To find how much paint you need, multiply the room’s perimeter by the ceiling height to get wall area. Subtract doors and windows. Multiply by the number of coats, then divide by about 350 square feet per gallon. A typical 12 by 12 foot room with 8 foot ceilings needs about 2 gallons for two coats.

Whole-house numbers follow from the same math. Here is the method, a worked example, and rough rules of thumb for bigger homes.

How do you calculate how much paint you need?

Use four steps for each room.

  1. Wall area. Add up the length of all walls (the perimeter) and multiply by the ceiling height.
  2. Subtract openings. Take out doors, windows, and large built-ins.
  3. Multiply by coats. Two coats is standard for most repaints.
  4. Divide by coverage. Most paints cover about 350 to 400 square feet per gallon. Use 350 to stay safe.

Check the can, since coverage varies by product and by surface. Rough, textured, or unprimed walls soak up more paint.

How much paint does a 12x12 room need?

Here is the worked example.

  • Perimeter: 12 + 12 + 12 + 12 = 48 feet
  • Wall area: 48 ft x 8 ft ceilings = 384 square feet
  • Subtract one door and one window: about 36 square feet
  • Paintable area: 384 - 36 = 348 square feet
  • Two coats: 348 x 2 = 696 square feet
  • Divide by 350 square feet per gallon: about 2 gallons

So plan on 2 gallons for the walls of a 12 by 12 room. One coat would take about 1 gallon, but most repaints need two, and a big color change may need more.

If you are wondering what a pro would charge for the same room, our interior painting cost guide has typical Front Range ballparks.

How much paint for a 1,500 sq ft house interior?

Rough rule of thumb: many single-story homes around 1,500 square feet need somewhere near 8 to 12 gallons of wall paint for two coats. That assumes typical 8 foot ceilings and a normal number of windows and doors.

This is only an estimate. Floor area does not equal wall area. A home with many small rooms has far more wall than an open plan of the same size, and vaulted ceilings add a lot. Measure the rooms you plan to paint if you want a real number.

What are the rough gallon rules for larger homes?

Treat these as starting points for walls only, two coats, 8 foot ceilings, and average openings. They are not measurements.

Home size (sq ft)Wall paint, two coats (rough)
1,0005 to 8 gallons
1,5008 to 12 gallons
2,00010 to 16 gallons
2,50013 to 20 gallons

Open floor plans, two-story great rooms, and lots of windows change these numbers. Homes with 9 or 10 foot ceilings need more. Measure before you buy.

How much paint do ceilings need?

Ceiling area equals floor area. For the 12 by 12 room, that is 144 square feet. One coat needs about half a gallon, and two coats need about a gallon.

Ceiling paint is usually a flat, white product made to hide flaws and roll on without splatter. Buy it separately from wall paint. If your ceilings have stains, they may need a stain-blocking primer first.

How much paint do trim and doors need?

Trim uses less paint than walls, but it takes more labor. A single room’s baseboard and window casings often need only a quart. A full house of trim and doors can take a few gallons.

Use a durable semi-gloss or satin for trim so it wipes clean. Our sheen guide explains which finish fits which surface. Doors count by size: figure a quart for several interior doors, and more for both sides of a solid door.

Do you need primer?

Sometimes. Buy primer when you are:

  • Painting new drywall or bare patches.
  • Making a big color change, such as dark to light.
  • Covering stains, smoke, or water marks.
  • Painting over glossy surfaces.

New drywall soaks up paint unevenly, so prime it first. If you have repairs to make, read our drywall repair before painting guide. Primer covers about the same area as paint, roughly 300 to 400 square feet per gallon.

Paint-and-primer products can help with small changes. They are not a replacement for dedicated primer on stains or bare surfaces.

What if your walls are not standard?

Adjust the numbers for the room you have.

  • Tall ceilings. A 10 foot ceiling adds 25 percent to the wall area of an 8 foot room. Recalculate with the real height.
  • Dark colors and deep tints. Reds, deep blues, and dark greens often need a third coat, or a tinted primer, to look even.
  • Fresh drywall or heavy repairs. Unprimed surfaces drink paint, so budget for primer plus extra finish paint.
  • Gable walls and stairwells. Measure the triangle: half the base times the height.

Write down each room’s number and add them up. It takes ten minutes and can save a second trip to the store.

What should you buy extra?

Plan a small cushion.

  • Add 10 percent. It covers waste, drips, and thirsty patches.
  • Buy all paint for one room in one batch. Mix the cans together (called boxing) so the color matches from can to can.
  • Keep a quart for touch-ups. Label it with the room and sheen.
  • Get supplies too. Tape, drop cloths, roller covers, and brushes add up. Cheap covers shed lint.

Return policies vary. Ask the store before you buy tinted paint, since it usually cannot be returned once mixed.

Does Colorado change how much you need?

A bit. Dry Front Range air makes paint dry quickly, so rollers and brushes may need reloading more often. Older Boulder and Loveland homes often have textured walls and patched plaster, which use more paint per square foot. Rough textures can soak up 10 to 20 percent more.

Before you start, our guide on how to prepare for interior painting covers the steps that make paint go further and look better. For color ideas that suit local light, see interior paint colors for Colorado homes.

When is it worth hiring it out?

A single bedroom is a fair weekend project. A whole house is a different scale of work, including moving furniture, prep, cutting in, and cleanup. Our post on how long it takes to paint a house gives a realistic timeline. Pros also buy paint at contractor pricing and know exactly how much a job needs.

Not sure how much paint your house needs, or whether to do it yourself? We will measure it and give you a written number. We serve Loveland, Boulder, and Estes Park. Call 720-849-7654 or request a free estimate through our contact form. See our interior painting page for what we handle.