Paint Calculator

Calculate an approximate amount of paint needed for interior rooms or exterior surfaces based on dimensions, doors, windows, and number of coats. This tool provides an estimate for planning and comparison purposes only.

Surface Type

Units

Room Dimensions (ft)

Length

Width

Wall Height

Openings

Doors

Windows

Number of Coats

Enter dimensions and click Calculate

to see a detailed breakdown.

The assumptions behind the math

The calculator uses 350 sq ft per gallon and starts at 2 coats. Interior walls subtract 21 sq ft per door and 15 sq ft per window, then add ceilings, doors, baseboards, and trim. Interior trim uses 6 sq ft per door and 4 sq ft per window. Exterior mode subtracts 21 sq ft doors, 15 sq ft windows, and 64 sq ft garage doors, then adds 10 sq ft door trim, 10 sq ft window trim, 15 sq ft shutter pairs, and 64 sq ft garage doors. Porous drywall can use 25 to 30% more. Textured walls, drastic color changes, cabinets, and trim may need different math. Round up to the next quart for touch-ups.

How to calculate paint

Enter room dimensions or exterior surface width and height.
Set doors, windows, garage doors, shutters, and coats.
Calculate, then round up for touch-ups before buying paint.

Paint calculator FAQ

How much does a gallon of paint cover?

A gallon usually covers 300 to 400 sq ft for one coat on a smooth surface. This calculator uses 350 sq ft per gallon. Rough walls, heavy texture, and deep colors can lower that.

Do I need two coats?

Usually, yes. This calculator defaults to 2 coats because the second coat gives truer color and better coverage. One coat may be enough when you refresh the same color and sheen.

Do I need primer?

Use primer on bare drywall, sealed repairs, stains, and dark-to-light color changes. A tinted primer can help under strong reds, deep blues, and dark greens. Primer is not counted as finish paint here.

Should I buy extra?

Yes, buy about 10% extra for future touch-ups. Paint mixed later can vary by batch, even with the same formula. Extra from the original job blends better on scuffs and patches.

How much paint does a 12x12 room need?

A 12x12 room with 8 ft ceilings needs 2 gallons of wall paint for two coats. The wall perimeter is 48 ft, so 48 x 8 = 384 sq ft, minus one door at 21 sq ft and two windows at 30 sq ft leaves 333 sq ft per coat. Two coats make 666 sq ft, and 666 / 350 rounds up to 2 gallons because the calculator rounds to whole quarts.

Should the ceiling be calculated separately?

Yes, calculate the ceiling separately because ceiling paint is a different flat, drip-resistant product and often a different color. Use length x width x coats / 350. A 12x12 ceiling at 2 coats is 288 sq ft, so one gallon covers it.

How much does it cost to paint a room?

Multiply the gallons from the calculator by the paint line you plan to buy. In 2026, budget lines run about $25 to $35 per gallon. Mid-range paint runs $40 to $60, and premium paint runs $70 to $90. A 2-gallon wall job costs roughly $50 to $180 in paint alone, and supplies such as tape and rollers add another $30 to $50 for a first-time setup.