How much area does a gallon of paint cover?

One gallon of interior wall paint covers about 350 to 400 square feet in a single coat on a smooth, primed surface. Rough, porous, or unprimed walls absorb more and cover less, so plan for the lower end.

How many coats of paint do I need?

Two coats is standard for an even finish and full coverage. Use a primer coat plus two finish coats when painting over a dark color, a glossy surface, or bare drywall.

Do I subtract doors and windows?

Yes. Subtract roughly 20 square feet for a standard door and 15 square feet for an average window from the wall area, since you do not paint those surfaces.

Home / Paint Calculator

Paint Calculator

A paint calculator tells you how much paint a room needs and what it costs. It takes the room dimensions, the number of coats, and the doors and windows to subtract, and returns the gallons to buy plus the cost. Pick a room preset below or type your own measurements. Everything runs in your browser, with no signup.

Updated for 2026

Your room

Pick a preset or type the dimensions. Results update instantly.

Room preset

Length

Width

Ceiling height

Coats

coats

Doors

x 20 ft²

Windows

x 15 ft²

Paint price

$/ gallon

Paint to buy

Paintable area

0 ft²

With coats

0 ft²

Exact gallons

Paint cost

Coverage assumed

350 ft² / gallon / coat

How much paint common rooms need

Gallons to buy for two coats with one door and two windows, at 350 square feet per gallon. Tap any row to load it into the calculator.

Room	Size	Paintable area	Gallons (2 coats)

How the paint estimate is calculated

The wall area is the room perimeter times the ceiling height. Subtract the doors and windows, multiply by the number of coats, then divide by how far a gallon goes:

Gallons = ( Perimeter × Height − Openings ) × Coats ÷ 350

The perimeter is twice the length plus twice the width. A standard door is about 20 square feet and an average window about 15. One gallon of wall paint covers roughly 350 to 400 square feet per coat, so 350 is a safe planning figure. Round up to whole gallons, since paint is sold by the gallon and a little extra is useful for touch ups.

Worked example (Stand: 2026)

Room12 ft × 14 ft, 8 ft ceiling
Perimeter (2 × (12 + 14))52 ft
Gross wall area (52 × 8)416 ft²
Openings (1 door + 2 windows)50 ft²
Net wall area366 ft²
Two coats (366 × 2)732 ft²
Exact gallons (732 ÷ 350)2.09 gal
Gallons to buy3 gal
Paint cost (3 × $35)$105

Frequently asked questions

How much paint do I need for a room?

Multiply the room perimeter by the ceiling height for the wall area, subtract about 20 square feet per door and 15 per window, multiply by the coats, then divide by 350 square feet per gallon. The calculator above does this for any room you enter.

How much does a gallon of paint cover?

About 350 to 400 square feet per coat on a smooth, primed wall. Rough, porous, or unprimed surfaces soak up more and cover less, so plan for the lower end and keep some paint for touch ups.

How many coats should I use?

Two coats is standard. Add a primer coat when you paint over a dark or glossy color, or over bare drywall, so the new color stays even.

Should I subtract doors and windows?

Yes. They are not painted, so subtract roughly 20 square feet per door and 15 per window. For a large picture window or sliding door, measure it and subtract the real area.

Does this calculator store my numbers?

No. Everything is calculated in your browser. Nothing you enter is sent to a server or saved.

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