Roofing Calculator
Roof area = footprint × pitch factor, and one square is 100 ft². A 50 × 30 ft footprint at 6/12 (factor 1.118) is 1,677 ft² of roof, or 16.77 squares: order 56 bundles of shingles with 10 % waste.
Shingle bundles to order
56 bundles
18.45 squares including 10 % waste
Estimate only. Check dimensions, loads and local code requirements with a qualified professional before you build.
- Roof surface
- 1,677 ft² (155.8 m²)
- Roofing squares (no waste)
- 16.77
- Slope factor
- × 1.118
- Footprint
- 1,500 ft²
Hips, valleys, dormers and a starter course add waste: complex roofs often need 15 % or more.
How it works
Slope factor = √(1 + (rise ÷ 12)²) or 1 ÷ cos(angle)
Roof area = footprint (length × width, with overhangs) × slope factor
Squares = roof area ÷ 100
Bundles = squares × (1 + waste %) × bundles per square, rounded up
Assumptions
- All roof planes share the same pitch. For mixed pitches, run the calculator once per section.
- Three bundles per square, the packaging Owens Corning gives for laminate (architectural) shingles: "3 bundles = 1 roofing square". Check the wrapper of your product.
- Default waste 10 %; Owens Corning suggests 10–15 %, more on complex roofs.
Worked examples
A 50 × 30 ft footprint at 6/12, 10 % waste
- Slope factor √(1 + 0.5²) = 1.118
- 1,500 ft² × 1.118 = 1,677 ft² of roof
- 1,677 ÷ 100 = 16.77 squares; × 1.10 × 3 = 55.3, so 56 bundles
Result: 16.77 squares, 56 bundles.
Checked by the automated test “roofing — 1,500 ft² footprint at 6/12 → 1,677 ft², 16.77 squares, 56 bundles with 10 %”.
Owens Corning's 2,400 ft² gable example
- 2,400 ÷ 100 = 24 squares × 3 = 72 bundles
- With 10 % waste: 26.4 squares × 3 = 79.2, so 80 bundles
Result: 72 bundles without waste, 80 with 10 %.
Checked by the automated test “roofing — 24 squares + 10 % waste = 80 bundles”.
Roofing is an estimate — confirm with a licensed professional. Owens Corning itself recommends hiring a roofing contractor for the installation; use these numbers to plan and to compare quotes.
Squares, bundles and waste
- Bundles are rounded up, after the waste is added. Owens Corning suggests 10–15 % for waste and applies the same allowance to underlayment.
- Pitch changes the order a lot. The same 1,500 ft² footprint is 16.77 squares at 6/12 and 21.21 squares at 12/12.
- Complex roofs waste more. Hips, valleys, dormers and walls mean more cut shingles: move the waste box toward 15 %.
Common mistakes
- Leaving out the overhangs. Owens Corning measures the length and span of the roof including overhangs; the house walls alone give too small a footprint.
- Shingling a low-slope roof. The International Residential Code (IRC R905.2.2) allows asphalt shingles only from a 2/12 pitch, and asks for two layers of underlayment from 2/12 up to 4/12. Below 2/12 the roof needs another covering, and the calculator flags it.
- Treating the floor area as the roof area. Living space, garages and porches under one roof do not match the roof outline.
- Forgetting the extras. Starter shingles, ridge caps, underlayment and nails are separate items. Owens Corning counts 320 nails per square for three-tab shingles at four nails each.
Frequently asked questions
How many bundles of shingles are in a square?
Three for most laminate (architectural) shingles, according to Owens Corning: 3 bundles cover one square of 100 ft². Other products can differ, so read the coverage on the wrapper and set the bundles-per-square box to match.
How do I calculate roofing squares from square footage?
Divide the roof surface in square feet by 100. Use the sloped surface, not the floor area: a 1,500 ft² footprint at 6/12 is 1,677 ft² of roof, so 16.77 squares, not 15.
How do I find my roof pitch?
Owens Corning's method: measure how many inches the roof rises over 12 in of level run. A 4 in rise is a 4/12 pitch. Owens Corning also suggests a pitch gauge or a smartphone app.
How much underlayment do I need?
The same number of squares as shingles, plus waste. Owens Corning notes synthetic underlayment comes in 10-square rolls and #15 felt in 4-square rolls. For 16.77 squares + 10 %, that is 2 synthetic rolls or 5 rolls of felt.
Sources
Formula, examples and sources checked on by the Yardstick Math team. How we check our formulas.
- DIY Roofing – how to measure a roof, slope correction factors, squares, bundles and underlayment — Owens Corning
- Handbook 44, Appendix C – General Tables of Units of Measurement — NIST
- Residential Code, Chapter 9 Roof Assemblies – R905.2.2 Slope (IRC text: asphalt shingles only from 2/12; double underlayment from 2/12 up to 4/12) — City of Seattle, Department of Construction and Inspections (2018 Seattle Residential Code)