Paint Calculator

Enter your room size, or the total wall area, to see how many gallons and quarts of paint and primer to buy. Doors and windows are subtracted for you.

Quick answers

One gallon of paint covers about 350 to 400 sq ft per coat; a quart covers about 100 sq ft (Sherwin-Williams). Primer covers less, about 200 to 300 sq ft per gallon.
A typical 12 × 12 ft bedroom with 8 ft walls has 384 sq ft of wall. After one door and two windows, two coats need about 2 gallons.
Plan on two coats for most repaints, and for any big color change. Rough, porous or textured walls can use 10% to 25% more paint.

How to use the paint calculator

Measure the length and width of the room and the wall height. If you only paint some walls, switch to I know the wall area and enter width × height of each wall added together.
Count doors and windows. The calculator subtracts about 20 sq ft per door and 12 sq ft per window.
Check the coverage on your paint can and enter it. Most say 350 to 400 sq ft per gallon.
Buy what the result says. Gallons plus a quart is cheaper than an extra gallon you will not use.

The formula

Wall area is 2 × (length + width) × height. Paintable area is wall area minus doors and windows, plus the ceiling (length × width) if you paint it. Gallons = paintable area × coats ÷ coverage per gallon.

Worked example: a 12 × 14 ft bedroom with 8 ft walls

2 × (12 + 14) × 8 = 416 sq ft of wall. Minus one door (20) and two windows (24) leaves 372 sq ft. Two coats at 350 sq ft per gallon: 372 × 2 ÷ 350 = 2.13 gallons. Buy 2 gallons and 1 quart.

Paint for common room sizes

8 ft walls, one door, two windows, two coats, 350 sq ft per gallon, walls only.

Room	Wall area	Paintable	Gallons	Buy
10 × 10 ft bedroom	320 sq ft	276 sq ft	1.58	2 gal
12 × 12 ft bedroom	384 sq ft	340 sq ft	1.94	2 gal
12 × 14 ft bedroom	416 sq ft	372 sq ft	2.13	2 gal + 1 qt
14 × 16 ft living room	480 sq ft	436 sq ft	2.49	2 gal + 2 qt
16 × 20 ft family room	576 sq ft	532 sq ft	3.04	3 gal + 1 qt

Tips

Buy all the paint for one room at the same time so the color batch matches. Mix cans together in a bucket if you need more than one.
Ceilings usually take a flat ceiling paint and one or two coats. Trim is measured separately and usually needs only a quart.

Common questions

How many gallons of paint do I need for a 12 × 12 room?

With 8 ft ceilings the walls are 384 sq ft. Take off one door (20 sq ft) and two windows (12 sq ft each) and you have 340 sq ft. Two coats at 350 sq ft per gallon is 1.94 gallons, so buy 2 gallons. Add about 0.4 gallons more if you also paint the ceiling once.

How many square feet does a gallon of paint cover?

Sherwin-Williams says about 350 to 400 sq ft per gallon, and Behr lists 250 to 400 sq ft depending on the surface. Smooth, primed drywall is at the high end; bare drywall, textured walls and brick are at the low end. This calculator defaults to 350 to stay on the safe side.

Do I need primer?

Use primer on bare drywall, patched areas, stained walls, glossy surfaces and big color changes such as dark to light. Many paint-and-primer products can skip a separate primer on walls that are already painted and in good shape.

How do I calculate wall area?

Add up the length of all walls (the room perimeter) and multiply by the wall height. A 12 × 14 ft room has a 52 ft perimeter; at 8 ft high that is 416 sq ft. Then subtract doors and windows.

Related calculators

Sources

Reviewed October 4, 2026 by ShedMath Editorial Team. See how we calculate.

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