How to calculate roofing squares
Roofing is measured in squares. One square is 100 square feet of roof surface. The calculation has three steps:
- Footprint. Multiply the length and width of the house, measured at the outside walls, plus the overhang on each side.
- Slope. Multiply the footprint by the pitch factor to get the real roof area.
- Squares. Divide the roof area by 100, then add a waste allowance for cut shingles.
Roof area = length × width × √(1 + (pitch ÷ 12)²)
Example: a 40 × 30 ft gable roof, overhangs included, at a 6/12 pitch. The footprint is 1,200 sq ft. The pitch factor is 1.118, so the roof is 1,342 sq ft, or 13.4 squares. With 10% waste that’s 14.8 squares, and at 3 bundles per square, 45 bundles of shingles.
Pitch factors
| Pitch | Angle | Multiply footprint by |
|---|---|---|
| 3/12 | 14.0° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 5/12 | 22.6° | 1.083 |
| 6/12 | 26.6° | 1.118 |
| 7/12 | 30.3° | 1.158 |
| 8/12 | 33.7° | 1.202 |
| 9/12 | 36.9° | 1.250 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45.0° | 1.414 |
Don’t know your pitch? The roof pitch calculator works it out from a rise and run or an angle, and shows which roof coverings the code allows at it.
Everything else on the roof
Shingles are only part of the order. The calculator also works out:
- Starter strip along the eaves and rakes, at about 100 linear feet per bundle (Owens Corning; GAF and CertainTeed bundles cover 116 to 120 ft).
- Hip and ridge caps at about 25 linear feet per bundle, the GAF Seal-A-Ridge figure. CertainTeed and Owens Corning caps cover 30 and 33 ft.
- Underlayment by the roll: a #15 felt roll covers 400 sq ft, #30 felt 204 sq ft, and a 10-square synthetic roll about 930 sq ft after laps.
- Drip edge, the metal edging along the eaves and rakes, as a total length.
- Nails, at four per shingle.
The hip roof option assumes a rectangular house with the same pitch on all four sides, so its ridge is the length minus the width. For L-shaped houses, dormers or several roof sections, calculate each rectangle on its own and add them up, and allow more waste for the valleys.
For the walls under the roof, the siding calculator adds the gable triangles to the wall area.
For the full price of a new roof, tear-off included, the roof replacement cost calculator works from the same measurements.