The Formula Behind This Calculator
Wall area = 2 x (length + width) x wall height
Paintable area = wall area - (doors x 21 sq ft) - (windows x 15 sq ft) + ceiling area if included
Gallons = (paintable area x coats / coverage per gallon) x (1 + waste %)
This is the same method paint counters and professional estimators use. A standard interior door with its trim works out to about 21 square feet, and an average window with trim to about 15 square feet, so subtracting them keeps the estimate honest instead of quietly padding it. The coverage rate of 350 square feet per gallon is the figure most major paint brands print on the can for smooth, previously painted walls. Rough texture, bare drywall and deep base colors all drink more paint, which is why the coverage field is editable and why a waste allowance is added at the end rather than hidden inside the math.
Worked Example: A 12 by 14 Bedroom
Say you are repainting a bedroom that measures 12 feet by 14 feet, with 8 foot ceilings, one door and two windows. You plan two coats and one coat of primer because the old color is a dark blue and the new one is a soft white.
- Wall area: 2 x (12 + 14) x 8 = 416 square feet.
- Subtract openings: 416 - 21 (door) - 30 (two windows) = 365 square feet per coat.
- Paint: 365 x 2 coats / 350 = 2.09 gallons. Add 10 percent waste: 2.29 gallons.
- Buy: round up to the nearest quart, so 2 gallons plus 1 quart (2.25 gallons is a hair short, and most people simply buy 3 gallons and keep the spare for touch ups).
- Primer: 365 / 300 coverage = 1.22 gallons, plus waste is about 1.34 gallons, so buy 2 gallons or 1 gallon plus a quart.
- Budget: paint at $30 to $60 per gallon lands around $70 to $140, primer around $30 to $50, for a materials total near $100 to $190 before rollers, tape and trays.
Type those same numbers into the calculator above and you will land in the same place, which is the point: the tool and the hand math should always agree.
A Complete Guide to Buying the Right Amount of Paint
Running out of paint two thirds of the way down the last wall is the classic DIY painting mistake, and it is worse than it sounds. A fresh can mixed later in the day can dry a shade off, especially with custom tints, and the seam where you stopped can stay visible forever. Buying far too much is the quieter mistake: quality interior paint runs $30 to $60 a gallon, so two spare gallons is a dinner out sitting on a garage shelf going thick. The goal is a small, deliberate surplus, and that is exactly what measuring properly plus a 10 percent allowance gives you.
Measure the Room, Not Your Memory of It
Rooms are rarely the size we assume. A bedroom you think of as 10 by 10 is often 11 by 13, and that difference is nearly 40 percent more wall. Measure both walls at floor level, note feet and inches separately, and measure the ceiling height in a corner, because older homes often slope. If the room has a bump out, a chimney breast or an alcove, measure that section as its own little rectangle and add it on. Vaulted or sloped ceilings need the actual sloped length measured with the tape following the angle. Two extra minutes with a tape now is what makes every number after it trustworthy.
Why Coverage Rates Vary So Much
The 350 square feet per gallon on the can assumes a smooth surface, a quality roller cover and one even coat. Real walls differ. Heavily textured walls can use 20 to 25 percent more paint because the roller has to fill every valley. Bare drywall and fresh joint compound act like a sponge until primer seals them. Cheap paint with low solids may need an extra coat to hide, which is why a $20 gallon is often not a bargain. Going from a dark color to a light one, or the reverse, plan on the full two coats and consider a tinted primer, which many paint stores will shade toward your finish color for free. If any of these apply to your room, lower the coverage setting to 300 or raise the waste to 15 percent and the estimate will reflect your walls instead of a laboratory wall.
Primer: When It Is Optional and When It Is Not
Primer is not watered down paint and it is not a sales trick. On new drywall it is close to mandatory, because unsealed compound absorbs finish paint unevenly and leaves dull patches called flashing. Over water stains, smoke damage or crayon, a stain blocking primer is the only reliable fix, since stains bleed through coat after coat of ordinary paint. For a simple refresh of the same color family over clean, sound paint, you can usually skip primer and put the money into a better finish paint. A middle path many pros use is a paint and primer in one product for the first coat on patched walls, then a straight finish coat on top.
Sheen Changes How Far Paint Goes and How It Looks
Flat and matte paints hide flaws and tend to cover well, which suits ceilings and low traffic bedrooms. Eggshell and satin add a gentle glow, wipe clean more easily, and are the workhorses for living rooms and hallways. Semi gloss belongs on trim, doors and kitchens and baths where moisture and scrubbing are routine. Higher sheens show every roller mark and wall defect, so the smoother the sheen, the more your prep matters. None of this changes the gallon math much, but buying the right sheen the first time saves a full repaint, which changes your budget a great deal.
What a Room Really Costs in 2026
For a typical 12 by 12 bedroom, materials usually land between $100 and $220 as of October 2026: two to three gallons of paint, a gallon of primer if needed, plus $25 to $40 of rollers, covers, tape, a tray and a drop cloth, based on national home center pricing. Hiring the same room out typically runs $300 to $600 depending on your market, prep needs and ceiling height, according to published contractor cost surveys. That gap is why painting remains the highest return DIY project in the house, provided you measure first and buy once.
Five Habits That Keep You Out of Trouble
- Box your paint: pour all cans of the finish color into one bucket and mix, so any tiny tint difference disappears before it reaches the wall.
- Buy everything for the job in one trip, including an extra roller cover, because a worn cover wastes more paint than it saves.
- Write the room, color name and date on the leftover can lid with a marker. Future you will be grateful at touch up time.
- Keep a wet edge and work in natural light where you can, so lap marks show up while they are still fixable.
- Store leftovers indoors, off concrete, with plastic wrap under the lid for a tight seal. Properly stored paint stays usable for two to five years.