How Much Paint for a Vaulted Wall With a Triangle Gable? The Triangle Math
A gable wall is a rectangle wearing a triangle hat, and the only reason these walls scare people is that nobody splits the two apart. Do that, and a wall that looks like geometry homework becomes two multiplications you can do in your head at the paint counter. I’ll walk a real 25-foot-wide gable end through both shapes, show you the two ways people get it wrong by nearly 20% in either direction, then size the cans for smooth siding and rough stucco.
Cut the wall in two at the eave line
Picture the gable end of a house: a flat rectangle from the ground up to the eaves, then a triangle that rises from the eave line to the roof peak. The clean move is to draw a horizontal line across the wall at eave height and treat what’s below and above as separate shapes.
For my example wall, the rectangle is 25 feet wide by 18 feet tall at the eaves. That’s 25 × 18 = 450 square feet, the easy part.
The triangle on top is where the area for a triangle = (base × height) ÷ 2 earns its keep. The base is the same 25 feet as the wall below it. The peak rises 8 feet above the eave line. So the triangle is (25 × 8) ÷ 2 = 200 ÷ 2 = 100 square feet. Half of the base-times-height, every time, because a triangle is exactly half the rectangle that would box it in.
Add the two shapes: 450 + 100 = 550 square feet of gable wall. That single number is the whole job. Everything from here is either checking it or turning it into gallons.
The two errors that bracket the right answer
When I first measured a gable like this I didn’t trust the triangle math, so I reached for a rectangle instead — and there are two rectangles you might grab, one too big and one too small. Watch how badly each misses.
| How you measured it | Math | Square feet | Off by |
|---|---|---|---|
| Right answer (rectangle + triangle) | 450 + 100 | 550 | — |
| Whole thing as a rectangle to the peak | 25 × 26 | 650 | +100, about 18% over |
| Just the rectangle, ignore the triangle | 25 × 18 | 450 | −100, about 18% short |
The 26 in that middle row is the full height from ground to peak (18 + 8), and squaring off the whole wall at 25 × 26 pretends the two triangular slivers beside the peak are solid wall. They aren’t — they’re sky. You’d buy paint for 100 square feet of air, roughly 18% too much. Going the other way and ignoring the triangle entirely leaves you 100 square feet short, the same 18% in the other direction, and that’s the one that strands you on a ladder with a dry roller and a bare peak. The correct triangle math sits exactly between the two blunders, which is a nice sanity check: if your answer isn’t between 450 and 650, you’ve slipped a digit somewhere.
A shortcut that lands in the same place
Once you’ve measured a few gables you stop drawing the line and start estimating, and there’s a one-line shortcut I cross-check against the real number: take the full width times the full height to the peak, then multiply by 0.60.
Here that’s 25 × 26 × 0.60 = 650 × 0.60 = 390 square feet — and that’s the catch. The 0.60 factor assumes a triangle-dominated wall, not a tall rectangle with a small hat. For a squat gable like mine, keep the rectangle whole and only halve the triangle — the long-hand math again. The 0.60 rule earns its keep on a steep, full-height end: a 25-foot-wide A-frame at 22 feet to the peak gives 25 × 22 × 0.60 = 330 square feet, and the true split lands within a few percent. Use 0.60 for triangle-heavy walls, split the shapes for squat ones, and when the two disagree by more than 10%, trust the split.
If you’d rather not eyeball the factor at all, drop your width, eave height, and peak rise into the paint calculator and it’ll keep the rectangle and triangle separate for you. But I want you holding the logic, because the second you change the peak height the answer moves and a black-box number won’t tell you why.
Turning 550 square feet into cans
Now the surface decides everything, the same way it does indoors — I lay out the full coverage range over in how many square feet a gallon of paint covers, and a gable end usually sits at one of two extremes: smooth lap siding near the top of the range, or rough stucco near the bottom.
| Surface | Coverage/gal | Two coats of 550 sq ft | Gallons | Cans to buy |
|---|---|---|---|---|
| Smooth siding or fiber cement | 350 sq ft | 1,100 sq ft | 3.14 | 3 gal + a quart |
| Rough stucco or stone | 250 sq ft | 1,100 sq ft | 4.40 | 4 gal + 2 quarts |
Two coats of 550 is 1,100 square feet of coverage either way; only the divisor changes. Smooth siding at 350 lands at 3.14 gallons — three gallons cover 1,050, leaving 50 short, so a quart (roughly 88 square feet of exterior paint) closes the gap with margin. Rough stucco at 250 jumps to 4.40 gallons — that texture wraps a roller around every pock and ridge, billing you better than a full extra gallon over the smooth version of the identical wall. Four gallons cover 1,000, leaving 100, so two quarts get you home with cut-in to spare. Round up; an unopened can goes back to the store, a half-painted peak does not.
Subtract the gable vent and any windows
Most gables have a vent up near the peak — a louvered triangle, half-round, or rectangle — and many have a window or two in the rectangle below. Those are holes you don’t paint the wall behind, so take them out before you buy.
Say my gable has a 2-foot-by-2-foot square vent (4 square feet) and one 3-foot-by-4-foot window (12 square feet). That’s 16 square feet of openings. Net wall drops from 550 to 534 square feet. Two coats is now 1,068 square feet: on smooth siding that’s 3.05 gallons — still three gallons plus a quart, the deduction just buys back your margin. On stucco it’s 4.27, still four-and-two-quarts. The openings rarely change the can count on a wall this size, but I subtract them anyway because on a tighter gable they can be the difference between three gallons and “three gallons plus a needless fourth.”
The vaulted-interior version
Flip all of this inside and it’s the same triangle, smaller. A vaulted living-room accent wall is a gable end you’re painting from the comfort of a step ladder — and if your room were boxy with flat walls you’d just be doing the four-wall arithmetic I run through in how to figure paint for a 12×12 room; the vault only adds one triangle to that. Suppose the accent wall is 14 feet wide, 9 feet to the eave line, and rises 4 feet more to the vault peak. Rectangle: 14 × 9 = 126 square feet. Triangle: (14 × 4) ÷ 2 = 28 square feet. Total 154 square feet. Two coats of smooth, previously painted drywall at 350 is 308 square feet of coverage ÷ 350 = 0.88 gallon — one gallon does the whole accent wall with paint left for touch-ups. That’s the payoff of the split: the exact same (base × peak-rise) ÷ 2 that sized an exterior gable also sizes the dramatic wall over your couch, just with a friendlier number at the end.
These figures are planning estimates, not promises — your siding’s age, your roller nap, and how hard you press all get a vote, which is why coverage comes as a band. Measure your real width, your real eave height, and your real peak rise; keep the rectangle and the triangle apart; subtract your vents and windows; multiply by two coats; divide by 350 for smooth or 250 for rough. Do the triangle once and the scary wall turns into the easy one.