Almost everyone buys the wrong amount of paint. Either you finish the second coat with two-thirds of a gallon left over, which then sits in the basement until it freezes, or you run out at four in the afternoon with one wall to go and the store already closed.
The math is not complicated. The reason people get it wrong is that the standard rule of thumb was built for a wall that does not exist in most houses.
Where the 350 to 400 Square Feet Figure Comes From
Manufacturers print coverage on the can, usually somewhere between 350 and 400 square feet per gallon. That number is real, and it assumes conditions that are close to laboratory conditions: a smooth, previously painted, primed surface in good repair, applied by roller at the recommended film thickness.
Almost nothing in an occupied house matches that. The moment your surface is textured, porous, or a significantly different color than what you are covering, the effective coverage drops, sometimes by a third.
That is the entire source of the error. The number on the can is not wrong; it just describes an ideal you rarely have.
The Four Things That Change the Answer
Texture. A knockdown or orange peel wall has more actual surface area than a flat one. Heavy texture can cut coverage by 20 percent or more, and popcorn ceilings are worse than anything on a wall.
Porosity. New drywall, bare wood, and old chalky paint all drink the first coat. This is what primer is for, and skipping it usually costs more in extra topcoat than the primer would have.
Color change. Going from dark to light, or covering a saturated red or deep blue, can turn a two-coat job into three. A tinted primer solves this far more cheaply than a third coat of finish paint.
Sheen. Flat paints generally cover in fewer coats than glossier ones, which is why ceilings are usually flat, and trim usually is not.
Doing the Measurement Properly
The method takes about ten minutes with a tape measure and a piece of paper.
Measure the perimeter of the room and multiply by the wall height. That gives you gross wall area. Then subtract openings: a standard door is roughly 20 square feet, and a typical window somewhere between 12 and 15. Do not subtract anything smaller than that, because the extra you would save is inside your margin of error anyway.
Divide by the coverage figure on the can, then multiply by the number of coats. Then add ten percent. That last step is the one people skip, and it is the one that keeps you from making a second trip.
Ceilings are calculated separately, as floor area, and trim is estimated by linear foot rather than by area.
Use a Calculator, and Then Sanity Check It
For a whole-house estimate, running the numbers by hand across eight rooms is tedious and error-prone. A paint calculator that asks for dimensions, openings, and number of coats will get you a working figure in a couple of minutes and will not drop a decimal the way a tired person with a phone calculator will.
What a calculator cannot see is your particular wall. If your surfaces are heavily textured, unusually porous, or you are making a dramatic color change, take the output and add to it rather than treating it as final.
Buy It All at Once
One practical point that costs people more than any measuring error: buy the whole job in a single purchase, from the same batch if you can.
Tinted paint varies slightly between mixes. On a wall painted from two batches weeks apart, you can sometimes see the seam in raking light. Professional crews order the full quantity up front and box it, meaning they combine the cans in a larger container and mix them together, precisely so the color is identical from the first wall to the last.
The Regulation Most Homeowners Have Never Heard Of
Here is the part that matters more than any of the above if your house is older, and it is not widely known.
Lead-based paint was banned for residential use in 1978, so any home built before then may have it. Disturbing that paint through sanding, scraping, or aggressive prep creates lead dust, and there is no safe level of lead exposure, particularly for young children. The EPA’s Renovation, Repair and Painting Rule requires that anyone paid to perform work disturbing painted surfaces in pre-1978 homes, child care facilities, and preschools be certified in lead-safe work practices.
The rule generally does not apply to homeowners working on their own home, though it does apply if you rent out all or part of it, run a child care business there, or are renovating to sell. If you are hiring someone for an older house, asking whether the firm is lead-safe certified is a reasonable and completely normal question.
For homeowners doing the work themselves in an older house, the EPA still recommends lead-safe practices: contain the work area, avoid dry sanding and scraping, and clean up thoroughly.
What the Leftovers Are Actually Worth
Assume you will have some paint left, and plan for it rather than pretending you will not.
Keep about a quart for touch-ups, in a properly sealed container, labeled with the room and the color code. Write the code on the can in permanent marker, because store records do not last forever and paint labels fade.
Do not keep the rest. Latex paint that has been through freeze-thaw cycles is unusable, and a shelf of half-empty gallons from 2014 is not a resource; it is a disposal problem. Most municipalities run household hazardous waste collection days, and many paint retailers accept leftover product through take-back programs.
The Short Version
Measure the walls, subtract the doors and windows, divide by the coverage on the can, multiply by coats, add ten percent. Add more if the surface is textured or porous or you are making a big color change. Buy it all at once.
And if the house predates 1978, sort out the lead question before anyone starts sanding. That is the one item on this list where getting it wrong costs more than a wasted afternoon.
