Enter your room dimensions and get the exact gallons needed. Add multiple rooms for a full project estimate.
Measure the room, not the wall you happen to be facing. You need four numbers per room, and three of them come from a tape measure.
The tool measures walls. Ceilings, trim, and the faces of doors are not included, and the FAQ below covers how to add them by hand.
Walls are 2 × (14 + 12) × 9, which is 468 square feet. Take out one door (20) and two windows (30) and the paintable area is 418 square feet. Two coats at 400 square feet per gallon is 2.09 gallons. The 10% buffer makes it 2.30, and the page rounds to 3 gallons. At the mid-range $45 a gallon that is about $135.
Now measure the ceiling, because this is where the purchase changes. The ceiling is 14 × 12, or 168 square feet. Two coats is 0.84 gallons, 0.92 with the buffer. Walls plus ceiling is 3.22 gallons, and the page now calls for 4 gallons, roughly $180. One measurement moved the order by a whole can.
| What you measure | Area | Gallons, 2 coats, with buffer |
|---|---|---|
| Walls (14 × 12 × 9 room) | 418 sq ft | 2.30 → buy 3 |
| Ceiling | 168 sq ft | 0.92 |
| Both | 586 sq ft | 3.22 → buy 4 |
Trim is a separate problem. Baseboard, casing, and crown are measured in linear feet, and 1 gallon usually covers a whole house's trim in two coats. Add it only if you are changing trim color, because that job needs its own can and its own sheen.
The headline number is whole gallons, rounded up, already carrying the 10% buffer. That extra covers color variation between cans, paint left in the tray and the roller, and touch-ups six months later. Rounding is not padding: you cannot buy 2.3 gallons, and stopping at 2 leaves a wall unfinished.
Compare the per-gallon cost before you buy four singles. Most brands price a 5 gallon pail below five singles, and if your job lands at 4 or more gallons the pail is usually the cheaper way to get there. Tinting is the catch: a pail is mixed as one batch, so it is ideal when the whole job is one color and awkward when each room is different.
The coverage box matters more than the price tier. The 350 / 400 / 450 presets are a range, not a promise. Fresh drywall drinks paint, so does stucco and any wall with heavy texture; smooth, previously painted walls land near the top of the range. The number printed on your can is the one to enter, and a cheap paint usually covers less than an expensive one, which quietly cancels out part of the savings.
Nothing here is a substitute for a lead test. If the house was built before 1978, sanding or scraping the old paint is a containment question before it is a quantity question.
With 8 foot ceilings, one door, and two windows: 384 square feet of wall, 334 after openings, 1.67 gallons at two coats, 1.84 with the buffer. Buy 2 gallons. A 9 foot ceiling pushes it to 2.08 after the buffer, and you would buy 3.
Only over an identical color and sheen in good condition. Repainting the same off-white over itself can work in one coat. Any color change, patches, or a sheen change needs two, and going light to dark usually needs a tinted primer plus two.
Run the same wall area through the formula with one coat. On the 14 by 12 room that is 418 ÷ 400, about 1.05 gallons, so one gallon with the buffer. Primer covers roughly like paint, but check the can, because shellac and stain-blocking primers are thinner.
Only if you are painting them. The 20 and 15 square foot deductions assume the door and window stay as they are. Painting a door's two faces and edge adds about 40 square feet, so count it as its own row and remove the deduction.
Titanium dioxide, the pigment that hides what is underneath, is what makes paint opaque, and deep bases carry less of it. Going from a light wall to navy or deep red commonly needs three coats or a tinted primer. Set the coat selector to 3 and buy the extra can in the same batch.
Water-based paint stored sealed and out of freezing temperatures is usually usable for years; oil-based lasts longer still. Label the can with the room and the date, and keep it for touch-ups rather than throwing away a half gallon you already paid for.
Coverage per gallon belongs to the manufacturer and is printed on the can. The lead rule is the EPA's.
Last updated: November 2, 2026. Reviewed by David Liu. The 20 and 15 square foot door and window allowances are this page's averages for standard openings, not a code requirement.