Paint Calculator: How Much Paint Do I Need for My Room?

How the Paint Calculator Measures Your Walls

The calculator finds your total wall area by multiplying the room's perimeter (length + width, times 2) by the ceiling height. It then subtracts the areas you will not paint: 20 square feet per door and 15 square feet per window, which are reliable averages for standard openings. If you include the ceiling, it adds length × width.

That paintable area is multiplied by the number of coats (two by default) and divided by coverage. Paint cans usually promise 350 to 400 square feet per gallon, but real-world coverage depends on texture, color, and how heavily you roll. The calculator uses 350 square feet per gallon so the estimate stays on the safe side.

A Worked Example

Say your bedroom is 12 by 14 feet with 8-foot ceilings, one door, and two windows.

1. Perimeter: (12 + 14) × 2 = 52 feet.
2. Wall area: 52 × 8 = 416 square feet.
3. Subtract openings: 416 − 20 − 30 = 366 square feet.
4. Two coats: 366 × 2 = 732 square feet.
5. Gallons: 732 ÷ 350 = about 2.1 gallons.

In practice, you would buy 2 gallons plus a quart. Adding the 168-square-foot ceiling at two coats needs another 336 square feet, or roughly 1 more gallon of ceiling paint.

When You Need Primer

Primer is not always necessary, especially with good-quality paint over a clean wall in a similar color. The calculator adds one coat of primer when you tell it any of these apply:

• Going dark to light. Covering navy or charcoal with a pale color without primer can take three or more topcoats.
• New drywall or patches. Bare joint compound absorbs paint unevenly and shows through as dull spots, known as flashing. If you have just patched walls, our guide to repair drywall after removing wallpaper explains how to prep them.
• Glossy surfaces. Paint struggles to grip high-gloss or semi-gloss finishes, so prime after a light scuff sanding.

Primer coverage is similar to paint, so the calculator uses the same 350 square feet per gallon. Ask the store to tint it toward your topcoat for deep colors.

Estimating Cost and Choosing a Finish

Enter your price per gallon and the calculator multiplies it by the gallons needed, adding primer at its own price if you included it. Premium paint costs more per gallon but often covers better and stands up to cleaning, which can make it cheaper per square foot over time.

Finish matters for both looks and practicality. Matte and flat finishes hide wall imperfections and read as rich and chalky, ideal for bedrooms and ceilings. Eggshell has a soft glow and wipes clean, a good default for living rooms and hallways. Satin handles kitchens and bathrooms, and semi-gloss is the classic choice for trim and doors. Need help picking the color itself? Browse our warm pastel bedroom paint colors and pastel colors for home, or build a full scheme with the room color palette generator.

Tips for a Professional-Looking Finish

• Buy all your paint at once, or mix cans together in a bucket, to avoid slight batch color differences.
• Cut in edges with a brush first, then roll while the edges are still wet to avoid visible banding.
• Let each coat dry fully per the label before recoating.
• Keep leftover paint sealed and labeled with the room and date for future touch-ups.

Painting is one of the highest-impact changes you can make. For more ways to get a high-end look, see our list of renter friendly upgrades.