Roofing Calculator
Estimate total roof area, number of squares, bundles of shingles, and underlayment rolls — adjusted for roof pitch.
What is Roofing Calculator?
The roofing calculator converts a home's footprint plus roof pitch into the actual sloped surface area of the roof, then into U.S.-standard roofing squares (100 sq ft), shingle bundles, and rolls of underlayment.
Use it to size a shingle order or to sanity-check a contractor bid.
Formula
Pitch factor = √(1 + (rise/12)²). A 6/12 roof has a pitch factor of 1.118 — so a 1,000 sq ft footprint has 1,118 sq ft of actual roof surface.
Pitch factors for common roofs:
- 3/12: 1.031
- 4/12: 1.054
- 6/12: 1.118
- 8/12: 1.202
- 12/12: 1.414
Squares = surface area ÷ 100. U.S. asphalt shingles pack 3 bundles per square.
Worked example
A 30×40 ft ranch with a 6/12 pitch roof and 10% waste:
- Footprint = 1,200 sq ft
- Pitch factor = 1.118 → surface = 1,342 sq ft
- Adjusted for waste = 1,476 sq ft = 14.8 squares
- Shingles = 14.8 × 3 = 45 bundles
- Underlayment = ~1.5 rolls of synthetic (10-square rolls)
How to use this calculator
- Measure the building footprint below the eaves — length × width. For complex roofs with multiple wings, calculate each section separately and add.
- Estimate pitch: a speed square or pitch gauge on the fascia gives rise/12. Most U.S. homes are 4/12 to 9/12.
- Use 10% waste for simple gable roofs, 15% for hip roofs, 20%+ for roofs with many valleys and dormers.
Frequently asked questions
How do I measure pitch without climbing on the roof?
Stand on a ladder at the gable end and hold a level horizontally against the rake. Measure how many inches of vertical drop you get over 12 inches of horizontal run — that ratio is your pitch (e.g., 6 inches of drop over 12 inches = 6/12).
Why 3 bundles per square?
Standard 3-tab asphalt shingles and most architectural shingles are packaged 3 bundles per 100 sq ft. Heavier "designer" or premium architectural shingles may be 4 or even 5 bundles per square — check the wrapper.
Do I need starter strip and ridge caps too?
Yes, but they're priced and counted separately. Rules of thumb: starter strip = 1 bundle per 120 linear ft of eave; ridge caps = 1 bundle per 35 linear ft of ridge (or 1 carton of cap shingles per 25 ft).
What life span should I expect?
3-tab: 15–20 years. Architectural: 25–30 years. Premium (SBS-modified, copper, slate, tile): 40+ years. Manufacturer warranties are longer than realistic lifespan in most U.S. climates.