How Much Paint Do I Need? Room Paint Calculator + Buying Guide

Running out of paint halfway through a wall is annoying; ending up with three spare gallons is expensive. The math is simple once you know the three numbers that matter, and the calculator below does it for you. Then read on for the two mistakes that make the calculation wrong.

The quick answer

A gallon of interior wall paint typically covers about 350 to 400 square feet per coat on a smooth, primed surface (check the label; manufacturers state coverage on every can). A typical 12 × 12 foot bedroom with 8-foot walls, one door and two windows needs roughly 2 gallons for two coats on the walls, plus about a gallon if you are painting the ceiling.

The calculator subtracts about 20 square feet per standard door and 15 per average window. If your windows are large, add them as two.

How the math works

1. Perimeter: add up the length of all walls. For a rectangle, 2 × (length + width).
2. Wall area: perimeter × ceiling height.
3. Subtract openings: doors and windows.
4. Multiply by coats: two coats is standard for a color change.
5. Divide by coverage: the square feet per gallon on the label.

For the 12 × 12 room: perimeter 48 ft × 8 ft = 384 sq ft, minus 50 for openings = 334 sq ft. Two coats = 668 sq ft. At 350 sq ft per gallon, that is 1.9 gallons, so buy 2.

The two mistakes that throw off the estimate

1. Ignoring surface texture and porosity

Label coverage assumes a smooth, sealed surface. Fresh drywall, bare plaster, heavily textured walls and masonry soak up paint and can cut coverage substantially. New drywall should get a coat of primer first; that also means you buy primer rather than a second coat of expensive finish paint.

2. Underestimating dramatic color changes

Going from dark red to pale gray, or vice versa, often needs a tinted primer plus two coats. Ask the store to tint your primer toward the finish color; it reduces the coats you need.

How many coats do you need?

• Same or similar color, good condition: often one coat, though two gives a more even, durable finish.
• Color change: two coats.
• Big light/dark change or new drywall: primer plus two coats.
• Paint-and-primer products: can help hide, but still usually need two coats on a big change.

Gallons vs quarts: buying smart

If the calculator says 2.3 gallons, buy 2 gallons plus a quart, or check whether a 5-gallon bucket is cheaper per gallon for larger jobs. Ask the store to mix all your paint at once, and if you have multiple cans, "box" them (pour them together into a bucket) so the color is perfectly uniform across walls.

Keep about a quart of leftover paint, labeled with the room and date, for future touch-ups.

Choosing the sheen

• Flat/matte: hides imperfections best; less washable. Ceilings and low-traffic rooms.
• Eggshell: slight sheen, easier to wipe. Living rooms and bedrooms.
• Satin: more durable and washable. Hallways, kids' rooms, kitchens.
• Semi-gloss: very washable. Trim, doors, bathrooms.

Higher sheen shows wall flaws more, which is another reason to patch first.

Prep checklist

• Move furniture to the center and cover it
• Fill holes and dents; sand smooth once dry
• Wash greasy or dirty walls (kitchens especially) and let dry
• Tape trim if you are not confident cutting in freehand
• Lay drop cloths along every wall
• Prime patches and any bare drywall
• Stir paint thoroughly; box multiple cans

Painting order that saves time

Ceiling first, then walls, then trim. Cut in along edges with a brush about two inches wide, then roll the walls while the cut-in edge is still wet to avoid visible lines. Roll in a "W" pattern in roughly 3 × 3 foot sections and finish with light vertical strokes.

How much for a ceiling, trim and doors?

• Ceiling: length × width. A 12 × 12 ceiling is 144 sq ft, under half a gallon per coat.
• Trim: a quart of semi-gloss often covers baseboards and window casings in an average room.
• Doors: about 20 sq ft per side; a quart covers several doors.

Estimating a whole house interior

Add each room's paintable area and divide as above. Hallways, stairwells and rooms with vaulted ceilings take more than floor area suggests, so measure walls, not floors.

Worked examples for common rooms

These assume 350 sq ft per gallon, standard doors and windows, and smooth previously painted walls. Kitchens need less wall paint than their size suggests because cabinets and backsplash cover so much wall; measure only the exposed wall sections.

Rollers and tools also affect how much you use

A roller nap that is too thick for smooth walls holds more paint than you need and leaves an orange-peel texture; too thin a nap on textured walls leaves bare spots, so you apply extra coats. As a guide, 3/8-inch nap suits smooth drywall, 1/2-inch suits light texture, and 3/4-inch suits heavily textured surfaces or masonry. Load the roller from a tray or bucket grid rather than dunking it, and roll off the excess. Paint sprayers can use noticeably more paint than rolling because of overspray, so add a margin if you spray.

What to do with leftover paint

Seal the can tightly (tap the lid down with a mallet over a rag), store it upside down for a few seconds to seal the rim, then right side up somewhere that does not freeze. Write the room, color name, sheen and date on the lid. Latex paint that has frozen and thawed repeatedly or smells sour should be disposed of according to your local household hazardous waste rules; many areas run paint take-back programs.