Paint Coverage Calculator: Square Feet to Gallons

How a paint coverage calculator converts square feet to gallons

A paint coverage calculator turns square feet into gallons with one core formula. Wall area divided by the coverage rate per gallon, multiplied by the number of coats. Wall area is the room perimeter times the ceiling height. Sherwin-Williams puts typical coverage at about 350 to 400 square feet per gallon, enough for a 10 ft by 10 ft room. Two coats is the residential default. So a room with 360 sq ft of wall painted twice needs about 2 gallons.

Painting is a $49.1 billion U.S. industry in 2026, per the IBISWorld Painting and Wall Covering Contractors report. The Bureau of Labor Statistics counted 342,200 construction and maintenance painters in 2024, at a median wage of $23.40 an hour. Plenty of them still size paint by eye at the store counter. A paint coverage calculator removes the guess. SimplyWise Cost Estimator turns a photo into an itemized estimate in about 6 seconds, with material quantities you can check against the numbers above.

The formula in one line

Gallons equals wall area in sq ft, divided by coverage per gallon, times the number of coats. Take 360 sq ft of smooth wall at 375 sq ft per gallon for two coats. That is 360 divided by 375, times 2, which equals 1.92 gallons. Round up to 2. Always round each line up to the next whole gallon. Running short mid-job costs more in a return trip than the extra gallon costs at the counter.

Step 1: Measure wall area

The calculator needs wall area first, and wall area is the room perimeter times the ceiling height. Add the length of all four walls to get the perimeter. Then multiply by the ceiling height in feet. A 12 ft by 15 ft room has a perimeter of 54 ft. At an 8 ft ceiling that is 54 times 8, or 432 sq ft of gross wall area before deductions.

For an L-shaped or bumped-out room, walk the baseboard. Add every straight run as its own length, including the short returns. Skipping them is a common way to undershoot wall area by 5 to 10 percent.

Step 2: Subtract doors and windows

Next, decide whether to deduct openings. Leave windows and doors in the wall area unless total openings exceed 15 percent of the wall. Cutting in around the trim eats the surface area you would have saved. A standard interior door is about 20 sq ft. A typical window is about 15 sq ft.

The 15 percent rule worked out

On the 432 sq ft room above, 15 percent is about 65 sq ft. One door and two windows come to about 50 sq ft, so they stay in the wall area. They fall under the threshold, and the cutting-in labor offsets the saved paint. A sunroom with 120 sq ft of glass on the same wall clears the threshold. There, deduct the glass and quote the trim cutting-in as its own labor line.

Size the paint and price the job from a photo, free

Step 3: Apply the coverage rate per gallon

Now divide the net wall area by the coverage rate for the surface. Coverage drops sharply on textured, porous, or rough surfaces. A calculator that uses one flat number for every surface will underbuy on stucco and overbuy on smooth drywall.

The smooth-drywall figure is Sherwin-Williams’ published typical coverage. The other rows are working trade ranges, not manufacturer claims. Pull the technical data sheet for the exact product on hand and use its published spread rate whenever the buy is large.

Step 4: Multiply by the number of coats

Multiply the gallons figure by the number of coats. Residential repaint prices two finish coats by default. Color changes, going lighter, painting over stains or repairs, and warranty exposure all push scope to two coats. Single-coat pricing is reserved for a same-color refresh on premium paint over a clean substrate.

When the calculator needs three coats

Some jobs need three. A dark-to-white color change is the classic case. So are bright reds and yellows, whose pigments hide poorly. Bare or stained surfaces pull the first coat in unevenly and need another. A deep accent wall going back to white is where two coats leave a ghost and the painter eats the third coat for free. Add the third coat to the calculator instead of hoping.

Step 5: Add a primer line

Primer is its own line, not part of the finish-paint math. It belongs on bare drywall, bare wood, patched repairs, stained surfaces, and dramatic color changes. Primer covers less per gallon than finish paint, because the substrate is more absorbent. Bare drywall and patched walls run 200 to 300 sq ft per gallon of primer. Size primer on the same wall area, at the lower rate.

When you can skip primer

You can skip a dedicated primer coat on a clean, previously painted surface in a similar color with a self-priming paint. Self-priming finish paints still spread at the finish rate, not the primer rate, so the calculator does not change. The decision is binary. Either add a primer line at 200 to 300 sq ft per gallon, or rely on the finish paint and keep the two-coat math.

