Calculate roof area, squares, and shingle bundles needed for your roofing project.
| Pitch | Angle | Slope Factor | Category |
|---|---|---|---|
| 2/12 | 9.5° | 1.0138 | Low Slope |
| 3/12 | 14.0° | 1.0308 | Low Slope |
| 4/12 | 18.4° | 1.0541 | Moderate |
| 5/12 | 22.6° | 1.0833 | Moderate |
| 6/12 | 26.6° | 1.1180 | Moderate |
| 7/12 | 30.3° | 1.1577 | Moderate |
| 8/12 | 33.7° | 1.2019 | Moderate |
| 9/12 | 36.9° | 1.2500 | Steep |
| 10/12 | 39.8° | 1.3017 | Steep |
| 12/12 | 45.0° | 1.4142 | Steep |
| Material | Amount |
|---|---|
| 3-tab shingles | 3 bundles |
| Architectural shingles | 3 bundles |
| Hip & ridge cap | 1 bundle per 35 LF |
| Roofing nails | 2–2.5 lbs |
| Underlayment | 1 roll per 4 squares |
| Ice & water shield | 1 roll per 2 squares |
A 40 by 30 foot house covers 1,200 sq ft of ground, but its roof does not. At a 6/12 pitch the shingled surface is 1,342 sq ft, and with 10% waste the order is 45 bundles, not the 40 the footprint suggests. Five bundles short means a second trip and possibly a different dye lot. This calculator applies the slope factor for you and rounds up to whole bundles.
The formulas: slope factor = √(1 + (rise ÷ 12)²); roof area = length × width × slope factor; area with waste = roof area × (1 + waste%); squares = area with waste ÷ 100; bundles = area with waste ÷ coverage per bundle, rounded up. Pitch angle is arctan(rise ÷ 12).
Typical case: a 40 × 30 ft footprint, 4/12 pitch, 10% waste, 33.3 sq ft per bundle. Slope factor √(1 + (4 ÷ 12)²) = 1.0541. Roof area 1,200 × 1.0541 = 1,264.9 sq ft, shown as 1,265. With waste that becomes 1,391.4 sq ft, which is 13.91 squares. Divided by 33.3 it is 41.79, so the page asks for 42 bundles. Pitch angle is 18.4°. Note that the Squares tile already includes the waste.
Edge case one: a pitch of 0 is accepted and gives a slope factor of exactly 1.0000. The same footprint is 1,200 sq ft, 1,320 with waste, and 40 bundles.
Edge case two: typing a pitch in degrees. The box wants rise per 12 inches of run, so 26.6 is read as a 26.6/12 roof, with a slope factor of 2.4318, 2,918 sq ft of roof and 97 bundles, against 45 for a true 6/12 (which is 26.6°). There is no warning for this, so use the dropdown beside the box if you only know the angle. If length or width is zero or blank, the results disappear, and a cleared coverage box quietly falls back to 33.3.
The footprint, in plan view, including any eave overhang. The slope factor then lifts that flat area to the real surface, and it covers both sides of a gable roof in one go.
From a ladder or attic, hold a level horizontal and measure the vertical gap at 12 inches along it. If your phone gives an angle instead, rise per 12 = 12 × tan(angle), so 26.6° is a 6/12 roof.
Not in one pass. The page assumes one rectangle at one pitch. Run each plane separately and add the results, and raise the waste factor for hips, valleys and dormers.
A square is 100 sq ft, and 100 ÷ 3 is 33.3 sq ft per bundle. Many architectural shingles are sold that way, but some are four bundles to a square, so read the package and change the coverage box.
Ridge and hip caps, starter strips, underlayment, drip edge, flashing and nails. The reference table on the page gives per-square allowances for some of these, but measure the ridge length and eaves yourself.