How Much Paint Do I Need? Room-by-Room Guide

A standard 10 by 12 foot bedroom with 8 foot ceilings needs about 2 gallons of paint for two coats on the walls. A 12 by 16 foot living room needs 3 gallons. A whole 2,000 square foot house exterior typically takes 12 to 18 gallons depending on siding texture and trim work. These numbers assume one gallon covers 350 square feet per coat, which matches most major brands.

If you want exact numbers for your specific room or project, the how much paint calculator handles the math in seconds. This guide explains the formula, walks through real examples at common sizes, and covers waste, primer, and ordering decisions that trip people up.

The Paint Quantity Formula

Every paint calculation comes down to one equation:

Gallons needed = (Paintable area × Number of coats) / Coverage per gallon

Paintable area is the total wall surface minus doors and windows. Coverage is printed on the can label, usually 350 to 400 square feet per gallon for smooth interior walls. Coats are almost always 2 for finish paint, sometimes 1 for touch-ups, and 1 for primer.

Here’s how it works on a real wall. A 12 foot long wall that’s 8 feet tall has 96 square feet. Two coats means 192 square feet of paint coverage. Divide by 350 square feet per gallon and you get 0.55 gallons. Round up to the nearest quart or gallon at the store.

Subtracting Doors and Windows

A standard interior door takes up 20 square feet. A standard window takes up 15 square feet. Subtract these from your total wall area before calculating.

Most rooms have one door (20 sq ft) and one or two windows (15 to 30 sq ft). That’s 35 to 50 square feet you don’t need to paint. Across two coats, that saves roughly a quart of paint, which is the difference between buying 3 gallons and 2 gallons plus a quart on a medium room.

Room-by-Room Examples

These examples assume 8 foot ceilings, two coats, and 350 sq ft per gallon coverage. All calculations include a 10% waste factor for touch-ups and second coats over uneven coverage.

Small Bedroom (10 by 12 feet)

The four walls total 352 square feet. Subtract 20 for the door and 15 for one window. You’re left with 317 square feet of paintable area.

Step	Calculation	Result
Wall perimeter	2 × (10 + 12)	44 ft
Wall area	44 × 8	352 sq ft
Minus openings	352 - 35	317 sq ft
Two coats	317 × 2	634 sq ft
Gallons needed	634 / 350	1.81 gal
With 10% waste	1.81 × 1.10	2.0 gal

Buy 2 gallons. If you want to paint the ceiling too, add another gallon. Ceilings need their own gallon because ceiling paint is a different product than wall paint in most cases.

Living Room (12 by 16 feet)

A medium living room with one door, two windows, and 8 foot ceilings needs about 3 gallons for the walls.

The perimeter is 56 feet. Wall area is 448 square feet. Subtract 20 for the door and 30 for two windows, leaving 398 paintable square feet. Two coats brings it to 796 square feet. Divide by 350 and add 10% waste, and you land at 2.5 gallons. Round up to 3 gallons because paint stores sell in whole gallons and quarts, and 3 gallons gives you about a quart of leftover for touch-ups over the next few years.

Whole House Exterior (2,000 sq ft, single story)

Exterior paint coverage is lower than interior, usually 250 to 300 square feet per gallon because siding is textured. A typical single-story 2,000 square foot home has about 1,400 to 1,800 square feet of paintable exterior wall after subtracting windows, doors, and the roof footprint.

At 275 sq ft per gallon, two coats on 1,600 square feet of siding needs about 12 gallons. Add 2 gallons for trim and another 2 gallons for primer if your siding hasn’t been painted in over 8 years. Total: 14 to 16 gallons for the project.

Why Coverage Varies

The 350 sq ft per gallon figure is for smooth drywall with one coat over similar color paint. Real coverage depends on three things:

• Surface texture. Smooth drywall: 350 to 400 sq ft per gallon. Knockdown texture: 300 to 350. Heavy stucco or rough cedar siding: 200 to 250.
• Color change. Painting white over dark red? You’ll need a primer coat plus two finish coats, or three finish coats of a high-hide paint. Coverage effectively drops by a third.
• Paint quality. Premium paints (Benjamin Moore Aura, Sherwin-Williams Emerald) cover in one coat over similar colors. Builder-grade paints often need two coats even on freshly primed walls. Read the label for the manufacturer’s coverage claim.

