How Much Paint Do I Need? Complete Room Paint Calculator Guide

01 / The details

Measure paintable wall area

For a rectangular room, measure length, width, and wall height in feet. Calculate gross wall area with 2 × (length + width) × wall height. If the room is irregular or has different wall heights, measure each wall as width × height and add the results.

Subtract doors, windows, tile, or built-ins you will not paint. Measure each opening and multiply its width by its height. A 36 × 80 in. door is about 20 sq ft; a 36 × 48 in. window is 12 sq ft. Measure in inches, then divide by 144 to convert square inches to square feet.

Net paintable wall area = gross wall area − unpainted openings. Keep ceilings and trim separate because they have their own area and may use different paint.

• Rectangular room: 2 × (length + width) × wall height.
• Each opening: width × height; subtract only surfaces that will not receive wall paint.
• Add separate wall areas for alcoves, angled walls, or rooms that are not rectangular.

02 / The details

Turn square feet into gallons

Use this formula: gallons = paintable wall area × number of finish coats ÷ coverage per gallon. Find the coverage on the exact paint label or product page; it is an estimate per coat, not a guarantee.

For context, BEHR lists coverage of 250–400 sq ft per gallon for some interior products, while Sherwin-Williams’ calculator FAQ gives a typical 350–400 sq ft range. Those figures vary by product and surface. Rough, textured, porous, unprimed, or strongly changing-color walls can use more; use the label for your paint and plan from its lower stated coverage when conditions are difficult.

The examples below assume 350 sq ft per gallon per coat, two finish coats, 8 ft walls, smooth prepared surfaces, and the same door and window sizes. They exclude primer, ceiling paint, and trim.

03 / The details

Example: a 10 × 12 ft room

With 8 ft walls, gross wall area is 2 × (10 + 12) × 8 = 352 sq ft. Subtract one 20 sq ft door and two 12 sq ft windows: 352 − 20 − 24 = 308 sq ft of paintable wall.

For two coats, 308 × 2 = 616 sq ft of coverage. At 350 sq ft per gallon, 616 ÷ 350 = 1.76 gallons, so plan to buy 2 gallons of finish paint. If your product label says 250 sq ft per gallon, the same room needs about 2.46 gallons, so round up to 3.

• Gross walls: 352 sq ft.
• Openings: 44 sq ft; net walls: 308 sq ft.
• Two coats at 350 sq ft per gallon: about 1.76 gallons; buy 2 gallons.

04 / The details

Example: a 12 × 12 ft room

With the same 8 ft walls and openings, gross wall area is 2 × (12 + 12) × 8 = 384 sq ft. After subtracting a 20 sq ft door and two 12 sq ft windows, paintable wall area is 384 − 44 = 340 sq ft.

Two coats cover 340 × 2 = 680 sq ft. At 350 sq ft per gallon, 680 ÷ 350 = 1.94 gallons, so buy 2 gallons. At a label coverage of 250 sq ft per gallon, the estimate is 2.72 gallons, which rounds up to 3.

• Gross walls: 384 sq ft.
• Openings: 44 sq ft; net walls: 340 sq ft.
• Two coats at 350 sq ft per gallon: about 1.94 gallons; buy 2 gallons.

05 / The details

Plan coats and primer separately

Two finish coats are a useful starting assumption when you want even color and hide, but the paint label, current wall color, surface condition, and application directions determine the actual plan. A color change or a wall that absorbs paint unevenly may need additional coverage.

Primer is most useful on new drywall, bare or patched areas, stains, porous surfaces, or when the product system calls for it. A clean, sound, previously painted wall may not need a separate primer coat. Primer is not the same as a finish coat: estimate it separately using its own coverage and recoat instructions.

• New drywall and repaired patches need a suitable primer before finish paint.
• Use stain-blocking primer for stains that could bleed through.
• Follow the paint and primer labels for preparation, coats, and drying time.

06 / The details

Add the ceiling and trim as separate surfaces

A flat ceiling is usually its own estimate: ceiling area = room length × room width. A 10 × 12 ft ceiling is 120 sq ft. Multiply that area by its own coat count and divide by the coverage for the ceiling paint you choose; do not include it in the wall perimeter calculation.

For trim, measure the total length and the exposed face width, then multiply them to estimate square feet. For example, 40 linear feet of 6 in. trim has about 20 sq ft of face area because 6 in. is 0.5 ft. Estimate baseboards, door and window casing, and other trim separately, using the coverage and finish specified for that product.

07 / The details

Choose a practical shopping quantity

Round up to the next package size instead of rounding a gallon estimate down. If the calculation says 1.76 gallons, two gallons covers the assumed area; if your label-based calculation is 2.46 gallons, plan for three gallons unless the product is available in another size that fits the total.

Buy enough of the same paint, color, and sheen for the planned coats, and keep the label for later touch-ups. Recheck the label coverage if you switch products or if the walls are rough, stained, or unprimed.