How to use
- Enter the building footprint length and width.
- Set pitch as rise per 12 units of horizontal run.
- Enter a uniform overhang on every side, or zero if your dimensions already include the eaves.
- Choose a waste allowance and copy bundles per square from the product wrapper before calculating.
How the estimate is calculated
First add twice the overhang to each footprint dimension. Multiply the enlarged rectangle by √(1 + (pitch rise ÷ 12)²) to get the sloped area. Divide square feet by 100 to get roofing squares. Order squares include the extra percentage, and bundle count is order squares × bundles per square, rounded up.
Example: a 40 ft × 30 ft building at 6/12 pitch with no added overhang has 1,341.6 sq ft of roof area. That is 13.42 roofing squares before extra, or 14.76 squares with 10% extra. At 3 bundles per square, order 45 bundles of field shingles.
What a square and a bundle represent
| Quantity | Value used | Status |
|---|---|---|
| One roofing square | 100 sq ft | Trade area unit |
| Bundles per square | 3 initially | Editable planning figure; read wrapper |
| Extra allowance | 10% initially | Editable planning allowance |
| Area at 6/12 pitch | Footprint × 1.1180 | Calculated geometric factor |
The bundle and waste values shown here are planning starting points, not product requirements. Bundle coverage can vary, so the wrapper is the source to use when entering your shingle’s value.
Gables, hips, and accessories
A simple gable provides a useful first estimate because its two faces share one slope and the footprint is rectangular. Hips, valleys, dormers, and mixed pitches change the cutting layout. They can require a different allowance or a section-by-section takeoff rather than the same generic percentage.
Field-shingle bundles do not count starter strips, ridge caps, flashing, underlayment, fasteners, or ventilation components. Measure those separately according to the selected roof system. This page estimates material area only and does not decide which covering is appropriate for the slope.
Common mistakes
- Do not enter 6 for a 6-degree roof; the input expects a 6/12 rise ratio.
- Adding an overhang to eave-to-eave dimensions counts it twice.
- Do not assume every shingle needs exactly three bundles per square.
- Rounding each intermediate area before the bundle step can change the final count; this tool retains full precision internally.
Sources
Trade-unit and planning context: GAF bundle-and-square article, background reading for the terminology. All areas and multipliers are calculated; verify actual bundle coverage on the wrapper.
Related calculators
- Roof Pitch Calculator — derive pitch from rise and run
- Square Footage Calculator — measure projected roof sections
Frequently asked questions
Is the footprint the roof’s sloping area?
No. It is the horizontal length and width before the slope adjustment. Entering sloped area as footprint would increase it a second time.
Where do I enter an eave overhang?
Use the overhang field only when the footprint dimensions exclude it. The calculator adds that distance on both ends of both dimensions.
Are ridge caps included in the bundles?
No. The output is a field-shingle count. Measure ridge caps, starter material, and other accessories separately.
Can I use this for a complex roof?
Only as a rough comparison when the assumptions fit. Mixed pitches, valleys, and dormers need separate measurements and a cutting allowance suited to the layout.