Interior & Finishing · Calculator
Paint Calculator
Use this paint calculator to find how many gallons of paint and primer you need for a room, after doors and windows. It turns the result into cans you can actually buy and estimates the cost.
Your room
21 sq ft each (3 x 7 ft)
15 sq ft each (3 x 5 ft)
Two coats is standard, and a must for color changes.
Smooth, previously painted drywall or plaster
Check the can: most say 350 to 400.
- Wall paint2 gallons
- Ceiling paint1 gallon
How much paint do I need for a room?
For most bedrooms the answer is 2 gallons for the walls and 1 for the ceiling. To get your exact number, measure the length of each wall, add them up, and multiply by the wall height. Subtract the doors and windows, multiply by the number of coats, and divide by the coverage on the can.
Use the low end of the coverage range. Cans often say "up to 400 square feet," which assumes smooth, primed walls and a perfect roller technique. We default to 350 because real walls, real rollers and cutting in eat paint.
Measuring an L-shaped or odd room? Get the floor area first with the square footage calculator, then add up the wall lengths here.
Paint calculator chart for common room sizes
Figured with the calculator above: 8 ft walls, one door, two windows, two coats, smooth walls and 350 sq ft per gallon. Use it as a quick shopping list, then run your own numbers.
| Room | Wall area (sq ft) | Wall gallons | Buy for walls | Buy for ceiling |
|---|---|---|---|---|
| 10 x 10 ft | 269 | 1.5 | 2 gallons | 1 gallon |
| 10 x 12 ft | 301 | 1.7 | 2 gallons | 1 gallon |
| 12 x 12 ft | 333 | 1.9 | 2 gallons | 1 gallon |
| 12 x 15 ft | 381 | 2.2 | 2 gallons + 1 quart | 1 gallon + 1 quart |
| 14 x 16 ft | 429 | 2.5 | 3 gallons | 2 gallons |
| 15 x 20 ft | 509 | 2.9 | 3 gallons | 2 gallons |
| 20 x 20 ft | 589 | 3.4 | 1 five-gallon bucket | 3 gallons |
Paint coverage by surface
Texture adds surface area and porous surfaces soak up paint. The calculator adds about 10 percent for light texture and 25 percent for heavy texture or new drywall, on top of whatever coverage you enter.
Prime new drywall first. Bare paper and joint compound absorb paint at different rates, which shows up as dull patches called flashing. A coat of drywall primer evens that out and is cheaper than a third coat of finish paint. Hanging new board? The drywall calculator covers sheets, mud and tape.
Going from dark to light (or to a deep red or yellow) usually takes a tinted primer plus two coats. Budget for it before you start rather than making a second trip mid-project.
| Surface | Coverage per gallon |
|---|---|
| Smooth, painted drywall | 350 to 400 sq ft |
| Light texture (orange peel) | 300 to 350 sq ft |
| Heavy texture or popcorn | 250 to 300 sq ft |
| Bare drywall (primer) | 200 to 300 sq ft |
| Wood trim and doors | 350 to 400 sq ft |
| Brick, block, stucco | 150 to 250 sq ft |
Which paint sheen to use
Sheen changes durability more than coverage. Flatter paints hide wall flaws, glossier paints wipe clean. Most homes use eggshell or satin on walls, flat on ceilings and semi-gloss on trim.
| Sheen | Look | Best for | Washability |
|---|---|---|---|
| Flat / matte | No shine, hides flaws | Ceilings, adult bedrooms, low-traffic rooms | Low |
| Eggshell | Soft, low glow | Living rooms, bedrooms, hallways | Medium |
| Satin | Pearl-like sheen | Kids rooms, kitchens, bathrooms, family rooms | High |
| Semi-gloss | Noticeable shine | Trim, doors, cabinets, bathrooms | Very high |
| High gloss | Mirror-like, shows every flaw | Doors, trim, furniture accents | Highest |
Tips for buying and applying paint
- Buy all of one color at once. Tints vary slightly from batch to batch. If you run short and buy more later, the difference can show on a wall.
- Box your paint. When you have more than one can of a color, pour them together into a 5-gallon bucket and stir. Every wall gets the exact same mix.
- Keep a quart for touch-ups. Label it with the room, color and date. That is one reason the calculator rounds up.
- Spraying? Airless sprayers lose some paint to overspray, so buy about 20 to 30 percent more than the calculator says. Our guide to using a paint sprayer indoors covers masking and technique, and HVLP vs airless explains which wastes less.
- Cut in first, then roll. Keep a wet edge and roll each wall top to bottom in one session to avoid lap marks.
Common estimating mistakes
- Using floor area instead of wall area. A 12 x 12 room has 144 sq ft of floor but 384 sq ft of wall at 8 ft. Paint goes on the walls, so measure the perimeter and multiply by the height.
- Forgetting the second coat. Most of the "I ran out" trips happen because the estimate covered one coat. The calculator defaults to two for a reason.
- Trusting the best-case coverage. The number on the can is a lab figure. Textured walls, porous patches and a thick roller nap all cut it, which is why the surface setting adds 10 to 25 percent.
- Mixing ceiling and wall paint. Ceilings usually get a flat white ceiling paint, so the calculator lists them as separate cans instead of one total.
- Skipping tall walls. A 9 or 10 ft wall has 12 to 25 percent more area than an 8 ft one. Measure the real height, including stairwells and vaulted ends.
Rolling or spraying a whole house?
Frequently asked questions
How much paint do I need for a 12x12 room?
About 2 gallons for the walls of a 12x12 room with 8 foot ceilings, one door and two windows, at two coats. That is 333 square feet of wall. Add 1 gallon of ceiling paint if you are painting the ceiling too.
How many square feet does a gallon of paint cover?
One gallon covers about 350 to 400 square feet per coat on smooth, primed walls. Textured walls, bare drywall and dark-to-light color changes cut that to 250 to 300 square feet, so plan on more paint.
Do I need two coats of paint?
Usually yes. Two coats give even color and sheen and hide roller marks. One coat is only enough when you repaint the same color over a clean, sound surface, and paint-and-primer products do not change that.
Should I subtract doors and windows when calculating paint?
Yes. Subtract about 21 square feet for each standard door and 15 square feet for each window. In a small room with few openings the difference is small, but in a room with a slider or big windows it can save a gallon.
How much paint do I need for a ceiling?
Multiply the room length by width for the ceiling area, times the number of coats, then divide by 350. A 12x15 ceiling at two coats needs just over 1 gallon, so buy a gallon plus a quart.
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