Enter the length and width of the building, including the overhangs, and the roof pitch. The calculator turns the footprint into the real sloped roof area and gives the roofing squares, shingle bundles and underlayment rolls to order.
How to use the roofing calculator
- Measure the outside of the building and add the eave and rake overhangs, or measure the drip edge from the ground with a long tape.
- Enter the pitch as inches of rise per 12 in of run. Not sure? The roof pitch calculator finds it from the rise and run or the angle.
- Keep 10% waste for a simple gable roof and raise it to 15% for hips, valleys and dormers.
- Check the bundles per square on the shingle wrapper and the coverage on the underlayment roll.
Roofing formulas
Pitch factor = √(1 + (rise ÷ 12)²) Roof area (ft²) = footprint (ft²) × pitch factor Squares = roof area × (1 + waste %) ÷ 100 Bundles = squares × bundles per square, rounded up
A roofing square is 100 ft² of roof. GAF says most standard 3-tab and architectural shingles come three bundles to the square, though some heavy specialty shingles need four or five. The pitch factor is the length of the sloped rafter for every foot of level run; it works for gable, hip and shed roofs that share one pitch, because every face rises at the same angle.
Worked example: a 40 × 30 ft house with a 6/12 roof
- Footprint including overhangs: 40 × 30 = 1,200 ft².
- Pitch factor: √(1 + 0.5²) = 1.118.
- Roof area: 1,200 × 1.118 = 1,342 ft².
- With 10% waste: 1,476 ft², which is 14.76 squares.
- Bundles: 14.76 × 3 = 45 bundles.
Pitch factors
Multiply the footprint by the factor to get the roof area:
| Pitch | Angle | Pitch factor | Roof area per 1,000 ft² footprint |
|---|---|---|---|
| 3/12 | 14° | 1.031 | 1,031 |
| 4/12 | 18.4° | 1.054 | 1,054 |
| 5/12 | 22.6° | 1.083 | 1,083 |
| 6/12 | 26.6° | 1.118 | 1,118 |
| 7/12 | 30.3° | 1.158 | 1,158 |
| 8/12 | 33.7° | 1.202 | 1,202 |
| 9/12 | 36.9° | 1.25 | 1,250 |
| 10/12 | 39.8° | 1.302 | 1,302 |
| 12/12 | 45° | 1.414 | 1,414 |
How much waste for shingles?
GAF suggests adding 10 to 15% for trim and waste, with more for steep slopes and roofs with rakes, valleys and dormers. Starter strips and ridge caps are usually bought separately; measure the eaves for the starter and the ridges and hips for the cap.
Tips for ordering roofing
- Round up to whole bundles. Suppliers sell by the bundle; a spare bundle is useful for future repairs from the same colour run.
- Measure each section if the pitch changes. Run the calculator once per pitch and add the results.
- Check the underlayment label. Roll coverage varies widely between felt and synthetic products.
- Do the soffit and fascia too. The soffit calculator uses the same overhang measurements.
Questions
How many bundles of shingles do I need for 1,000 square feet?
1,000 ft² of roof is 10 squares, or 30 bundles at three per square, before waste. With 10% waste it is 33 bundles.
How do I calculate roof area from the footprint?
Multiply the footprint, including overhangs, by the pitch factor. A 6/12 roof has a factor of 1.118, so 1,200 ft² of footprint is about 1,342 ft² of roof.
How many square feet is a roofing square?
100 ft², a 10 × 10 ft patch of roof.
Does the roof shape change the area?
Not if every face has the same pitch: a hip roof and a gable roof over the same footprint have the same area. Hips and valleys do add waste, so use 15%.
Step-by-step guides
Sources
- How Many Bundles of Shingles Are in a Square?, GAF Roof Views blog, Aug 24, 2026.
Used for: 100 ft² per square, three bundles per square for most 3-tab and architectural shingles, 10–15% waste. - NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically, National Institute of Standards and Technology.
Used for: Exact inch and foot definitions; square metre conversions.