How It Works

For each wall, multiply height by width. A 12 ft × 9 ft wall is 108 sq ft. Add all walls together. In a rectangular room, that is 2 × (length + width) × height — the calculator does this the moment you type your room's length, width, and wall height.

Measuring an odd-shaped room? Add each wall individually in Detailed mode instead of relying on the rectangle. Room presets (bedroom, living room, kitchen, and more) set sensible starting sizes and openings that you can edit at any time.

These surfaces are not painted. A standard door is about 21 sq ft; a typical window around 15 sq ft. Subtract the total opening area from your wall area to get the true paintable surface. The calculator deducts doors and windows automatically — or use custom opening sizes in Detailed mode.

One coat rarely covers. Two coats is standard over primer; three if you are going from a dark color to a light one. Multiply your paintable area by the number of coats to get the true paint volume. The coat count applies to walls and ceiling alike in the calculator — you select the number of coats used in the estimate.

Coverage is the square feet a gallon covers per coat, and it varies by product, surface, and application. A gallon of interior paint covers roughly 350–400 sq ft per coat on smooth, primed walls — but check the manufacturer's product data for the specific paint you're using.

The calculator uses the coverage rate you enter, defaulting to 350 sq ft/gal for interior paint. Rough exterior siding and stucco absorb more, which is why the exterior estimate defaults to 300 sq ft/gal and lets you adjust it.

Need ceiling paint too? Enable "Include ceiling" to add the ceiling surface (length × width) to the estimate. Ceilings usually take a single coat, and the result separates wall and ceiling totals so you can buy accordingly. Primer is not included in any estimate — add it separately if your walls are bare or patched.

Paint needed (gallons) = (wall area − openings) × coats ÷ coverage. For a room, wall area is 2 × (length + width) × wall height. That is the entire calculation behind the tool — you type the numbers and it does the arithmetic for walls, ceiling, and exterior elevations, showing results in gallons and liters.

Quick Estimate treats the room as a rectangle: one length, width, and height, with standard door and window sizes. It's the fastest way to a ballpark number.

Detailed Estimate lets you model the room wall by wall — individual widths and heights, standard or custom-sized openings, optional ceiling, and optional trim paint (baseboard and crown). It's the better choice for irregular rooms and exact numbers.

In Detailed mode, Interior starts with a wall list. Add each wall, enter its width and height, set its surface, and count its standard doors and windows — or add custom openings with exact dimensions. Optional ceiling and trim paint are added to the total, each with its own settings.

For exterior projects, switch to Detailed → Exterior to calculate paint across multiple elevations, doors, windows, and optional trim. Add one elevation per side of the house with its width, height, and openings. Exterior coverage is separately configurable — it defaults to 300 sq ft/gal for siding, stucco, and brick, and you can adjust it to match your paint's label.

Paint is sold in whole cans, so the calculator rounds the estimated gallons up to whole 1-gallon cans for its "Buy" recommendation — for example, 2.4 gallons becomes 3 × 1 gal cans. That is a simple rounding for convenience, not a claim about the most economical mix of can sizes, and it is not a guarantee that the quantity is sufficient. Round up and keep one spare for touch-ups.

The amount of paint depends on wall area, ceiling area, openings, coats, and coverage — not just room floor size. For a 12 × 14 ft room with 9 ft walls and no openings, the walls are roughly 468 sq ft (2 × (12 + 14) × 9), so two coats need about 2.7 gallons at 350 sq ft/gal. Enter 12 ft length, 14 ft width, and your wall height in the calculatorand adjust doors, windows, and coats for your exact room.

Results are estimates based on the dimensions, openings, coats, and coverage rate you enter. Actual paint usage varies by product, surface, application, and conditions. Check the manufacturer's product data for the specific paint you're using.