Roof calculator square feet: the one-line method
For any roof whose planes all share one pitch, the roof area is the footprint (with overhangs) times the rafter factor, whether it is a gable, a hip or a shed. The slope only stretches the plan area in one direction, and the factor √(1 + (pitch ÷ 12)²) is that stretch. You can measure the footprint on the ground, which is safer than walking the roof.
| Pitch | Factor | Roof area | Squares |
|---|---|---|---|
| 3/12 | 1.031 | 1,031 sq ft | 10.31 |
| 4/12 | 1.054 | 1,054 sq ft | 10.54 |
| 5/12 | 1.083 | 1,083 sq ft | 10.83 |
| 6/12 | 1.118 | 1,118 sq ft | 11.18 |
| 7/12 | 1.158 | 1,158 sq ft | 11.58 |
| 8/12 | 1.202 | 1,202 sq ft | 12.02 |
| 9/12 | 1.25 | 1,250 sq ft | 12.5 |
| 10/12 | 1.302 | 1,302 sq ft | 13.02 |
| 12/12 | 1.414 | 1,414 sq ft | 14.14 |
Shingle calculator: squares and bundles
A shingle calculator works in squares: one roofing square is 100 sq ft of roof. Divide the area by 100, add a waste allowance for cutting at hips, valleys and rakes, and multiply by the number of bundles in a square, which the shingle wrapper states. Hips and valleys waste more than a plain gable.
- Asphalt shingles need a pitch of at least 2/12; from 2/12 to 4/12 the IRC requires double underlayment[1].
- Ridge cap and hip cap are ordered by length; the tool lists both.
- Drip edge runs along the eaves and the rakes.
Gable, hip and shed edges
| Roof | Eaves | Rakes | Ridge | Hips |
|---|---|---|---|---|
| Gable | 2 × length | 4 × rafter length | length | – |
| Hip | full perimeter | – | length − width | 4 × hip length |
| Shed | length | 2 × slope length | – | – |
Hip rafters on a regular hip roof rise at the same pitch over a run 1.414 times longer; the hip roof calculator gives their lengths. Below 3/12, the IRC wants ridges and hips designed as beams[2].
Measuring a complicated roof
Split it into rectangles that each have one pitch, run the calculator for each, and add them. Dormers, porch roofs and valleys between wings are separate planes. If the pitch is unknown, measure it first with the roof pitch calculator. For panel roofs the metal roof calculator counts panels instead of squares, and the rafter calculator gives the framing.
Stay off steep roofsFootprint measurements from the ground give the same area. Leave roof walking to people with fall protection.
Common roof measuring mistakes
Most roofing orders come up short because of the overhangs or the pitch:
- Footprint without overhangs. Add the eave and rake overhangs to the length and width.
- Squares of footprint. A square is 100 sq ft of roof surface, after the pitch factor.
- No waste for hips and valleys. Cut roofs waste more than a plain gable.
- Shingles on a low slope. Asphalt shingles need at least 2/12 (2021 IRC R905.2.2)[1].
Frequently asked questions
How do I calculate the square footage of my roof?
Multiply the footprint (length × width, including overhangs) by the rafter factor for your pitch. At 6/12 the factor is 1.118.
How many squares is a 2,000 sq ft roof?
20 squares, since one square is 100 sq ft. Add waste before ordering.
How many bundles of shingles do I need?
Squares (with waste) × bundles per square. The wrapper states how many bundles cover a square.
Is there a roof calculator app?
Yes: this page is built to work as one on a phone. It runs in any mobile browser with no sign-up, and you can add it to your home screen from the browser menu.
Does a hip roof have more area than a gable?
Not at the same pitch and footprint: both equal footprint × rafter factor. A hip roof does need more cutting, so allow more waste.
How many shingles do I need?
Roof area ÷ 100 gives squares; multiply by the bundles per square printed on the wrapper and add waste. A 1,400 sq ft roof is 14 squares: 42 bundles at 3 per square, before waste.
What is the minimum roof pitch for shingles?
2/12 under IRC R905.2.2, and between 2/12 and 4/12 two layers of underlayment are required.