I learned this on a kitchen project where I tried to paint navy blue walls white with two coats of mid-grade paint. The result was patchy after two coats and needed a third gallon plus a weekend of touch-ups. A tinted primer would have saved a gallon and several hours.

Always Order Extra

Add 10% to 15% to your calculated total. Here’s why:

• Second coat needs more. The first coat soaks into the surface. The second coat sits on top and covers a bit more area, but you’ll use about 95% of the first coat’s quantity, not less.
• Touch-ups happen. Scuffs, nail holes, and accidents during furniture moves all need touch-ups. Having the exact same can (same dye lot) saves a repaint later.
• Rollers and trays absorb paint. A fresh roller absorbs about 1/4 cup before it starts releasing paint to the wall. Tray liners trap another 1/4 cup. That’s a quart lost across a full house project.

For a 2 gallon job, buy 2 gallons and a quart. For a 12 gallon job, buy 14 gallons. The extra cost is small compared to a mid-project store run that breaks your flow.

Primer Is a Separate Calculation

Primer coverage is usually 200 to 300 square feet per gallon, lower than finish paint because primer is thinner and the surface is often porous. Calculate primer the same way as finish paint, but use 1 coat and the primer’s own coverage number.

A 400 square foot room that needs full primer takes about 1.5 gallons of primer at 275 sq ft per gallon. Don’t try to combine primer and paint quantities. Most paint cans labeled “paint and primer in one” still benefit from a true primer on bare drywall, stains, or major color changes.

When to Use the Calculator

For one room with simple shapes, the manual formula works fine. The paint calculator helps when:

• You have multiple rooms with different ceiling heights
• You’re including trim, doors, and baseboards separately
• You want primer quantities calculated alongside finish paint
• You’re switching between feet, inches, and meters
• You need to handle exterior walls with gables and dormers

The calculator subtracts openings automatically, applies waste factor, and rounds to gallons and quarts in the combinations stores actually sell. For trim quantities, you may also want the painting estimate calculator which includes labor and supply costs.

Frequently Asked Questions

How much paint do I need for a 12x12 room?

A 12 by 12 foot room with 8 foot ceilings has 384 square feet of wall area. After subtracting one door (20 sq ft) and one window (15 sq ft), you have 349 paintable square feet. Two coats need 2.2 gallons. Buy 2 gallons and a quart, or 3 gallons if you also want to paint the ceiling.

How many gallons of paint for a 1,500 square foot house exterior?

A single-story 1,500 square foot house has about 1,200 to 1,400 square feet of paintable exterior siding. At 275 sq ft per gallon coverage and two coats, plan for 9 to 11 gallons of body color paint. Add 1 to 2 gallons for trim and 2 gallons of primer if the siding is bare or chalky.

Does a gallon of paint cover 400 square feet?

A gallon of paint covers 350 to 400 square feet on smooth, primed drywall under ideal conditions. Textured walls drop coverage to 250 to 300 square feet. Bare drywall, stained surfaces, or major color changes can drop effective coverage to 200 square feet per gallon. Always check the can label and add 10% extra.

Should I buy paint in gallons or quarts?

Buy paint in gallons when you need more than 1.5 gallons total. Gallons cost less per ounce than quarts and store better once opened. Use quarts for trim, accent walls, and touch-ups under one gallon. A standard project mix is 2 gallons of wall paint plus 1 quart of trim paint.

How much extra paint should I buy for waste?

Add 10% to 15% extra paint for waste, rollers, trays, and touch-ups. On a 2 gallon job, buy 2 gallons and a quart. On a 12 gallon job, buy 14 gallons. The extra cost is small and saves a mid-project store run if you run short on the second coat or need touch-ups months later.