A 12 by 14 foot bedroom with 8 foot ceilings, one door and two windows has about 372 square feet of wall to paint. Two coats make that 744 square feet of rolling, and at the 400 to 450 square feet per gallon printed on current interior paint data sheets, the walls need 1.65 to 1.86 gallons. Buy 2 gallons for the walls.
That answer depends on three numbers you can check yourself: the wall area after openings, the number of coats, and the spreading rate the manufacturer prints for the product. The usual shortcut, "one gallon covers 350 to 400 square feet," is close, but it hides where the number comes from and when it stops being true.
Measuring the paintable area
Walls are the perimeter times the height. For the 12 by 14 room the perimeter is 12 + 14 + 12 + 14 = 52 feet, and 52 feet times 8 feet is 416 square feet of gross wall.
Then subtract the openings you will not paint. Use the real sizes if you can measure them. Where you cannot, these are the assumptions behind every room figure on this page:
| Opening | Assumed size | Area deducted |
|---|---|---|
| Interior door | 36 by 80 inches | 20 sq ft |
| Window | 3 by 4 feet | 12 sq ft |
One door and two windows take 20 + 24 = 44 square feet off, which leaves 372 square feet of wall.
Show the numbers behind this figure
| Item | Size | Area (sq ft) |
|---|---|---|
| Walls (perimeter x height) | 52 ft x 8 ft | 416 |
| Door | 36 x 80 in | −20 |
| Windows (2) | 3 x 4 ft each | −24 |
| Paintable wall area | 372 |
Small deductions such as outlets and switch plates are a fraction of a square foot each, far less than the rounding in the final purchase, so they can be ignored. Large openings are worth measuring: a sliding patio door 6 feet wide and 80 inches high is 40 square feet, twice the deduction for an ordinary interior door.
The same method applied to some common rooms, with two coats at the low end of the data-sheet range (400 square feet per gallon):
| Room (L x W x ceiling height) | Openings assumed | Net wall area | Gallons, 2 coats at 400 sq ft/gal | Ceiling area |
|---|---|---|---|---|
| 10 x 10 x 8 ft | 1 door, 1 window | 288 sq ft | 1.44 | 100 sq ft |
| 12 x 12 x 8 ft | 1 door, 2 windows | 340 sq ft | 1.70 | 144 sq ft |
| 12 x 14 x 8 ft | 1 door, 2 windows | 372 sq ft | 1.86 | 168 sq ft |
| 12 x 14 x 9 ft | 1 door, 2 windows | 424 sq ft | 2.12 | 168 sq ft |
| 14 x 16 x 8 ft | 1 door, 2 windows | 436 sq ft | 2.18 | 224 sq ft |
| 16 x 20 x 8 ft | 2 doors, 3 windows | 500 sq ft | 2.50 | 320 sq ft |
An accent wall splits the estimate in two. Paint one 14 foot wall with a window in a second colour and it is 112 − 12 = 100 square feet, or 200 square feet for two coats: 0.44 to 0.50 gallons, which is 2 quarts. The other three walls are 272 square feet, or 544 for two coats: 1.21 to 1.36 gallons, which rounds up to a gallon and 2 quarts. Splitting the colours leaves the total at 2 gallons (2 quarts of the accent colour plus a gallon and 2 quarts of the main colour), but with almost nothing spare in either colour; buying the accent colour as a full gallon would make it 2.5.
Ceiling height matters more than people expect. Going from 8 to 9 feet in the same 12 by 14 room adds 52 square feet of wall, and that pushes two coats at 400 square feet per gallon to 2.12 gallons, past the 2 gallons that were enough at 8 feet. The square footage calculator handles rooms that are not rectangles: split an L-shaped room into two rectangles and add the lengths of the outside walls only.
