🖌️ Paint Calculator

How to Calculate Paint for a Room

Knowing how much paint to buy prevents wasted trips to the store and leftover gallons cluttering your garage. Here's the industry-standard method.

Basic Formula

Step by Step

• Calculate wall area: Perimeter (2 × length + 2 × width) × wall height
• Add ceiling if painting it: Length × width
• Subtract openings: Doors (~21 sq ft each), windows (~15 sq ft each)
• Multiply by coats: Most color changes need 2 coats
• Divide by coverage: Typically 300–400 sq ft per gallon depending on finish

Coverage and Sheen

Sheen changes coverage a little and changes everything else a lot. The higher the sheen, the more washable the paint and the more mercilessly it shows every dent, patch and roller mark underneath.

Sheen names are not standardised between manufacturers — one brand's eggshell can be another's satin. Compare the gloss level on the can, not the word on the label. Our paint sheen guide covers the full ladder, room by room, including how sheen behaves on touch-ups and under raking light.

Painting outside? Exterior coverage works differently — stucco and rough-sawn cedar can take roughly double the paint of smooth siding. Use the exterior paint calculator, which adjusts the spread rate by substrate. For what the job costs, see how much it costs to paint a room.

Do I Need Primer?

Primer isn't always necessary, but it helps in several situations:

• New drywall: Always prime bare drywall to seal the surface
• Dark to light color change: A tinted primer reduces the number of top coats needed
• Stains or odors: Use a stain-blocking primer for water stains, smoke, or pet odors
• Glossy surfaces: Primer helps paint adhere to slick finishes

If you're painting a similar color over a previously painted wall in good condition, you can usually skip primer and go straight to two coats of paint.

Which primer matters as much as whether you use one. Water-based, oil and shellac primers block very different things — a water stain painted over with the wrong primer will bleed back through however many coats you apply. Our primer and stain-blocking guide covers what each family actually seals.

How Much Paint for a Room?

Wall area is the perimeter times the ceiling height. These are the common room sizes worked out for you, assuming 8 ft ceilings, two coats, and one door plus one window deducted:

A 5-gallon pail costs less than four separate gallons and removes any risk of color variation between cans. If you're at three gallons or more, price the pail.

If you're buying separate gallons of the same color, box them — pour all the cans into one bucket and mix before you start. Tinted paint varies slightly between cans, and boxing guarantees the last wall matches the first.

Ceilings, Trim, and Doors

• Ceiling: length × width. A 12 × 12 room is 144 sq ft — under half a gallon per coat. See the ceiling paint calculator.
• Trim and baseboards: roughly 1 quart per 100 linear feet, per coat.
• Doors: about 1 quart per 3 doors, per coat, painting both faces.
• Window casings: roughly 1 quart per 8–10 windows, per coat.

What Actually Changes Your Coverage

The 350–400 sq ft on the can assumes a smooth, primed, similarly-colored wall. Real walls rarely are, and these factors matter more than the brand you buy:

Deep reds, bright yellows, and strong oranges are the hardest colors to cover, because their pigments are naturally less opaque. Expect three coats over anything but a matched tinted primer — and have the primer tinted toward the final color, which usually saves a full coat.

Order of Operations

1. Ceiling first. Spatter falls downward, so do it before the walls.
2. Walls second. Cut in the edges with a brush, then roll while the cut-in is still wet so the textures blend.
3. Trim last. It's easier to cut a clean line on trim against a finished wall than the reverse.
4. Respect the recoat time. Most latex paints say two to four hours. Rolling a second coat too soon lifts the first and leaves streaks that no amount of extra paint will fix.
5. Keep a wet edge. Work in sections of about three feet and always roll back into paint that's still wet, or you'll see lap marks in raking light.

Frequently Asked Questions

How much does a gallon of paint cover?

One gallon covers approximately 350–400 square feet on a smooth surface with one coat. Rough or textured surfaces may require more paint per coat.

How many coats of paint do I need?

Two coats is standard for most projects. You may get away with one coat if you're painting the same color. Dramatic color changes (especially dark to light) may need three coats or a coat of primer first.

How much paint for a 12×12 room?

A 12×12 room with 8-foot ceilings has about 384 sq ft of wall area. After subtracting a door and two windows, you need roughly 2 gallons for two coats.

Should I buy extra paint?

Yes. Buy at least 10% extra for touch-ups and future repairs. Store leftover paint in a cool, dry place — it can last years when sealed properly.

How do I calculate paintable wall area?

Multiply the room's perimeter by the ceiling height, then subtract openings — about 21 sq ft per door and 15 sq ft per window. A 12 × 12 ft room with 8 ft ceilings has 384 sq ft of wall, or about 348 sq ft paintable after one door and one window.

Is a 5-gallon bucket cheaper than 5 gallons?

Almost always, and it has a second advantage: no color variation between cans. If your project needs three gallons or more, price the pail. Buying separate gallons is fine too, but box them — pour them all into one bucket and mix before starting.

What is boxing paint?

Combining multiple cans of the same color into one container and stirring them together before you start. Tinted paint varies slightly can to can, and boxing guarantees the wall you paint last matches the one you painted first.

Does texture change how much paint I need?

Considerably. Light orange-peel texture cuts coverage by 10–15%, and heavy knockdown by 20–30%. Spraying instead of rolling costs another 20–40% to overspray. The number on the can assumes smooth, primed drywall.

Which colors need three coats?

Deep reds, bright yellows, and strong oranges — their pigments are inherently less opaque. Have your primer tinted toward the final color and you'll usually save a coat.

How long should I wait between coats?

Check the can, but typically two to four hours for latex. Recoating too early lifts the first coat and leaves streaks you can't paint out. Full cure — when the paint reaches its final hardness and washability — takes up to 30 days.

What order should I paint a room in?

Ceiling, then walls, then trim. Spatter falls downward, so the ceiling has to come first, and it's much easier to cut a clean line on trim against a finished wall than the other way round.

How much paint do I need for trim and doors?

Roughly 1 quart per 100 linear feet of trim per coat, and about 1 quart per 3 doors per coat painting both faces. Window casings run around 1 quart per 8–10 windows.