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Paint Calculator
Use this free paint calculator to work out exactly how much paint to buy before you start a room. Enter the room size or the total wall area, subtract the doors and windows, choose the number of coats, and it tells you the gallons needed, the cans to buy, and the estimated cost.
A door counts as 21 sq ft and a window as 15 sq ft. Coverage of 350 sq ft per gallon is the usual label value for smooth interior walls.
| Item | Amount |
|---|---|
| Total wall area | 416 sq ft |
| Less doors and windows | 51 sq ft |
| Paintable area | 365 sq ft |
| Exact paint needed | 2.09 gal |
| Gallons to buy (1-gal cans) | 3 |
| Primer needed | None |
| Estimated paint cost | $105.00 |
How it works
Paint is sold by the gallon, but walls are measured in square feet, so the whole job is a conversion: find the paintable wall area, multiply by the number of coats, and divide by how far one gallon goes. Get any one of those three numbers wrong and you either run out halfway through the second coat or end up with a half-used can sitting in the garage for years.
The wall area for a rectangular room is the perimeter times the ceiling height. Add the length and the width, double that to get the perimeter, and multiply by the height. A 14 by 12 foot room with 8 foot ceilings has a perimeter of 52 feet, so the walls total 416 square feet. If you already know the total wall area — from a contractor quote, a floor plan, or a room with an awkward shape — choose the direct-area option and type it in instead.
Paintable area = wall area − (doors × 21 sq ft) − (windows × 15 sq ft)
Gallons needed = paintable area × coats ÷ coverage per gallon
Gallons to buy = gallons needed rounded up to the next whole gallon
Doors and windows are subtracted because you do not roll wall paint over glass or an open doorway. This calculator counts a standard interior door as 21 square feet and a typical window as 15 square feet. If your windows are unusually large, such as floor-to-ceiling windows or sliding glass doors, measure them and use the direct-area option with the area already subtracted.
Coverage is the number on the paint can, usually about 350 square feet per gallon for interior latex paint on a smooth wall. Rough, textured, or bare drywall drinks paint and may cover only 250 to 300 square feet per gallon, so lower the coverage number if your walls are new, rough, or unprimed. The result is always rounded up: you cannot buy 2.09 gallons, so you buy 3 one-gallon cans. If the exact amount is just over a whole number, consider a quart for the small remainder instead of a full extra gallon.
The primer box adds one coat of primer over the same paintable area, using the same coverage. Primer is worth it on bare drywall, patched walls, stained walls, or when you are changing from a dark color to a light one. It seals the surface so the top coats cover evenly and last longer. The cost line covers paint only; primer, rollers, brushes, tape, and drop cloths are separate purchases.
Worked example
You are repainting a bedroom that is 14 feet long, 12 feet wide, with 8 foot ceilings. It has 1 door and 2 windows, and you plan on 2 coats at 350 square feet per gallon, with paint at $35 per gallon.
Step 1: Find the wall area — 2 × (14 + 12) × 8 = 2 × 26 × 8 = 416 square feet.
Step 2: Subtract the openings — 1 door × 21 = 21, and 2 windows × 15 = 30, so 416 − 51 = 365 square feet of paintable wall.
Step 3: Multiply by the coats — 365 × 2 = 730 square feet to cover.
Step 4: Divide by coverage — 730 ÷ 350 = 2.09 gallons of paint needed.
Step 5: Round up and price it — buy 3 gallons, and 3 × $35 = $105.00.
The result is 3 gallons — 2.09 gallons needed, 2 coats, about $105.00. That is exactly what the calculator shows for these inputs.
FAQ
How many gallons of paint do I need for a 12 by 12 room?
A 12 by 12 foot room with 8 foot ceilings has about 384 square feet of wall. After subtracting one door and two windows, about 333 square feet remain. Two coats at 350 square feet per gallon need about 1.9 gallons, so buy 2 gallons. Add a third gallon if the new color is much darker or lighter than the old one.
How much area does one gallon of paint cover?
Most interior latex paint covers about 350 square feet per gallon for one coat on a smooth, primed wall. Rough, textured, or bare drywall absorbs more paint and may cover only 250 to 300 square feet per gallon. Always check the label on your can, because coverage varies by brand, finish, and surface.
Do I need one coat or two coats of paint?
Plan on two coats for almost every repaint. The first coat covers the old color and evens out the surface, and the second coat gives full, even color and a durable finish. One coat is usually enough only when you are repainting the same color on a wall that is already in good shape.
Should I subtract doors and windows from the wall area?
Yes. A standard interior door is about 21 square feet and a typical window is about 15 square feet, and you do not paint that glass or open space with wall paint. Subtracting them keeps you from buying an extra gallon you do not need, especially in a room with several windows.
How much does it cost to paint a room?
Paint itself usually costs $25 to $60 per gallon, so a typical bedroom needing 3 gallons costs about $75 to $180 in paint. Add primer, rollers, brushes, tape, and drop cloths, which often add $30 to $60. Hiring a painter is a separate cost and is usually charged per room or per square foot.