CALC Material Calc

Interior · Exterior

Paint Calculator: Wall Area to Gallons

Enter your room's dimensions, how many doors and windows to subtract, and how many coats you're planning. This tool converts that to gallons and an estimated cost.

Your room

Add up all four wall lengths. A 12×12 room = 48 ft.

Each standard door subtracts ~21 sq ft.

Each standard window subtracts ~15 sq ft.

You'll need

0.0
gallons, rounded up to the nearest quart
Paintable area 0 sq ft
Total coverage needed 0 sq ft
Estimated cost —

Estimates only, based on a standard 350 sq ft/gallon coverage rate. Textured walls, dark color changes, and unprimed drywall all reduce actual coverage — check your specific paint can's stated coverage rate.

Tools for this job

Paint Roller & Tray Kit A full kit with roller frame, extension pole, and tray — covers most wall-painting jobs without piecing tools together. Painter's Tape Clean-edge tape for trim, ceilings, and switch plates — makes the difference between crisp lines and touch-up work. Canvas Drop Cloth Reusable canvas holds up far better than plastic sheeting for protecting floors and furniture. Replacement Roller Covers Worth having a couple spares on hand — a worn roller cover leaves texture and lint in the finish.

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How the math works

Paint coverage is based on wall area, not room volume. The calculation:

wall area = perimeter (ft) × height (ft)
paintable area = wall area − (doors × 21) − (windows × 15)
total coverage needed = paintable area × number of coats
gallons = total coverage needed ÷ 350

The 350 sq ft figure is a standard single-coat coverage rate most paint brands use as a baseline — check your specific can, since some formulas (especially deep-base colors) cover less per gallon.

Coverage reference

ContainerCovers (1 coat, standard paint)
1 quart~87 sq ft
1 gallon~350 sq ft
5 gallons~1,750 sq ft

Common mistakes

The most common overspend: not subtracting doors and windows on a room with several of each, which can overstate a gallon need on a small room. The most common underbuy: assuming one coat is enough for a major color change — going from a dark to a light color, or vice versa, almost always needs two full coats to look even, not just touch-ups.

Frequently asked questions

How much wall area does a gallon of paint cover?

One gallon of paint typically covers about 350 square feet with a single coat, though this varies by paint brand, wall texture, and whether the surface is primed.

Do I need two coats of paint?

Two coats are usually recommended for a full color change, uneven surfaces, or lighter colors going over darker ones. A single coat can work when repainting a very similar color over a well-prepped surface.

Should I subtract doors and windows from my paint estimate?

Yes, for accuracy on larger rooms, though many painters skip it for small rooms since the leftover paint covers touch-ups and the math error is minor relative to a full gallon.