Understanding your result
The headline is the total gable wall area, net of openings, with the per-gable area and the area with waste beneath it. The stats show the single-gable area, the rise, the sheathing sheets and the rake trim. The breakdown goes from width and pitch to rise, to the triangle, to the total, to the waste, the sheets and the rakes.
The number to remember is the rake length per side: it is the same length the drip edge calculator and the starter strip calculator need for the rakes, and it is what the rafter length calculator calls the line length (less the ridge shortening). Measure the gable once and the rake comes with it.
For the area of the exterior wall below the gable, the paint it takes and the siding for the whole house, the companion site Paints Calculator has an exterior house paint calculator that takes the gables too; this page stops at the triangle.
How we calculate this
The step people skip is the waste. A rectangular wall wastes a few percent to cuts; a triangle wastes far more, because every course of siding is cut at the pitch angle at both ends and every sheathing sheet along a rake is cut on the diagonal. Fifteen percent is the usual allowance, and it assumes the diagonal offcut from one rake is flipped and used at the other. A gable with a window and a vent wastes a little more again.
When the peak height is measured instead of the pitch, the calculator reports the pitch it implies, which is a useful check: a 6-foot peak on a 28-foot gable is 5.1/12, and if the roof is supposed to be 6/12, something was measured wrong. The pitch calculator converts a pitch read from a level, and the ridge height calculator gives the peak from the span for a roof that is still on paper.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Gable width | 32 ft | The site’s example house; the width between the outside faces of the walls at the plate |
| Pitch | 6/12 | A typical US pitch; enter yours |
| Number of gables | 2 | A plain gable roof has two ends; dormers and wings add more |
| Siding and sheathing waste | 15% | Siding manufacturers’ estimating guides allow 10 to 20% for gable ends cut on the diagonal |
| Sheathing sheet | 4 × 8 ft (32 sq ft) | Standard panel size |
| Siding square | 100 sq ft | The unit siding is sold in |
Assumptions last reviewed October 7, 2026.
The calculator does not add the rectangular wall below the gable, does not model a gable with a raised (gambrel or clipped) peak, and does not price paint. The guide on why a roof is bigger than a house covers how gables, hips and dormers change the roofing, and roof pitch explained covers reading the pitch that sets the triangle’s height.
Two worked examples
Two gables on a 32-foot-wide house at 6/12
Width 32 feet, pitch 6/12, two gables, no openings, 15% waste.
- Rise: 32 ÷ 2 × 6 ÷ 12 = 8 ft
- Area of one gable: 32 × 8 ÷ 2 = 128 sq ft
- Two gables: 256 sq ft; with waste: 256 × 1.15 = 294 sq ft (2.94 squares of siding)
- Sheathing: 294 ÷ 32 = 9.2, so 10 sheets
- Rake: 16 × 1.118 = 17 ft 11 in per side; 71.6 ft of rake trim for both gables
Three squares of siding and ten sheets; the rake trim at 71.6 feet is eight 10-foot sticks with the laps.
One gable measured from the ground, with a window
Width 28 feet, peak measured at 6 feet above the plate, one gable, 12 square feet of window to subtract.
- Pitch implied: 6 ÷ 14 × 12 = 5.1/12
- Area: 28 × 6 ÷ 2 = 84 sq ft; net of the window: 72 sq ft
- With waste: 72 × 1.15 = 83 sq ft, 3 sheets of sheathing
- Rake: 14 × 1.087 = 15 ft 3 in per side, 30.5 ft of trim
The 5.1/12 pitch is the check: if the plans say 6/12, the laser measure found the underside of the barge rafter rather than the peak of the wall, which is about 10 inches lower on this roof.
Where to find your inputs
Gable width. Outside of wall to outside of wall at the top plate, the same width the span uses, from a tape along the gable wall at ground level.
Pitch or peak height. The roof pitch from a level on the rake board or from the plans; or the height of the peak above the top plate measured with a laser from the ground (point it at the underside of the sheathing at the peak and subtract the wall height).
Openings. Width times height of each window, louvre or vent in the gable, per gable.
Count. Every gable end on the house, including dormers and wings, each with its own width if they differ (run them separately).
Common mistakes
- Using the full width times the height. A gable is a triangle: divide by two.
- Measuring the height to the ridge of the roof. The gable wall stops at the underside of the sheathing; the ridge board and roofing sit above it. Subtract the rafter depth at the peak.
- Rectangular waste on a triangle. Five percent is a wall; a gable is fifteen. The diagonal cuts are real.
- Forgetting the rake overhang on the trim. The barge board runs to the end of the overhang, a foot or more past the wall at each end.
- Adding the gable to the roof area. It is wall. The roof area calculator already covers the roof above it.
- Different pitches on the two gables. A front gable over a porch is often shallower than the main roof. Run each width and pitch on its own.
Questions people ask
- How do I calculate the area of a gable?
- A gable is a triangle: area = width × height ÷ 2, where the height is the rise from the top plate to the peak. For a 32-foot-wide gable at a 6/12 pitch the rise is 16 × 6 ÷ 12 = 8 feet and the area 32 × 8 ÷ 2 = 128 square feet. Two gables are 256 square feet, before the 15 percent waste that cutting siding on the diagonal needs.
- How do I find the gable height without climbing?
- From the pitch: the height is half the gable width times the rise per 12 divided by 12, so a 28-foot gable at 5/12 rises 14 × 5 ÷ 12 = 5.83 feet. Or measure the peak with a laser measure from the ground and let the calculator work out the pitch, which it reports so you can check it against the roof.
- How much siding does a gable need?
- The triangle area less windows and vents, plus about 15 percent, because every course of siding is cut at an angle at both rakes and the offcuts rarely fit elsewhere. Siding is sold by the square (100 square feet), so two 128-square-foot gables with 15 percent waste are 294 square feet, three squares. The paint for the gable is a separate question the calculator does not answer.
- How many sheets of sheathing for a gable?
- Divide the area with waste by 32 square feet per 4 × 8 sheet and round up: 294 ÷ 32 = 9.2, so 10 sheets for two 32-foot gables at 6/12. Lay the sheets so the diagonal offcut from one rake is turned over and used at the other; the waste allowance assumes you do.
- How long is the rake of a gable?
- Each rake runs from the eave corner to the peak: half the width times the pitch factor, √(1 + (rise ÷ 12)²). A 32-foot gable at 6/12 has rakes of 16 × 1.118 = 17.9 feet, so a house with two gables has 71.6 feet of rake for trim, drip edge and starter. Add the rake overhang if the trim runs to the end of the barge board.
- Does the gable count toward the roof area?
- No. The gable is a vertical wall under the roof; the roof planes meet above it. The roof area calculator gives the roof; this calculator gives the triangle of wall the roof leaves exposed. They share only the rake length.