Paint Calculator

Quick reference for the most common room sizes. Assumes 8 ft ceiling, 1 door (21 sq ft), 1 window (15 sq ft), 350 sq ft per gallon coverage. Add 1 gallon if including the ceiling.

Bump up by 1 gallon if walls are textured (popcorn, knockdown), or going from dark to light colors (third coat needed).

Most interior latex paint covers around 350 square feet per gallon on smooth, primed drywall. The formula sounds simple:paint = (wall area − doors and windows) × coats ÷ 350. In practice, three things trip people up: forgetting to subtract openings, underestimating the number of coats (almost always 2 over a color change), and not accounting for surface texture.

Our paint calculator handles all of it. Enter your room's length, width, and ceiling height, count your doors and windows, choose how many coats you plan to apply, and we tell you exactly how many gallons and quarts to buy. Include the ceiling, switch to metric, tweak coverage for rough surfaces — all handled.

Step 1: Wall area

Multiply the room's perimeter by the ceiling height. Perimeter is 2 × (length + width). For a 12 ft × 10 ft room with 8 ft ceilings, walls are 2 × (12 + 10) × 8 = 352 sq ft.

Step 2: Subtract openings

A standard interior door is about 3 ft × 7 ft = 21 sq ft. A standard window is about 3 ft × 5 ft = 15 sq ft. Subtract these from the wall area. Our calculator uses these defaults; adjust if your doors and windows are notably larger or smaller.

Step 3: Multiply by coats

Almost every interior paint job needs two coats: one for coverage, one for an even finish. Three coats when going from dark to light (or vice versa), or over fresh drywall without primer.

Step 4: Divide by coverage

Standard interior latex covers 350 square feet per gallon on smooth surfaces. Bump this down for textured walls (250–300) or stucco (150). Round up to the nearest gallon, then check if a quart can cover the remainder.

• Smooth drywall, second coat: 400 sq ft / gallon
• Smooth drywall, first coat: 350 sq ft / gallon
• Textured drywall or popcorn ceiling: 250–300 sq ft / gallon
• Bare wood or rough lumber: 200 sq ft / gallon
• Brick or stucco: 150 sq ft / gallon
• Concrete block: 175 sq ft / gallon

Premium paints (Sherwin-Williams Emerald, Benjamin Moore Aura) can legitimately cover with one coat over similar colors, which doubles your effective coverage. Cheap paints often need a third coat to look right.

Primer counts as its own coat for paint calculation purposes. Self-priming paints (most premium brands) can skip a dedicated primer step on most surfaces, but you still need two coats of the self-priming paint.

1. 1

   Prep

   Move or cover furniture. Remove switch plates and outlet covers. Patch holes with spackle, sand smooth, wipe with a damp cloth. Tape edges and trim.
2. 2

   Cut in

   Use a 2.5 in angled brush around the ceiling line, baseboards, and corners. Paint a 2–3 in band that the roller cannot reach.
3. 3

   Roll

   Use a 9 in roller with a 3/8 in nap for smooth walls or 1/2 in for textured. Load the roller, roll a W pattern, then fill it in. Work in 3 ft × 3 ft sections.
4. 4

   Second coat

   Wait the can-recommended dry time (usually 4 hours for latex). Repeat cut-in and roll.
5. 5

   Clean up

   Latex paint cleans up with warm water. Soak brushes, work the bristles to release paint, rinse, store. Pull tape while paint is still slightly tacky for a clean edge.

How many gallons of paint do I need for a 12 × 12 room?

A 12 × 12 room with 8 ft ceilings, one door, and one window needs about 1.7 gallons for two coats. Buy 2 gallons.

Should I buy paint by the gallon or quart?

Gallons are cheaper per ounce. Quarts make sense only for the last 15–30 sq ft when you are between sizes, or for very small projects (closet, accent wall). Our calculator tells you the optimal mix.

Why does paint coverage vary so much?

Three reasons: surface texture (textured walls eat more paint), paint quality (cheap paint needs more coats), and application method (sprayers waste 20–30% to overspray). The 350 sq ft default is for a brush or roller on smooth, primed drywall with quality paint.

Can I mix two cans of the same color?

Yes, and you should. Mix them together (called "boxing") before starting so any tiny color variations average out. Just pour them into a larger bucket and stir.

How long does paint last?

Unopened: 2–10 years depending on type (latex shorter, oil longer). Opened: 1–2 years if sealed and stored cool. Test a small patch on cardboard before using stored paint on a wall.