Roofing calculator: how many bundles of shingles do I need?
39 bundles
1,300 square feet of roof · pitch factor 1.08 · for 40 × 30 ft
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3/8 or 10' 6".
39 bundles
1,300 square feet of roof · pitch factor 1.08
A 40 by 30 ft footprint at a 5 in 12 pitch is 1,300 square feet of roof, which takes 39 bundles covering 33.3 square feet each. The pitch factor is 13/12, straight from Pythagoras.
A worked example
A 28 by 19 ft footprint at 8/12 pitch, bundles of 25 sq ft, with a 12% waste allowance needs 29 bundles. 532 square feet of footprint at a pitch factor of 1.2 is 639.4 square feet of roof, and at 25 square feet a bundle with 12% waste that is 29 bundles.
What else it is called
Pitch is also written as a slope in degrees or as a ratio like 5:12, and the surface is sold by the square, meaning 100 square feet of roof. Whatever the label, what this page needs is the rise in 12 and the coverage on the bundle.
Common sizes
| Footprint | 4/12 | 5/12 | 6/12 | 8/12 | 10/12 |
|---|---|---|---|---|---|
| 30 × 20 ft — 600 sq ft | 632.5 | 650.0 | 670.8 | 721.1 | 781.0 |
| 40 × 25 ft — 1,000 sq ft | 1,054.1 | 1,083.3 | 1,118.0 | 1,201.9 | 1,301.7 |
| 40 × 30 ft — 1,200 sq ft | 1,264.9 | 1,300.0 | 1,341.6 | 1,442.2 | 1,562.0 |
| 50 × 30 ft — 1,500 sq ft | 1,581.1 | 1,625.0 | 1,677.1 | 1,802.8 | 1,952.6 |
| 60 × 40 ft — 2,400 sq ft | 2,529.8 | 2,600.0 | 2,683.3 | 2,884.4 | 3,124.1 |
Square feet of roof surface, footprint times the pitch factor from Pythagoras.
Where the numbers come from
- The pitch factor is not a figure we looked up: it is Pythagoras. A pitch of 5 in 12 means the roof climbs 5 inches for every 12 it runs, so the rafter covers √(5² + 12²) = 13 inches for every 12 of footprint, and the surface is 13/12 of the plan area. Other pages print a table of factors; this one takes the square root of the pitch you type.
- Coverage per bundle is the figure printed on the wrapper, and you select it. This page does not use the trade unit of the square, nor assume three bundles to one, because the bundles per square vary with the product and that would be a figure we have to source.
This is the surface of a simple gable or hip read off its footprint. It does not add the extra courses that hips, valleys and ridges eat, it does not count starter or ridge cap, and it does not subtract chimneys or skylights. A cut-up roof needs a bigger waste allowance than a plain one: that allowance is yours to set.
By Javier Miralles · Last updated . How this site is made.
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