Texture is the whole calculation
The number on a can of exterior paint is somewhere between 350 and 400 square feet per gallon. That figure is real, and it is measured on a smooth, sealed, non-absorbent surface. Almost nothing on the outside of a house is smooth, sealed and non-absorbent.
Square footage measured with a tape is plan area. What the paint has to cover is actual surface, and on a rough sawn board, a shake, or a sand-finish stucco the actual surface is substantially larger than the plan area, because it is folded into grain, saw marks and aggregate. On heavy stucco or split-face block the difference is not subtle: a gallon that covers 375 square feet of smooth siding may cover 125 of split face, a factor of three.
That is why the spread rate here is an input with the ranges shown rather than a constant. Pick the texture that matches what you are painting, or override the rate outright if you have a figure from the product data sheet for the exact surface. Everything downstream is just division.
| Surface | Square feet per gallon | Against smooth |
|---|---|---|
| Vinyl, aluminium, smooth fibre cement, sound old paint | 350 to 400 | 1.0x |
| Wood-grain fibre cement, planed cedar lap | 300 to 350 | about 1.15x |
| Rough sawn wood, T1-11, shakes and shingles | 200 to 250 | about 1.7x |
| Sand-finish stucco, light render | 150 to 200 | about 2.1x |
| Heavy stucco, split-face block, exposed brick | 100 to 150 | about 3.0x |
Those are working ranges rather than specifications. The data sheet for the product you have bought, applied to the surface you actually have, is the authority. Where a range is wide, the low end is the honest planning figure.
The first coat is not the same size as the second
On a sealed surface every coat costs roughly the same. On a porous one the first coat is partly absorbed and the second sits on top of it, so the first can take twenty-five to fifty percent more material than the ones that follow. Bare wood, weathered and chalky old paint, and new masonry all behave this way.
The calculator handles that with a multiplier on the first coat only. It matters more than people expect on a repaint of a weathered wall: two coats on a chalky surface is not twice one coat, it is about 2.25 times, and the difference on a large house is a full pail.
Priming is the other half of that question and it is a different decision from coats. Spot priming bare and repaired areas is what most repaints need, and the share of the wall involved is usually somewhere between ten and thirty percent. A full prime is for a wall that is bare, that has been stripped, or where the colour or the substrate demands it. The calculator lets you set the share rather than guessing for you, because it is the one input that varies most between two houses that look the same.
Trim, and why it is priced separately
Trim is a small area and a large share of the work. Four hundred linear feet of six inch trim is only two hundred square feet of face, about a gallon and a half at two coats, but it is where the brush time goes and it is usually a different product from the body. Counting it into the body gallons produces a single number that cannot be bought, because half of it is one sheen and half another.
Note that the trim area here is face only. If you are finishing the backs and edges as well, the surface roughly doubles, and whether that is appropriate depends on the material and its instructions rather than on any general rule. The exterior trim and corner board calculator reports both figures from a full trim takeoff, which is the cleaner way to get the linear footage for this page.
What decides whether the paint lasts
None of it is in the gallon count. Exterior paint fails from below and behind far more often than it fails from above: moisture moving out through the wall lifts a film that is stuck to damp wood, and no amount of surface preparation fixes a wall that is wet for a reason. If paint is peeling in a pattern that follows the framing, or only on one elevation, or only above a particular detail, the paint is reporting a water problem rather than causing one. The wall condensation risk calculator is the better place to start on that.
The other thing worth measuring before you buy anything: on a house painted before the late 1970s, the existing coating may be lead-based, and scraping, sanding or power washing it is what turns a stable film into an exposure. That is a testing question with a clear first step and no shortcut. The same era of house may also have asbestos in the cladding itself. Neither has an answer that comes out of a paint calculator, and both are worth resolving before a scraper touches the wall.
Finally, most of an exterior repaint happens above head height, and falls are the thing that actually hurts people on this job. That is worth as much planning as the colour. For the interior version of this calculation see the paint calculator, for washing the wall down first the pressure washing calculator, and for stain rather than paint on wood the fence stain calculator uses the same absorption logic on a different surface.
Questions people ask
Why does my rough siding take so much more paint?
Because there is more surface inside the same square footage. A tape measures the plan area of the wall; the paint has to coat every face of every saw mark, grain valley and aggregate particle in it. Rough sawn cedar has perhaps 1.7 times the developed surface of a planed board, and heavy stucco or split-face block can be three times. It is not that the paint is being applied more thickly, it is that the area is genuinely larger than the number on your tape. This is the single largest source of underbuying on exterior jobs.
Should I deduct windows and doors from the area?
For exterior body paint, yes, and in full. The argument for not deducting is a cutting-in allowance, which matters on interiors where openings are a large share of a small wall. On an exterior the openings are a modest share of a large area and the waste built into the spread rate already covers the cutting in. Where people go wrong is in the other direction: forgetting to add the gables back. Two gable ends on a moderate roof can add two hundred square feet, and they are the part of the wall nobody stands next to with a tape.
How much primer do I actually need?
It depends on how much bare surface there is, which is why the calculator asks rather than assuming. A sound repaint that needs a few scraped patches might be ten percent of the wall; a wall that has been extensively scraped could be half of it. Primer also spreads differently from finish paint and usually covers a little less on a porous surface, hence its own rate field. If the whole wall is bare or has been stripped, set it to a full prime, in which case the primer becomes roughly a third of your total paint volume.
Can I use the same paint on body and trim?
You can, and there is a reason most people do not. Trim usually takes a higher sheen for durability and for the way it defines the edges of the elevation, and the two products then differ in more than colour. If you do use one product throughout, count the areas together and the gallon figure is simply the sum. If you use two, keep them separate from the start, because a combined gallon count cannot be turned into an order at the counter.
Does the calculator allow for touch-ups and drips?
Only through the spread rate, which is where the allowance genuinely belongs. Using the low end of the range for your texture rather than the middle builds in roughly a ten to fifteen percent cushion, which is enough for spillage and a touch-up pass. A separate waste percentage on top would be double counting, because published spread rates are already achievable figures rather than theoretical ones. What is worth doing is buying the last gallon in the same trip rather than later, since a tinted colour matched weeks apart can differ enough to show.