What the data sheets actually say
Benjamin Moore and PPG both publish a technical data sheet (TDS) for each of the products below, and each sheet gives a spreading rate per coat along with the conditions behind it. We read the current sheets for two widely sold interior lines: Benjamin Moore's Regal Select (US sheets for four sheens) and two PPG Sico interior paints sold in Canada. They are cited here as published data, not as a recommendation of any product.
| Product (sheet) | Spreading rate per coat | Wet film | Dry film | Volume solids |
|---|---|---|---|---|
| Benjamin Moore Regal Select, flat (N547) | 400 to 450 sq ft per gallon | 3.6 to 4.0 mils | 1.6 to 1.9 mils | 46% |
| Benjamin Moore Regal Select, matte (N548) | 400 to 450 sq ft per gallon | 3.6 to 4.0 mils | 1.5 to 1.7 mils | 43% |
| Benjamin Moore Regal Select, satin/pearl (N550) | 400 to 450 sq ft per gallon | 3.6 to 4.0 mils | 1.3 to 1.5 mils | 38% |
| Benjamin Moore Regal Select, semi-gloss (N551) | 400 to 450 sq ft per gallon | 3.6 to 4.0 mils | 1.3 to 1.5 mils | 37% |
| PPG Sico Plus, flat (341-50X) | approximately 400 sq ft per 3.78 L | 4.0 mils | 1.3 mils | 32% |
| PPG Sico Clean Surface Technology (383-50X) | up to 400 sq ft per 3.78 L | 4.0 mils | 1.7 mils | 44.7% |
A mil is a thousandth of an inch. The fine print is as important as the number.
- The Sico Plus sheet gives its 400 square feet "on primed, smooth, nonporous surfaces," and says the coverage figures "do not include material loss due to application."
- Benjamin Moore's sheets print the spread rate with the note "Depending on surface texture and porosity," and ask the buyer to "estimate the right amount of paint for the job," because doing so will "ensure color uniformity."
- The Sico sheets list their film thickness "according to the maximum spreading rate". The rate and the film are the same fact stated two ways, which is the key to the next section.
None of the four Regal Select sheets makes one coat the recommendation. Each says that while the product "sometimes makes one-coat coverage achievable," Benjamin Moore "recommends two coats to achieve full color development," that two coats are recommended on bare substrates, and that previously painted surfaces "can be finished with 1 or 2 coats". The Sico sheets warn that "some colours, or drastic colour changes, may require an additional coat of paint."
Where 400 square feet per gallon comes from
The spreading rate on a data sheet is geometry. A US gallon is 231 cubic inches. Spread it in a layer one mil thick (0.001 inch) and it covers 231 ÷ 0.001 = 231,000 square inches, which is 1,604 square feet. So:
coverage per gallon (sq ft) = 1,604 ÷ wet film thickness (mils)
| Wet film thickness | Coverage per gallon |
|---|---|
| 3.6 mils | 446 sq ft |
| 4.0 mils | 401 sq ft |
| 4.5 mils | 356 sq ft |
| 5.0 mils | 321 sq ft |
That is why the Benjamin Moore range of 400 to 450 square feet lines up with its 3.6 to 4.0 mil wet film, and why the Sico sheets, at 4.0 mils, say about 400. The rate on the sheet is the wet film the maker specifies, converted into area. Put the paint on thicker and the gallon covers less, in exact proportion. The popular "350 square feet" corresponds to a wet film of about 4.6 mils. Read against the 4.0 mil target, it sets aside about an eighth of each gallon (401 minus 350, divided by 401) for absorption, texture and the roller: a planning margin, not a measured loss.
The dry film is the wet film times the volume solids, the share of the can that stays on the wall once the water leaves. At 4.0 mils wet, the 46% flat leaves 1.84 mils of dry paint on the wall and the 32% Sico Plus leaves 1.28 mils, in line with the dry films those sheets list. The two paints cover the same area per gallon, but the first leaves about 44% more dry material behind. That is the hidden variable in "coverage": two paints with the same printed spreading rate leave different amounts of film on the wall, so the printed square feet say nothing on their own about how well a coat hides the colour underneath.
