Roofing Calculator – Roof Area, Squares, Bundles and Metal Panels
This roofing calculator works out how much roofing to buy from measurements you can take from the ground. Enter the building's footprint, the overhangs, the roof style and the pitch, and it returns the true sloped roof area, the number of roofing squares, every edge length, and a shopping list: shingle bundles and accessories, a metal panel cut list, or pieces of tile, slate or shakes. It also draws each roof in plan, in section and unfolded flat.
Measuring a Roof From the Ground
You don't need to climb up. Measure the outside walls, then the overhangs: how far the roof deck sticks out past the wall, measured level, at the eaves and at the gable ends (rakes). For an L-shaped house, enter the main block as one section and only the part of the wing that sticks out as another, so the footprints don't overlap. Porches and garages with a different pitch are separate sections too.
Roof Pitch and the Slope Factor
Pitch is rise over run, written as inches of rise per 12 inches of run. A sloped plane is longer than its footprint by the slope factor k = √(1 + p²), where p = rise ÷ run. Because every plane of a gable, hip or shed roof shares that factor, the whole roof's area is simply footprint with overhangs × k, however the planes are arranged. Hips and valleys between planes of equal pitch meeting at 90° are longer still: run × √(2 + p²). To find the pitch from the ground, measure the rise from the wall top to the ridge at a gable end and use the Rise & span mode, or read the angle off a phone level held against a rafter in the attic.
| Pitch | Angle | Slope factor k |
|---|---|---|
| 3/12 | 14.04° | 1.0308 |
| 4/12 | 18.43° | 1.0541 |
| 5/12 | 22.62° | 1.0833 |
| 6/12 | 26.57° | 1.1180 |
| 7/12 | 30.26° | 1.1577 |
| 8/12 | 33.69° | 1.2019 |
| 9/12 | 36.87° | 1.2500 |
| 10/12 | 39.81° | 1.3017 |
| 12/12 | 45.00° | 1.4142 |
Squares, Bundles and Waste
Roofing is sold by the square, 100 ft² of sloped surface. Most 3-tab and architectural shingles come 3 bundles to a square; heavier designer shingles take 4 or 5. Add waste for cutting: about 10% for a simple gable roof and 15% for hip roofs or any roof with valleys, because every hip and valley is cut on both sides. The total is rounded up once, so bundles = ⌈area × (1 + waste) ÷ coverage per bundle⌉.
Starter, Ridge Cap and Drip Edge
Starter strip runs along the eaves and usually the rakes. Hip and ridge cap covers every ridge and hip; a shed roof's high edge gets flashing or a vent instead. Metal drip edge lines the eaves and rakes and is sold in 10 ft or 3 m pieces, so the calculator adds 5% for laps and corners. Ridge vents and soffit vents are sized from the attic floor area, not the roof, and are not included here.
Underlayment and Ice Barriers
Underlayment goes under the whole roof; roll coverages are quoted net of laps. In cold climates, codes require an ice and water shield from the eave edge to a point about 24 in (610 mm) inside the exterior wall, plus a strip in each valley. With a deep overhang or a steep roof that band is wider than one roll, so it takes two courses.
Metal Panels: Coverage Width vs Sheet Width
Metal panels overlap at the side, so plan with the coverage width, not the overall sheet width: a sheet about 38 in wide typically covers 36 in. Panels run from eave to ridge, and on a hip roof each one is cut at an angle along the hip, so the calculator lists the stock length of every panel, rounded up to the next inch or 10 mm, and shows the offcuts. Exposed-fastener panels commonly need at least 3/12.
When to Call a Professional
Roof work is dangerous. OSHA treats any roof steeper than 4/12 as a steep roof requiring fall protection, and above about 8/12 most roofers use roof jacks and harnesses and charge more. Tile and slate are also far heavier than shingles, so have the structure checked before switching materials. Use this estimate to plan and compare quotes, and confirm quantities with your supplier.