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Roofing Calculator: Roof Area, Squares and Bundles
Enter the footprint of the building, the eave overhang and the roof pitch. The calculator converts plan area into sloped roof area, expresses it in roofing squares and estimates bundles of shingles with waste.
Why sloped area is larger than plan area
A roof is not flat, so the surface you cover is bigger than the footprint seen from above. For a pitch written as rise over a 12 unit run, the slope length per unit of run is the hypotenuse of a right triangle, which gives the pitch factor √(1 + (rise ÷ 12)²). Multiply the plan area by that factor to get the sloped area. A 6/12 pitch has a factor of about 1.118, and a 12/12 pitch has a factor of about 1.414.
One roofing square is 100 square feet of roof. Shingle packs are commonly sold as bundles, and the number of bundles per square is printed on the packaging. The calculator divides the sloped area by 100, applies your waste percentage, and multiplies by the bundles per square.
Measuring pitch
Pitch can be measured by holding a level horizontally 12 inches along a rafter or the underside of the roof and measuring the vertical distance to the rafter at the end of the level. That distance is the rise per foot of run. A table of familiar pitches helps with checking your number.
| Pitch | Angle | Pitch factor |
|---|---|---|
| 3/12 | 14.0° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 6/12 | 26.6° | 1.118 |
| 8/12 | 33.7° | 1.202 |
| 12/12 | 45.0° | 1.414 |
Steep and low-slope roofs are installed differently, and some coverings have minimum pitch limits, so check the maker's instructions.
Waste, hips and valleys
A simple gable roof wastes little material beyond the ridge, starter and cut ends, and about 10 percent is a reasonable planning figure. Hip roofs, valleys, dormers and many small planes generate more offcuts, so raise the percentage. This calculator treats every plane as having the same pitch, which is exactly right for a plain gable and a good approximation for a regular hip roof. Complex roofs are better measured plane by plane.
Ridge caps, drip edge, underlayment, flashing, vents and nails are separate items. Underlayment is usually bought by area, so the sloped area is the starting point for that figure too.
Limits of this estimate
The calculator is for estimating and learning. It does not replace local building codes, the roofing manufacturer's instructions or the judgement of a qualified roofer or structural professional. Roof work involves falls, load limits and wind uplift requirements that no online tool can assess. Measure safely from the ground or from plans when possible, and have a contractor confirm quantities and fixings before ordering or fitting materials.
Worked example
A 40 ft by 30 ft house with a 1 ft overhang all round, a 6/12 pitch, a 10% allowance and shingles sold 3 bundles to the square.
Inputs
| Units | Imperial (feet) |
|---|---|
| Building length (feet or meters) | 40 |
| Building width (feet or meters) | 30 |
| Overhang on each side (feet or meters) | 1 |
| Pitch: rise per 12 units of run | 6 |
| Waste allowance | 10 % |
| Bundles per square | 3 |
Result
Sloped roof area of 1,503 ft² (139.6 m²), without waste.
| Plan area including overhang | 1,344 ft² |
|---|---|
| Pitch factor | 1.1180 |
| Slope angle | 26.6° |
| Squares with waste | 16.53 |
| Bundles to buy | 50 (at 3 per square) |
- This assumes one pitch on every plane (gable or simple hip). Dormers, valleys, chimneys and skylights are not modelled.
Estimating note: Quantities and ratings here are estimates for planning and learning. Products, local codes and site conditions differ, so check manufacturer data and local regulations, and use a qualified professional for electrical, structural or safety-critical work.
Frequently asked questions
How do I calculate roof area from the house size?
Find the footprint including overhang, then multiply by the pitch factor, which is the square root of one plus the square of rise divided by twelve. The result is the sloped area.
What is a roofing square?
A roofing square is 100 square feet of roof surface, which is about 9.29 square meters. Shingles and many other coverings are priced and packaged around this unit.
How many bundles of shingles are in a square?
Many standard shingles use three bundles per square, but products differ. Enter the figure from your product label so the bundle count matches what you will buy.
Does a steeper pitch cost more to roof?
The steeper roof has a larger surface and often slower, more careful installation, so material and labour both tend to rise. The pitch factor shows the extra area directly.
Is a gable roof area the same as a hip roof area?
For the same footprint and one pitch on every plane, the surface area is the same. A hip roof usually creates more cutting waste because of its angled hips.
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References and sources
- Shingle and underlayment manufacturers publish coverage per bundle, minimum pitch and fastening instructions in their installation guides.