HomecalcTool

Paint Calculator for a Room

Calculate exactly how much paint you need for walls, ceiling, and trim in a single room. Includes 10% waste.

Your Price (optional)

Enter your paint's per-gallon price for an exact cost estimate.

How to Use This Calculator

Enter your room's length and width in feet. Set the wall height — 8 feet is standard for most homes, 9 feet for newer construction, 10 to 12 feet for great rooms. The calculator handles walls automatically by computing 2 × (length + width) × height.

Select the number of coats. Two coats is standard for most room repaint jobs. Choose three coats if you're going from a dark color to a light one or painting new unprimed drywall. Enter the number of interior doors and windows so the calculator can subtract those areas and avoid overbuying.

The wall result is for wall paint only. For ceiling paint, multiply your room length × width to get ceiling square footage, then use: gallons = ⌈(ceiling sq ft × 2 coats × 1.1) ÷ 350⌉. A 12 × 12 ceiling needs 1 gallon, a 15 × 20 ceiling needs 2 gallons. Buy ceiling paint separately — it's a different formula and different product than wall paint.

How to Calculate Paint for a Room

Wall paint formula: gallons = ⌈(paintable wall area × coats × 1.1) ÷ 350⌉

Paintable wall area = 2 × (length + width) × height − (doors × 21) − (windows × 15). Each door subtracts one 3 × 7 ft opening. Each window subtracts one 3 × 5 ft opening. The 1.1 factor adds 10% waste for roller nap absorption and touch-ups.

Ceiling paint formula: ceiling gallons = ⌈(length × width × coats × 1.1) ÷ 350⌉

Example: 14 × 12 bedroom, 8 ft walls, 2 coats, 1 door, 2 windows. Wall area = 2 × (14 + 12) × 8 = 416 sq ft. Paintable = 416 − 21 − 30 = 365 sq ft. Wall gallons = ⌈365 × 2 × 1.1 ÷ 350⌉ = ⌈2.30⌉ = 3. Ceiling: 14 × 12 = 168 sq ft. Ceiling gallons = ⌈168 × 2 × 1.1 ÷ 350⌉ = ⌈1.06⌉ = 2. Total for the room: 3 gallons wall paint + 2 gallons ceiling paint.

Room Painting Tips

Cut in before rolling, every coat. Cutting in means painting a 2–3 inch brush stroke along all edges — ceiling line, baseboard, corners, door and window casings. Do it for every coat, not just the first. Cutting in the first coat and then rolling over the edge on the second coat creates visible texture differences between the cut-in strip and rolled section, especially under raking light.

Use a 3/8-inch nap roller for smooth walls, 1/2-inch for textured walls. A nap that's too thick leaves excessive texture — orange-peel effect — on smooth drywall. A nap that's too thin doesn't deposit enough paint on textured surfaces and requires more passes. For ceilings, use a long-nap roller (1/2 to 3/4 inch) even on smooth surfaces to avoid reaching and re-reaching the same spots.

Keep a wet edge at all times. Roll in a W or M pattern, starting 12 inches from the previous section and rolling back into it while both sections are still wet. Stopping in the middle of a wall and picking up after the edge has dried creates a lap mark that shows through the final coat. If you must stop, finish at a corner — the corner line hides where one section ended and the next began.

What to Buy

Wall paint for living areas: Benjamin Moore Regal Select or Sherwin-Williams SuperPaint in eggshell. Both cover 350–400 sq ft per gallon on previously painted surfaces, have excellent washability, and require only two coats over most existing paint colors.

Ceiling paint: any major brand's dedicated ceiling paint — Sherwin-Williams Ceiling Bright White or Benjamin Moore Waterborne Ceiling Paint. Both are thick enough to minimize drips, dry flat to hide texture shadows, and cost $30–45 per gallon, less than premium wall paint.

Trim paint: use semi-gloss or gloss finish for all doors, casings, and baseboards. Sherwin-Williams Emerald Urethane Trim Enamel or Benjamin Moore Advance are the benchmark products — both level beautifully and dry hard enough to withstand daily contact. Buy trim paint in quarts for a single room, gallons if painting multiple rooms in the same color.

Frequently Asked Questions

How much paint do I need for a 12×12 room? +
A 12 × 12 room with 8-foot walls, 2 coats, 1 door, and 2 windows needs 3 gallons for the walls. For the ceiling (144 sq ft, 2 coats, 10% waste): ⌈144 × 2 × 1.1 ÷ 350⌉ = 1 gallon. Total for the full room: 3 gallons of wall paint and 1 gallon of ceiling paint. Trim adds roughly 1 quart for baseboards, door casing, and window trim in a room this size.
Should I use the same paint for walls and ceiling? +
No. Ceiling paint is formulated specifically for overhead application — it is thicker, which reduces drips, and dries to a flat finish that hides imperfections under overhead lighting. Wall paint in flat finish can technically go on a ceiling, but ceiling paint is cheaper per gallon and performs better overhead. Use dedicated ceiling paint for the ceiling, wall paint for the walls.
How many gallons do I need for just the ceiling? +
Ceiling paint covers 350–400 sq ft per gallon, the same as wall paint. For a 12 × 12 ceiling: 144 sq ft. At 2 coats and 10% waste: ⌈144 × 2 × 1.1 ÷ 350⌉ = 1 gallon. For a 20 × 16 ceiling: 320 sq ft. At 2 coats: ⌈320 × 2 × 1.1 ÷ 350⌉ = 2 gallons. New construction ceilings often need 3 coats of ceiling paint the first time due to drywall porosity.
How much paint do I need for trim in one room? +
Trim paint quantity depends on the linear footage of baseboard, door casing, and window trim. As a rule of thumb: one quart of trim paint covers a small to medium bedroom's baseboards and window and door casings. One gallon covers a large room or several rooms. Measure baseboard linear footage and multiply by 0.5 ft width for exact square footage, then divide by 350 sq ft per gallon.
In what order should I paint a room? +
Paint in this order: (1) ceiling first, (2) walls second, (3) trim and doors last. Painting ceiling first means any drips or splatter land on unpainted walls and can be covered. Painting walls before trim means trim paint covers any wall paint that strays onto unpainted trim. This order eliminates the need to tape as extensively and produces cleaner lines at every intersection.
Do I need to prime before painting a room? +
Prime when: the room has new drywall, you're changing color dramatically (dark to light or vice versa), you're painting over glossy surfaces, or the existing surface has stains, water marks, or smoke damage. Skip primer when: the existing paint is the same color in good condition, and the new paint is going on sound latex over latex. Self-priming paints exist but perform better as a second coat over a dedicated primer than as a primer substitute on bare drywall.
What sheen level should I use for walls in different rooms? +
Flat or matte for low-traffic bedrooms and ceilings — hides texture and imperfections best. Eggshell for living rooms, dining rooms, and hallways — washable without looking glossy. Satin for kids' rooms and high-traffic areas — more durable and easier to scrub. Semi-gloss for kitchens, bathrooms, trim, and doors — maximum durability and moisture resistance. High-gloss for furniture and detail work only — reveals every imperfection on walls.
How long does a room take to paint? +
A standard 12 × 12 bedroom takes a single experienced painter 3–4 hours for two coats of walls, including moving furniture, taping, cutting in, and rolling. Budget 6–8 hours if the room needs ceiling and trim as well. The biggest time factor is drying time between coats: water-based latex needs 2–4 hours in normal conditions before a second coat. Rushing the second coat over tacky first coat causes roller marks and poor adhesion.

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