The formula also explains the gap between the sheet and real walls. Paint soaked into porous drywall paper, paint that fills the valleys of a textured wall, and paint left in the tray and on the roller cover all leave the can without adding film to the face of the wall. The Sico Plus sheet says as much: its coverage figures exclude material lost in application, and porous surfaces "may require additional coats of paint to achieve a uniform finish." Wherever those losses are large, a real gallon covers less than the printed rate.
Show the numbers behind this figure
| Net wall area (sq ft) | 1 coat (gal) | 2 coats (gal) |
|---|---|---|
| 100 | 0.22 to 0.25 | 0.44 to 0.50 |
| 200 | 0.44 to 0.50 | 0.89 to 1.00 |
| 300 | 0.67 to 0.75 | 1.33 to 1.50 |
| 372 | 0.83 to 0.93 | 1.65 to 1.86 |
| 400 | 0.89 to 1.00 | 1.78 to 2.00 |
| 500 | 1.11 to 1.25 | 2.22 to 2.50 |
| 600 | 1.33 to 1.50 | 2.67 to 3.00 |
| 800 | 1.78 to 2.00 | 3.56 to 4.00 |
| 1000 | 2.22 to 2.50 | 4.44 to 5.00 |
Why one coat rarely covers
A single coat at the specified film leaves 1.3 to 1.9 mils of dry paint on these products. Whether that hides the colour underneath depends on the colour change, not on the spreading rate.
- Same colour, sound surface. A repaint in the same or a similar colour is the case where the Regal Select sheets allow one coat: previously painted surfaces "can be finished with 1 or 2 coats."
- A large colour change. Both makers name it as a reason for another coat: "drastic colour changes" in the Sico sheets, "full color development" in Benjamin Moore's. Benjamin Moore positions its Fresh Start High-Hiding All-Purpose Primer (046) as "the go-to for significant color change," and its Multi-Purpose Latex Primer sheet (N023) says that "for best hiding results" the primer should be "tinted to the approximate finish coat color."
- Some custom colours. The Regal Select sheets carry a special note that "Certain custom colors may require a Deep Base Primer tinted to a special prescription formula to achieve the desired color."
Rolling one coat on thicker is not a shortcut. By the formula above, a coat twice as thick uses exactly the paint of two normal coats, and the Sico sheets note that "thick films" extend drying time. So the default for a colour change is two coats, and the shopping question is whether to plan a third. The arithmetic is linear: a third coat on the 12 by 14 room is another 372 square feet, or 0.83 to 0.93 gallons more. One way to avoid over-buying is to buy for two coats plus a quart and decide after the second coat dries. A tinted primer under the first coat is the other route, covered next.
Rough, porous and new surfaces
The data-sheet rate assumes a smooth, sealed surface. Each of these takes more paint or an extra product, and the sheets themselves say so:
New drywall. The Regal Select sheets list the paint as self-priming on drywall but recommend two coats on bare substrates. The Sico Plus sheet takes the other approach: "Prime all bare wood, gypsum, plaster, masonry, metal, and porous surfaces with an appropriate primer." Benjamin Moore's Fresh Start High-Hiding All-Purpose Primer (046) has its own sheet: 400 to 450 square feet per gallon at 3.6 to 4.0 mils wet, the same geometry as the finish. On a new room with a separate primer, budget the primer as a full coat of area: for the 12 by 14 room, 372 square feet of primer (0.83 to 0.93 gallons) plus 744 square feet of finish. Use the drywall calculator to size the board itself, and count the primer separately from the paint.
Patched walls. The Sico Plus sheet says that on "very porous or absorptive repairs," for optimal results, "locally apply one coat" of primer before the finish, and Benjamin Moore's N023 sheet says previously painted surfaces "may be primed or spot primed as necessary." A quart of primer at the 046 sheet's rate covers 100 to 113 square feet, which is a lot of patches.
