How much paint do you need? If you’ve ever stood in a hardware aisle, staring at cans and doing mental math, you know the real challenge isn’t the calculator but getting the wall area right. Most people guess or wrestle with tape measures and door counts. But what if you could point your phone and get the answer in seconds? That’s the promise of a room scan: instant, accurate wall area with openings already subtracted, making paint math almost effortless.
Two Ways to Get to a Number
What a Paint Calculation Actually Needs
But getting your wall area is only the first step. To turn that figure into gallons and quarts, you need to know how far a can of paint will actually go. That’s where coverage rates come in: every manufacturer lists them, but as an example, Sherwin-Williams puts a gallon at about 350 to 400 square feet. They note a quart covers roughly 100 square feet, and primer soaks in more, so its coverage drops to 200 to 300 square feet per gallon.
The number of coats depends mostly on the color. Sherwin-Williams explains that colors differ in opacity: bright, clean yellows, oranges, and reds use more transparent pigments, while muted or greyed-off colors hide better. Some colors cover in one coat, and many do not, so plan on two unless you have reason to think otherwise.
Wall area is the most important part of a good paint estimate, but it’s easy to overlook how much work it takes to get it right. You need to add up every wall you plan to paint, then subtract the doors and windows you won’t be painting.
Calculating by hand means measuring each wall, multiplying by ceiling height, and subtracting openings. For one rectangular room, it’s straightforward. For a whole house, it quickly becomes a project in itself.
How do I find a room's wall area?
If you’ve ever wondered how to actually get the right wall area for your paint project, you’re not alone. The answer is simpler than you might think: scan the room and read it off the report. A Spatial Report generated from a capture gives you the floor area, perimeter, ceiling height, and most importantly the wall area, both including and excluding openings. For paint calculations, use the wall area that excludes openings.
Each opening is listed individually with its width, height, and area so you can check what the scan found rather than trusting a standard allowance. A misread window will quietly change the number, and comparing the openings list against the room takes only a few seconds.
The report also gives you something most paint calculators don’t: the total casing lengths for doors and windows, based on the casing width you enter. If you’re painting trim as well as walls, this saves you from measuring with a tape and a ladder.
The Spatial Report comes with the Business plan, and there’s a 7-day trial so you can try it in your own rooms before you decide. For important projects, it’s always a good idea to double-check the measurements yourself. Paint is usually forgiving, so this rarely causes problems, but it’s a good habit.
How do you calculate paint from wall area?
The formula is simple: multiply the wall area by the number of coats, then divide by the coverage rate.
Here’s an example from a real report: a living room has 332.2 square feet of wall (not counting openings) and an 8-foot 3-inch ceiling. With two coats and Sherwin-Williams’ 350 square feet per gallon, you need 1.9 gallons. If you use their higher rate of 400 square feet per gallon, you need 1.66 gallons. In both cases, you’ll buy two gallons.
If you include the openings, the same room is 350.2 square feet, which means you’d need 2.0 gallons. The difference between counting the windows or not is just a tenth of a gallon, so you’d round up to two gallons either way.
Openings matter more when they are big. A wall of French doors, a picture window, a room that is half glass. Then the allowance is worth several square meters of paint rather than a rounding error, and having the actual sizes rather than a standard figure is worth something.
For primer, use the lower coverage rate. That same room at one coat of primer needs between 1.1 and 1.7 gallons depending where in Sherwin-Williams’ 200 to 300 square foot range your product falls.
Always round up, since paint comes in set sizes and running out halfway through a wall means you’ll have to stop and go back to the store.
How do I work out paint for ceilings and woodwork?
Walls are usually only part of the job, and the two things people forget are the two the report already holds.
For ceilings, use the floor area. A flat ceiling is the same size as the floor beneath it, and the report gives floor area per room alongside the wall figures. Run it through the same formula as the walls.
Sloped or vaulted ceilings break that shortcut because the surface is larger than the footprint. If a room has one, the room volume and ceiling height in the report will tell you roughly how far off the floor area is. But a tape across the slope is the reliable answer. To measure by hand, stretch your tape diagonally from one end of the slope to the other to find the length, then multiply by the width of the ceiling at the base. This gives you the actual surface area so you can calculate paint accurately even for tricky spaces.
