Roofing Calculator
Roof area, squares and shingle bundles from the footprint and pitch (x/12, degrees or percent) for gable, hip and shed roofs, with waste, underlayment, ice and water shield, ridge cap, starter, drip edge, nails, tear-off and an installed cost estimate.
How it works
slope factor = √(1 + (rise ÷ 12)²) · angle = atan(rise ÷ 12) · roof area = footprint × slope factor (+ extra sections) · squares = roof area ÷ 100 · bundles = squares × bundles per square × (1 + waste%) · gable ridge = length, rakes = 4 × (width ÷ 2) × slope factor · hip ridge = long side − short side, each hip = (short ÷ 2) × √(2 + (rise ÷ 12)²)
A pitched roof has more surface than the ground it covers, so the flat footprint (with overhangs) is multiplied by a slope factor. A 6/12 roof rises 6 in per foot of run, which is 26.6° and a factor of 1.118. When every plane has the same pitch, the area rule works for gable, hip and shed roofs alike. Their edges differ, and the edges set the ridge cap, starter strip and drip edge. A gable has a full-length ridge and four rakes. A hip roof has a short ridge, four hips and eaves all round. A shed roof has one plane. Roofers count in squares of 100 sq ft, and most shingles run three bundles to the square. Waste covers cuts and starter courses. Underlayment is counted by roll coverage and doubled on 2/12–4/12 roofs. Ice and water shield follows the eaves, and nails are counted at four (or six) per shingle. Labor and tear-off are priced per square of roof surface.
Worked example
A 40 × 30 ft gable footprint is 1,200 sq ft. At 6/12 the slope factor is 1.118, so the roof is about 1,342 sq ft, or 14 squares. 13.42 squares × 3 = 41 bundles, and 45 with 10% waste. The ridge is 40 ft (2 ridge cap bundles). Eaves plus rakes come to 147 ft (2 starter bundles, 16 drip edge pieces), and you need 2 rolls of synthetic underlayment. At $38 a bundle, $200 a square for labor and $100 a square to strip one layer, the estimate is about $6,365.
Sources: Inch Calculator — pitch multipliers, felt roll coverage and nails per square · Calculator.net — slope correction factors 1/12 to 24/12 · RoofingCalculator.com — installed cost ranges by material
Frequently asked questions
What is a roofing square?
One hundred square feet of roof surface (9.29 m²). Shingles, underlayment and labor are all priced by the square, so converting your roof area to squares is the first step in any roofing estimate.
How do I find my roof pitch?
Measure the vertical rise over 12 inches of horizontal run. A 6-inch rise is a 6/12 pitch, which is 26.6° or a 50% slope. If you have an angle from a phone app or plans, choose Custom and enter degrees or percent instead.
Does a hip roof need more shingles than a gable?
With the same footprint and pitch, the surface area is the same. Hip roofs need more ridge cap, because the hips are capped too, and more waste from the angled cuts. Use about 15% waste instead of 10%.
How many bundles cover a square?
Three bundles per square for standard 3-tab and most architectural shingles. Heavier designer shingles can run four or five. Always check the bundles-per-square figure on the product wrapper.
What does a new roof cost?
Asphalt shingle roofs commonly run about $4–$7 per square foot installed, including tear-off. Steep, complex or multi-story roofs cost more. Enter your local bundle price and labor rate for a closer estimate, and get written quotes before you commit.
Related calculators
Estimates only — add for waste and confirm against local building codes and supplier specs.