Textured walls. Knockdown, orange peel and popcorn ceilings have more surface than their flat outline. The sheets give no figure for texture, only the note that the spread rate depends on surface texture and porosity, so treat the lower end of the range, or below it, as your planning rate. Using 350 square feet per gallon instead of 400 means buying about 14% more paint.
Masonry. Rough or pitted block is a different system. For it, the Regal Select sheets list a high-build masonry block filler (Ultra Spec 571) as the primer, not the finish paint. The filler fills the pores rather than coating them, and the finish coats go on over it.
Stains and slick surfaces. The Regal Select sheets suggest asking about special-purpose primers "if the surface to be painted is water stained, smoke damaged, grease stained or very slick," and list Fresh Start primers (032 or 046) for bleeding woods such as redwood and cedar. The 046 sheet adds that "two coats of primer may be required on severe stains." Treat each primer coat as an extra coat of area in the estimate.
Ceilings, trim and doors measured separately
Walls, ceiling and trim often take different products, so measure them separately rather than adding everything into one number.
Ceiling. The area is length times width: 12 x 14 = 168 square feet. Two coats need 336 square feet, or 0.75 to 0.84 gallons, so one gallon of ceiling paint covers it with some left over. Benjamin Moore's flat sheet describes that finish as "ideal for ceilings and low-traffic areas."
Baseboard and casing. Trim is measured as length times face height. In the 12 by 14 room there are about 49 feet of baseboard (the 52 foot perimeter less the 3 foot door). Assuming a 4.5 inch baseboard, that is about 18 square feet. Door casing, assumed here as 17 feet of 3.5 inch casing around each side of the door, adds about 5 square feet per side.
Doors. A flat slab door is 20 square feet a side, or 40 if you paint both faces.
For the whole trim package in the example room (baseboard, both faces of the door and both sides of the casing, about 68 square feet), two coats need about 137 square feet of paint. That is 1.4 quarts at 400 square feet per gallon, so 2 quarts of trim paint. The Sico Plus sheet recommends "a glossier finish" on trim and doors, and the Regal Select semi-gloss sheet lists interior trim and doors among its uses. The wall estimate is unchanged, because the trim was never inside the 372 square feet.
Quarts, gallons and 5-gallon pails
A US gallon is 4 quarts or 3.785 litres, so a quart is about 0.946 litres. The Sico sheets state their coverage per 3.78 L, which is the same volume as a US gallon to within rounding. At the 400 to 450 square feet per gallon on the sheets above:
| Container | Volume | Coverage per coat at 400 to 450 sq ft/gal | Two-coat wall area it covers |
|---|---|---|---|
| Quart | 0.25 gal (0.946 L) | 100 to 113 sq ft | 50 to 56 sq ft |
| Gallon | 1 gal (3.785 L) | 400 to 450 sq ft | 200 to 225 sq ft |
| 5-gallon pail | 5 gal (18.9 L) | 2,000 to 2,250 sq ft | 1,000 to 1,125 sq ft |
Turning a fractional answer into cans is a rounding decision:
- Take the gallons from the arithmetic, for example 1.86.
- Whole gallons first: 1.
- Convert the rest to quarts and round up: 0.86 gallons is 3.44 quarts, so 4 quarts.
- Four quarts are a gallon, so the order becomes 2 gallons. When the remainder rounds to three quarts or more, a single gallon can is the simpler purchase and leaves paint over for touch-ups.
A pail holds as much as five gallon cans, and one colour needs that much once its two-coat wall area passes about 800 to 900 square feet (4 gallons at 400 to 450 square feet per gallon cover 800 to 900 square feet twice), for example two or three bedrooms in the same colour. Whatever the container, both makers address colour consistency across cans. Benjamin Moore asks you to estimate the full amount for the job to "ensure color uniformity," and the Sico Plus sheet says: "When using more than one container of the same colour, intermix to ensure colour uniformity." Pouring several cans together into one larger bucket before you start does exactly that.