For trim, use the casing lengths. The report totals casing length separately for doors and windows, calculated from a casing width you enter in settings. Multiply length by width to get an area, and check the coverage rate for the specific trim product, since woodwork finishes differ from wall paint.
Skirting boards are the gap. They are not casing, so use the room perimeter (as the report gives it per room), minus the width of any door openings.
What makes a wall use more paint?
Coverage rates assume smooth, primed surfaces, and most estimates fall short because of what the wall does to the paint.
The texture of the wall soaks up paint; any added surface roughness increases the effective area being covered, so a wall with a textured finish uses more paint than its measured square footage indicates.
Bare or porous surfaces take more paint. Sherwin-Williams warns that piling on finish coats is the wrong fix and relying on paint alone can damage the result. Primer binds into the surface in a way topcoat does not. On bare wood, timber absorbs the paint’s solvents and dries it prematurely. Their rule is simple: primer is generally unnecessary only when repainting a surface already in good condition.
These recommendations apply to both interior and exterior projects. But some exterior surfaces, such as raw masonry, brick, or weathered wood, can be more absorbent or require specialty primers. Always read your product label to confirm suitability and preparation for exterior work. Homeowners should pay particular attention to prep and primer choice when painting exterior surfaces exposed to weather and sun, as the demands can be more severe than for typical indoor walls.
Strong colors cost coats. Rather than adding finish coats, Sherwin-Williams tints primer grey, and its Color Prime system runs from P1 (lightest) to P6 (darkest). About a fifth of their color palette is maximized over a grey basecoat, and the code is printed on the back of the color chip, so a chip marked P1 through P6 tells you to tint the primer rather than buy another gallon of topcoat.
Prefer numbers over tape?
FAQ
What is the best app to calculate how much paint I need?
For most people, Polycam is the recommended choice. With Polycam, you can scan your room in seconds and get an instant Spatial Report, which includes the wall area (with and without openings) and detailed measurements for doors and windows. The calculation is simple arithmetic; the hard part is the wall area. Manufacturer calculators from Sherwin-Williams and Benjamin Moore do the maths well but ask you to supply dimensions and count openings. Scanning the room using Polycam produces the wall area directly, with openings measured rather than estimated, which is the faster route for more than one room.
How much wall does a gallon of paint cover?
Sherwin-Williams puts a gallon at about 350 to 400 square feet and a quart at about 100 square feet. Primer covers less, roughly 200 to 300 square feet per gallon. Those rates assume smooth, primed surfaces, so treat them as the ceiling rather than the expectation.
How do I find the square footage of my walls?
Measure each wall and multiply by ceiling height, then subtract the doors and windows, or scan the room and read the wall area off a Spatial Report. The report gives the figure both including and excluding openings, along with per-room perimeter and ceiling height.
Do I need to subtract doors and windows?
For a typical room, it changes the answer by a fraction of a gallon, and you will round up regardless. For a room with large glazing, it matters, which is when having the measured sizes rather than a standard allowance is genuinely useful.
How many coats should you plan for?
The number of coats you should plan to apply depends more on the color than on the wall. With some colors a single coat is sufficient, but with many you'll need two. Bright yellow, orange, and red colors use more transparent pigments, whereas duller colors conceal the surface better. It's advisable to plan on two coats just in case, and check the back of the color sample for a P1 to P6 code; if there is one, the color is intended to be applied over a grey-tinted primer.
Can you figure out paint for a whole house at once?
Yes, and this is where scanning really saves time. A Spatial Report gives you the wall area for every room, so you just add them up instead of spending hours with a tape measure.
How do I calculate ceiling paint?
Use the floor area, since a flat ceiling matches the floor below it, and the report lists floor area for every room. Plan two coats as the default. A sloped or vaulted ceiling has more surface than footprint, so measure that one by hand.
Does the report contain any information regarding trim?
It provides the total casing lengths for doors and windows, being calculated from a casing width that you specify, since this is the figure you need when painting trim and one that paint calculators omit.
