Roofing Shingle Calculator — squares, bundles and pitch area
Roofing squares and shingle bundles from a building footprint, with the pitch multiplier that makes a roof larger than the house beneath it.
The number people get wrong is the area. A roof is bigger than the house it sits on, and by exactly how much depends on the pitch. Everything else follows from that one multiplier.
Quick answer: pitch multiplier and squares for a 40 × 28 ft house
| Pitch | Multiplier | Squares, +10 % | Bundles |
|---|---|---|---|
| Flat | 1 | 14.66 | 44 |
| 4:12 | 1.0541 | 15.46 | 47 |
| 6:12 | 1.118 | 16.39 | 50 |
| 8:12 | 1.2019 | 17.62 | 53 |
| 10:12 | 1.3017 | 19.09 | 58 |
| 12:12 | 1.4142 | 20.74 | 63 |
Footprint 40 × 28 ft with 18 in. overhangs: 43 × 31 = 1,333 ft². Roof area = footprint × √(rise² + 144) ÷ 12. Bundles at 3 per square, rounded up.
| Footprint incl. overhang | — |
|---|---|
| Pitch multiplier | — |
| Actual roof area | — |
| With waste | — |
| Squares | — |
| Shingle bundles | — |
| Underlayment rolls | — |
The footprint is not the roof. The correction is exact, not an estimate. Multiply plan area by √(144 + rise²) / 12, where rise is inches per 12 of run. A 6/12 is 1.1180 times its footprint. Over a 2,000 sq ft footprint a 12/12 is 2,828 sq ft. That is eight squares the plan never shows.
The multiplier is blind to roof shape. Every plane scales by the same factor, so a hip and a gable over the same 40 × 30 rectangle at 6/12 come to the identical 1,341.6 sq ft, 13.42 squares. What differs is sold by the linear foot and never enters that count. The gable has one 40 ft ridge. The hip, all four planes at 6/12, has a 10 ft ridge and four 22.5 ft hips: 100 linear feet of cap, two and a half times as much, on a count that did not move. GAF Seal-A-Ridge covers 25 lin ft per bundle, four bundles against two. TimberTex covers 20, five against two. Cap is always a separate order. The Timberline HDZ data sheet names five hip and ridge products, and none of them is the field shingle. Cut waste follows the same lines, since hips and valleys are where shingles get cut, so one percentage cannot serve both roofs. The 10% and 15% the trade uses are rules of thumb, not published figures.
Three bundles to the square is a product fact, not a constant. Timberline HDZ prints Bundles/Square: 3, 64 pieces at 5 5/8 in. exposure. CertainTeed's Grand Manor prints 50 shingles and 425 lb per square at 8 in. exposure: five bundles. Order in squares and let the wrapper convert.
Below 4/12 the code changes the order, not the shingle. IRC R905.2.2 allows asphalt shingles down to 2/12 and requires double underlayment from 2/12 up to 4/12. A 3/12 porch adds 3.1% to the square count and doubles the felt beneath it.
What I check on a job
Before one pitch covers the whole roof. I mark the pitch of every plane on the roof plan, taken from the framing section rather than an elevation label, and where a main roof meets a porch or shed at a different pitch I split the footprint and run each part on its own, because the calculator applies one multiplier to everything. Overhang is where a split goes wrong. The box defaults to 18 inches and adds it to both ends of both dimensions, so I zero it and add each rake and eave to its own dimension by hand, only on edges that really are eaves or rakes, never on the line where the porch meets the main roof. The pitch list also jumps from flat straight to 4/12. A 2/12 or 3/12 porch can still take shingles under R905.2.2, but picking Flat drops its multiplier to 1.000 and voids the bundle math, so I work that part by hand with the page's own formula, √(rise² + 144) / 12.
A roofing square is 100 ft² of roof surface. That is a definition,
not an estimate. The pitch multiplier is √(rise² + 12²) ÷ 12: a 6:12 roof is
about 1.118 times its footprint, a 12:12 roof about 1.414. Measuring the house and forgetting the
multiplier is how orders come in short.
Bundles per square and underlayment coverage vary by product. Three bundles per square is
ordinary for architectural asphalt shingles but it is not universal, so check the wrapper. Waste
percentages here are for the field only; hip, ridge and starter courses are separate items and are not
counted above. This estimates material, not the tear-off, decking, flashing or labour.
Worked example
A 40 × 28 ft house with a 6:12 roof and 18-inch overhangs: how many squares?
- Footprint with overhangs = (40 + 3) × (28 + 3) = 1,333 ft².
- 6:12 multiplier = 1.118, so the roof surface is 1,490 ft².
- With 10% for a simple gable: 1,639 ft² = 16.4 squares, ordered as bundles of a third of a square.
About 16.4 squares before rounding up to whole bundles. The multiplier is why a roof is always bigger than the plan says.
2 values on this page were driven in a browser and compared with independently derived figures on 2026-10-29 (2808 values across 228 calculators site-wide).
Questions
What is a roofing square?
100 square feet of roof surface. Shingles are sold three bundles to the square for standard architectural products.
Why is the roof larger than the house?
Because of pitch and overhangs. A 6:12 roof is 1.118 times its footprint, and 18-inch overhangs add 3 ft to each dimension.
Does a hip roof take more shingles than a gable?
Not in the field: over the same 40 × 30 ft rectangle at 6/12 both are 1,341.6 sq ft, 13.42 squares. The difference is cap, sold by the linear foot: the gable has a 40 ft ridge, the hip 100 ft of ridge and hips. At 25 lin ft a bundle that is four bundles against two.
Is it always three bundles to a square?
No; it is a product fact. Timberline HDZ prints three bundles, 64 pieces at 5 5/8 in. exposure. CertainTeed's Grand Manor prints 50 shingles and 425 lb per square at 8 in. exposure, which is five bundles. Order in squares and let the wrapper convert.