square feet per coat on smooth, primed walls, according to Sherwin-Williams. Textured, porous or new drywall can drop coverage to about 250 to 300 square feet per gallon.

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

About 2.10 gallons for two coats, if the walls are 8 feet tall with one door and two windows. That is 334 sq ft of wall at 350 sq ft per gallon plus 10% for touch-ups, so buy 2 gallons + 1 quart.

How many coats of paint do I need?

Two coats is the standard for even color, especially when you change colors. Covering a dark color with a light one may need a primer coat or a third coat.

How much primer do I need?

A gallon of primer covers about 200 to 300 square feet, according to Sherwin-Williams. For a primer estimate, set coverage to 250 and coats to 1 in the calculator.

Should I subtract doors and windows?

Yes. This calculator subtracts 20 sq ft for each door and 15 sq ft for each window. For large sliding doors or picture windows, add extra doors or windows to match their area.

Does the estimate include the ceiling?

No, the calculator covers walls only. A flat ceiling is length × width; a 12 × 12 ft ceiling is 144 sq ft, about 0.8 gallons for two coats at 350 sq ft per gallon.

Sources

  1. Sherwin-Williams, How to Use Paint Calculator
  2. Benjamin Moore, Paint Calculator
  3. Bob Vila, How Much Paint Do I Need?
  4. Painter's Solutions, Understanding Paint Coverage: How Much Do You Need?