Paint Calculator

How to use the Paint Calculator

1. Choose US (feet, gallons) or Metric (meters, liters).
2. For a simple room, enter its length, width and wall height. To paint only some walls, switch to Individual walls and add each wall's width and height.
3. Enter how many doors and windows you will paint around. The sizes start at about 20 sq ft per door and 15 sq ft per window; change them to match yours.
4. Set the number of coats and the coverage printed on your paint can.
5. Tick Paint the ceiling too if needed, then read how many gallons or liters to buy.

How it works

Wall area in room mode is the perimeter times the wall height: 2 × (length + width) × height. In individual-wall mode each wall is width × height and the walls are added together. The calculator then subtracts the doors and windows (count × size of each), multiplies by the number of coats, and divides by the coverage rate to get the amount of paint. The answer is rounded up because paint is sold in whole cans.

The default coverage of 350 sq ft per gallon (about 10 m² per liter in metric mode) is a common middle value for interior wall paint on smooth, previously painted drywall. Manufacturers usually list a range such as 250–400 sq ft per gallon. Fresh drywall, textured surfaces, deep colors and big color changes use more paint, so lower the coverage figure or add a coat.

Example: a 12 × 14 ft room with 8 ft walls has 2 × (12 + 14) × 8 = 416 sq ft of wall. Removing one door (20 sq ft) and two windows (30 sq ft) leaves 366 sq ft. Two coats need 732 sq ft of coverage, and 732 ÷ 350 = 2.09 gallons, so you would buy 3 gallons (or 2 gallons plus a quart).

The ceiling is listed separately because ceilings are often painted with a different, flat paint. Trim and doors are also usually done with a separate product and are not included.

Frequently asked questions

How much paint for one coat vs. two?
Two coats need twice the coverage of one. Two coats are standard for a color change or new drywall (often with a primer first); one coat can be enough for a fresh coat of the same color.

Do I need primer?
Primer helps on new drywall, patched areas, stains and dramatic color changes. Estimate primer the same way, using the coverage on the primer can and usually one coat.

What size are standard doors and windows?
A typical interior door is about 3 × 6.7 ft, roughly 20 sq ft, and an average window is around 15 sq ft. Measure yours if they are much larger, such as sliding patio doors.

Is it better to round up or down?
Round up. Running out mid-wall can leave a visible line if the next can is mixed slightly differently. A little extra is handy for touch-ups later.

Can I use this for exterior walls?
Yes, enter each side of the house as a wall in Individual walls mode, but use the coverage for your exterior paint, which is often lower on rough siding, brick or stucco.