Roof measurement and quantities
Gable Roof Calculator
Enter measured dimensions to calculate roofing quantities.
Your quantity record
A personal result appears after you enter valid inputs and calculate.
Formula, diagram and sources
Measured surface needs no pitch factor. Footprint surface = length × width × √(1 + (rise / 12)²). For separate planes, sum each projected area × its own factor. Roofing squares = surface / 100.
Equation version 2.0, checked October 14, 2026. Method and source records · Corrections
Last updated October 14, 2026 · Method · Corrections
For an equal-sided gable, use the full eave span once. The level run of each side is half that span. Multiply the whole footprint by the pitch factor to get total surface, or calculate each side and add them. Both methods agree. The sizes must include overhangs and exclude joined dormers. A roof name alone cannot show its measured inputs.
Sections7
The gable roof calculator finds the surface of a simple roof with two equal slopes. Enter the eave footprint's length and full span, then the common rise-in-12. The result shows each slope's length and area, plus the total roof surface and squares. It is an area worksheet, not a rafter cutting plan.
Gable roof calculator: simple symmetrical geometry
A simple gable consists of two rectangular slopes that meet along one ridge. The two eave lines run parallel to the ridge. The span runs across the roof from one eave to the other. This worksheet assumes both sides have the same pitch and that the ridge sits over the middle of the span.
Do not use it for a cross-gable, a roof with a raised ridge extension or an offset ridge. Those roofs need separate planes. Use the roofing calculator with a measured surface or a plan divided into separate plan areas. The same house can have one simple main gable and a separate porch roof.
Enter eave length, span and pitch
Enter length along the eave, not a diagonal rake. Enter the full level span between eaves, not the slope length from ridge to gutter. Choose the common rise per twelve level units from a measured report or plan. Do not infer it from a roof photograph.
Wall width may leave out the overhang at each side. A measured eave footprint already includes it. Do not add overhang a second time to that record. Indoor floor area is a different measure and can include several stories beneath one roof.
| Quantity | Symbol | Basis |
|---|---|---|
| Eave footprint length | L | Feet parallel to ridge |
| Full eave footprint span | W | Horizontal feet across both sides |
| Rise-in-12 | p | Common slope, not degrees |
| One-side horizontal run | W / 2 | Half the full span |
| One-side slope length | (W / 2) × factor | Sloped distance, not a cut length |
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For mixed units, convert before entry. Twelve inches is one foot. A pitch ratio uses the same unit for rise and run. A 6:12 slope can be expressed in inches or feet with the same ratio. It is not a six-degree angle.
Gable roof calculator method and example
The area factor is √(1 + (p / 12)²). One-side slope length is W / 2 × factor. Each side's area is L × W / 2 × factor. Combined surface is L × W × factor. Dividing by 100 gives roofing squares.
Assume L is 40 feet, W is 30 feet and pitch is 6:12. Each level run is 15 feet. Slope length is about 16.77 feet. Each side has about 670.82 square feet of surface. Together they have about 1341.64 square feet, or 13.4164 roofing squares.
That total already includes both sides. Multiplying the full-footprint result by two would double the roof area. Using only one half-span without adding the second side would miss half the covering. Check the separate side measurements shown in the result.
The example sizes are assumed. No building was inspected. Rafter stock length, ridge-board allowance and cuts are not calculated. A sloped measured length is not a construction schedule. The worksheet also leaves ridge caps, starter and flashing counts outside its surface total.
Use the surface in a quantity or cost worksheet
The shingle calculator accepts the total measured surface. It asks for exact bundle coverage and your stated ordering allowance. It counts starter and cap separately. An installed price worksheet uses surface instead of ordered bundle coverage, so no second waste uplift belongs there.
For comparison, read the hip roof calculator. A hip with the same rectangular footprint and equal pitch has the same net area. Its cuts and extra product lengths differ. That distinction explains why equal surface does not guarantee equal order counts or installation scope.
Sources and measurement limits
GAF's square guide confirms the surface-area unit. The slope formula follows the right-triangle relationship and was checked independently on October 14, 2026. See method and sources for the equation record or use Contact and Corrections to report an arithmetic error.
Frequently asked questions
How do you calculate the area of a gable roof?
For two equal slopes over a rectangular eave footprint, multiply eave length by full level span and the common pitch factor. This gives both sides together. You can also calculate each side using half the span and add them. Neither method should apply pitch again to an already measured sloped surface.
How long are the rafters on a gable roof?
The worksheet shows measured slope length from half-span and pitch. Actual rafter length also depends on the construction detail, overhang, ridge connection and cuts. Those are outside this area calculator. Treat the slope output as a covering measure and use the measured construction plan for any framing or timber order.
Can I use this for an uneven gable?
Use separate roof-plane measured inputs when the slopes or runs differ. This form assumes equal pitch and equal runs about a central ridge. An offset ridge, joined addition or dormer breaks that assumption. The general roofing calculator can add known plane areas without forcing the roof into an equal-sided gable model.
Should I add waste to the roof area?
Keep net surface unchanged. A cuts allowance describes ordered covering, not extra roof surface. Add it only in a material-order worksheet when you have a stated basis. A contractor's installed surface-area rate may already cover buying and cuts. Do not increase that installed area again just because the roof has gable edges.
Verdict: check both slopes once
Use this gable roof calculator when the measured footprint and equal-slope assumptions match the roof. Keep the full span separate from half-run and slope length. Return the total area to the roofing calculator or replacement estimator with the same unit basis.