Pre-1978 homes: the lead-safe rules
The lead warning printed on these data sheets is about preparation, not the paint in the can: "If you scrape, sand, or remove old paint, you may release lead dust." EPA puts the same point this way: renovation, repair and painting activities "can create hazardous lead dust when surfaces with lead paint, even from many decades ago, are disturbed."
The EPA's Renovation, Repair and Painting (RRP) Rule, in 40 CFR 745 Subpart E, sets the rules for paid work. As of the eCFR text current on October 26, 2026:
- Who it covers. The subpart applies to "all renovations performed for compensation in target housing and child-occupied facilities." Target housing is "any housing constructed prior to 1978," with exceptions for housing for the elderly or persons with disabilities and 0-bedroom dwellings, unless a child under 6 lives or is expected to live there. EPA says the requirements "became fully effective April 22, 2010," and that firms doing this work "must be EPA- or state-certified and must use certified renovators who follow specific work practices."
- What counts as a renovation. Any modification of an existing structure "that results in the disturbance of painted surfaces," which includes the scraping and sanding that come before painting.
- The small-job exception. Minor repair and maintenance that disrupts "6 square feet or less of painted surface per room for interior activities or 20 square feet or less of painted surface for exterior activities" is exempt, but only where none of the prohibited or restricted practices are used and the work "does not involve window replacement or demolition of painted surface areas."
- Prohibited and restricted practices. Open-flame burning or torching of painted surfaces; machine sanding, grinding, power planing, needle guns or abrasive blasting unless the machine has a shroud or containment system with a HEPA vacuum attachment; and heat guns, which are permitted "only at temperatures below 1,100 degrees Fahrenheit."
- Before work starts. "No more than 60 days before beginning renovation activities," the firm must give the owner EPA's lead pamphlet and get a written acknowledgment (or a certificate of mailing at least 7 days before), and must also give it to an adult occupant if the owner does not live in the unit.
- Testing out. The rule does not apply where a certified inspector or risk assessor, a certified renovator using an EPA-recognized test kit, or an EPA-recognized lab testing paint chips has determined that the components affected are free of lead-based paint at the regulatory levels.
EPA's page for do-it-yourselfers says that "generally speaking, the RRP Rule does not apply to homeowners doing RRP projects in their own home," but that it does apply if you rent out all or part of your home, run a child care centre in it, or buy, renovate and sell homes for profit. For homeowners doing their own work in a pre-1978 home, the same page recommends lead-safe work practices, and it also recommends hiring a lead-safe certified contractor. In such a home, the six-square-foot threshold is a useful yardstick when reading a contractor's quote: scraping a whole room of peeling trim is well beyond it. This summary of the rule is not legal advice; the regulation text and your state's program, where a state runs its own, govern a specific job.
None of this changes the gallons. It changes the preparation, the time and the cost of the job, which in an older house can be larger than the paint.
Keeping enough for touch-ups
The paint left in the last can is worth planning for, not just tolerating. A later touch-up is easiest with paint from the same mix, and the Sico Plus sheet is frank that touch-ups do not always vanish: "If the touch-up remains apparent, apply an additional coat on same area or on entire surface."
For the 12 by 14 room, 2 gallons against a need of 1.65 to 1.86 gallons leaves 0.14 to 0.35 gallons, or about 0.6 to 1.4 quarts, before losses to the tray and roller. That is a reasonable reserve if the walls take the paint at the sheet's rate. On a textured or new-drywall room, where a planning rate of 350 square feet per gallon is more realistic, two coats need 2.13 gallons, and the same 2 gallons will run short. That room needs 2 gallons and a quart, or a third gallon if you want paint left over.
Three habits make the leftover useful later:
- Write on the can lid the room, the colour name and formula code, the sheen and the date it was tinted, so a future touch-up or repaint can be matched.
