Roofing calculator
Roofing Calculator
Convert a horizontal roof footprint and uniform pitch into sloped area, squares, and shingle bundles.
Planning estimate, not a contractor quote. Review the assumptions and replace presets with local prices when available.
Method
How this calculator works
Enter the rectangular footprint before overhangs and add the horizontal overhang on each side. The calculator applies the slope factor once to the complete projected area. Use separate estimates for different pitches or separate roof sections, and keep ridge caps and starter products outside the field-shingle bundle count.
Projected area = (length + 2 × overhang) × (width + 2 × overhang). Slope factor = √(1 + (rise per 12 ÷ 12)²). Sloped area = projected area × factor. Bundles = ceil(sloped area × allowance factor ÷ bundle coverage).
Worked example
A realistic starting point
A 20 × 10 ft footprint with 1 ft overhang on each side projects to 264 ft². At a 6:12 pitch the sloped area is about 295.16 ft². A 10% allowance gives 324.68 ft², or 10 bundles if each covers 33⅓ ft².
What changes the answer
Factors worth checking
Footprint versus slope
Use horizontal dimensions in this mode. Entering already-sloped dimensions applies the pitch factor twice and overstates the roof.
Bundle coverage
Check installed coverage on the selected product. Three bundles per square is not universal; the calculator accepts product-specific coverage.
Separate roof components
Field shingles do not automatically include starters, hips, ridges, valleys, flashing, underlayment, or waste from complicated roof geometry.
Common questions
Roofing FAQ
Should I multiply by two for a gable roof?
Not when length × width covers the whole building footprint. That projected area already covers both sides; the slope factor converts the total once.
Can I use it for several roof pitches?
Calculate each section separately using its own horizontal footprint and pitch, then reconcile overlaps and shared edges.
Does it estimate roof replacement labor?
No. It estimates field-shingle material quantities and entered bundle cost. Obtain a separate scope for other materials and labor.
Evidence
Sources & review
- Owens Corning — Woodcrest product dataIllustrates why installed bundle coverage is product specific; not all shingles use three bundles per square.
- NIST — SI units: volumeUnit relationships; supplier product coverage and prices remain separate inputs.
Published by Calcstead. Report a correction.
Method and source links reviewed September 6, 2026. Price presets are editable planning inputs and should be replaced with current local information.