Step 6: Calculate ceilings and trim separately

Ceilings and trim never share the wall-paint line. A calculator that lumps walls, ceiling, and trim into one number will misbuy on at least one of them. Each uses a different product and a different unit.

Ceiling math

Ceiling area equals room length times width. A 12 ft by 15 ft room has a 180 sq ft ceiling. At 375 sq ft per gallon, that ceiling needs about half a gallon for one coat, or just under a gallon for two. Ceilings often take one coat on a same-color refresh. A repair or a smoke-stained ceiling needs two coats plus a stain-blocking primer.

Trim and door math

Trim converts from linear feet. Add up baseboard, casing, and crown, then divide by 350 to 400 linear feet per gallon. A room with about 94 linear ft of trim needs well under a gallon of enamel for two coats. That is 54 ft of baseboard plus 40 ft of casing. Doors count per door, both sides, two coats. A typical six-panel door uses roughly a quarter of the per-gallon trim coverage.

Worked example: a 12 by 15 ft bedroom, color change

Take a 12 ft by 15 ft bedroom with 8 ft ceilings, one door, two windows, smooth drywall, and a beige-to-navy color change. The math runs through every line: walls, primer, ceiling, and trim.

1. Wall area

   Perimeter is 12 plus 15 plus 12 plus 15, which equals 54 ft. At an 8 ft ceiling, gross wall area is 432 sq ft. One door and two windows total about 50 sq ft, under the 65 sq ft threshold, so leave them in. Net wall area stays 432 sq ft.

2. Wall paint

   432 divided by 375 equals 1.15 gallons per coat. The beige-to-navy change needs two coats, so that is 2.3 gallons. Round up to 3 gallons to cover the dark-over-light hide and cutting-in waste.

3. Primer

   The dramatic color change calls for a tinted primer coat over the full 432 sq ft. At 250 sq ft per gallon, that is 1.7 gallons. Round up to 2 gallons of primer.

4. Ceiling

   Ceiling area is 12 times 15, which equals 180 sq ft. At 375 sq ft per gallon for two coats, that is about 0.96 gallon. Round up to 1 gallon of flat ceiling paint.

5. Trim and door

   About 94 linear ft of trim plus one door, at 350 to 400 linear ft per gallon for two coats, is well under a gallon. Buy 1 gallon of trim enamel for the trim and both door faces.

6. Shopping list

   The calculator outputs 3 gallons of wall paint, 2 of primer, 1 of ceiling paint, and 1 of trim enamel. That is the full buy for one bedroom with no return trip, because every line rounded up.

Quick-reference paint coverage table

For a fast on-the-truck conversion, the table below pairs common scenarios with a gallons estimate for two coats. Treat these as starting points. Confirm against the calculator math above for the surface and color change in front of you.

The whole-home figure is the one most often underbought. It is easy to forget closets, hallways, and stairwells. Walk every paintable room and add each perimeter before you run the whole-home line.

Why a paint coverage calculator protects your margin

Mis-sizing paint cuts both ways. Overbuying eats material margin directly. Underbuying triggers a return trip that burns an hour of labor. A calculator that lands the buy on the first trip protects both lines at once. Once gallons are locked, paint cost flows into the material line and the coat count flows into application hours. That is why the calculator is the first input to the full estimate covered in the SimplyWise guide on how to estimate a painting job.

From coverage calculator to a same-day quote

Plenty of painters still size paint the same way. Walk the room, scribble dimensions on a clipboard, do the division in the truck, then guess at the store. That workflow leaves room for an arithmetic slip on every line. The painter who hands over a clean, itemized quote on site wins jobs from the one who promises a number tomorrow.

Photo to estimate with SimplyWise

SimplyWise Cost Estimator builds the estimate from a phone photo. The painter photographs the wall, ceiling, or trim and adds the room measurements in the project description. The app returns an itemized estimate in about 6 seconds, with material quantities. Every line item carries unit, quantity, and price, so you can edit any material, labor, or fee line on the spot. On Pro iPhones, a LiDAR room scan captures measurements. You can then hand over a professional, branded quote the client signs to approve. To put the numbers on paper afterward, pair it with the free SimplyWise painting invoice template.

What is included

• Photo to itemized estimate in about 6 seconds.
• Estimates that come back with material quantities.
• Editable material, labor, and fee line items.
• Unit, quantity, and price on every line item.
• LiDAR room scans on Pro iPhones to capture measurements.
• Professional, branded quotes the client signs to approve.