- Store it sealed and frost-free. Every data sheet cited on this page carries the instruction "protect from freezing". The Sico Plus sheet gives a storage range of 10 to 30°C (50 to 86°F); the Regal Select sheets give 40 to 90°F.
- Wait out the cure before washing a touched-up wall. The Regal Select sheets say painted surfaces "can be washed after two weeks," and the Sico sheets give 30 days for full curing.
The paint touch-up and waste calculator adds an application-waste allowance and a sealed touch-up reserve to the basic quantity. For the main estimate, the paint calculator takes your net wall area (after the door and window deductions worked above), the number of coats and a coverage rate, and rounds up to whole gallons. Its default coverage is 350 square feet per gallon, so the 12 by 14 room's 372 square feet at two coats comes out at 2.13 gallons, or 3 cans; enter 400 to match the data sheets and it returns 1.86, or 2. Other home-project guides, from concrete for posts and slabs to the tools for renovation and landscaping, are collected on the homeowners hub. Treat every estimate on this page as a planning figure: the label on the can and the product's own data sheet govern how a specific paint should be applied.
Sources
- Benjamin Moore, Regal Select Premium Interior Paint and Primer technical data sheets (US): Flat N547 (sheet code N547 US 032525), Matte N548 (N548 US 032525), Satin/Pearl N550 (N550 US 040225), Semi-Gloss N551 (N551 US 062025), for spread rate, wet and dry film, volume solids, coats, primer systems, storage and the lead warning: https://media.benjaminmoore.com/WebServices/prod/assets/production/datasheets/TDS_0547/N547_TDS_US.pdf, https://media.benjaminmoore.com/WebServices/prod/assets/production/datasheets/TDS_0548/N548_TDS_US.pdf, https://media.benjaminmoore.com/WebServices/prod/assets/production/datasheets/TDS_0550/N550_TDS_US.pdf, https://media.benjaminmoore.com/WebServices/prod/assets/production/datasheets/TDS_0551/N551_TDS_US.pdf (checked October 28, 2026)
- Benjamin Moore, Fresh Start High-Hiding All-Purpose Primer 046 technical data sheet (046 US 030325): https://media.benjaminmoore.com/WebServices/prod/assets/production/datasheets/TDS_0046/046_TDS_US.pdf (checked October 28, 2026)
- Benjamin Moore, Fresh Start Multi-Purpose Latex Primer N023 technical data sheet (N023 US 052220): https://media.benjaminmoore.com/WebServices/prod/assets/production/datasheets/TDS_0023/N023_TDS_US.pdf (checked October 28, 2026)
- PPG Architectural Coatings Canada, Sico Plus 341-50X technical data sheet (edition 06/2024): https://buyat.ppg.com/rep_pafpainttools_files/Sico/SC_34150X_CAEN.pdf (checked October 28, 2026)
- PPG Architectural Coatings Canada, Sico Clean Surface Technology 383-50X technical data sheet (edition 11/2025): https://buyat.ppg.com/rep_pafpainttools_files/Sico/SC_3835XX_CAEN.pdf (checked October 28, 2026)
- US EPA, Lead Renovation, Repair and Painting Program Rules (last updated December 15, 2025): https://www.epa.gov/lead/lead-renovation-repair-and-painting-program-rules (checked October 28, 2026)
- US EPA, Lead-Safe Renovations for DIYers (last updated August 20, 2026): https://www.epa.gov/lead/lead-safe-renovations-diyers (checked October 28, 2026)
- eCFR, 40 CFR 745.82 (applicability), 745.83 (definitions: renovation, minor repair and maintenance), 745.84 (pamphlet), 745.85 (prohibited and restricted practices), 745.103 (target housing), text current as of October 26, 2026: https://www.ecfr.gov/current/title-40/chapter-I/subchapter-R/part-745 (checked October 28, 2026)
- Room areas, gallon ranges, the wet-film conversion and the container table are CalculatorVillage calculations with the assumptions stated in each table.