How to Calculate the Square Footage of a Gabled Wall
A gabled wall is the end wall of a building with a pitched roof — a rectangle of wall with a triangle of gable sitting on top, forming a five-sided “house” pentagon. It is one of the most frequently estimated surfaces in construction, because siding installers, painters and insulation contractors all need its area to order materials. The calculation breaks neatly into the rectangle below and the triangle above.
The gabled wall formula
Add the rectangular wall to the triangular gable:
Area = (Base × Eave height) + ½ × Base × (Peak height − Eave height)
The rectangle is the wall up to the eaves (where the roof starts), and the triangle is the gable from the eaves up to the peak. The calculator takes the base width, the eave height and the total peak height, and combines the two parts for you.
The three measurements you need
- Base width: The full width of the wall at its base.
- Wall height to eaves: The vertical height from the ground (or floor line) up to where the sloping roof begins — the top of the rectangular section.
- Peak height: The vertical height from the same baseline all the way to the tip of the gable.
The triangular gable's height is simply the peak height minus the eave height, which the calculator works out, so you never measure the slope directly.
Worked example
For a gable end 20 feet wide, with walls 8 feet to the eaves and a 14-foot peak:
Rectangle: 20 × 8 = 160 sq ft. Gable triangle: ½ × 20 × (14 − 8) = ½ × 20 × 6 = 60 sq ft. Total = 220 square feet.
For siding, add about 10% for waste and trim; for paint, 5–10% and remember to subtract large windows or vents separately if you need a precise material order.
Practical estimating notes
For a rough material order you can leave small windows in and let the waste factor absorb them, but for an accurate paint or siding count, calculate the area of any large windows, doors or vents and subtract them from the result. Siding is usually sold by the square (100 sq ft) or by the piece, so convert the area accordingly, and always round up — running short mid-job is far costlier than a little surplus.
Related shape calculators
The gable triangle on its own is an irregular triangle if you only know its edges, or a base-and-height triangle. To estimate the interior walls of the same room, use the room walls calculator, and for plain rectangular wall sections the rectangle calculator.