Key takeaways
- A roofing square is 100 square feet of roof surface — the unit roofing is sold by.
- A roof's surface is larger than its footprint; the steeper the pitch, the bigger the gap.
- Architectural shingles come 3 bundles per square.
- Add ~10% waste for a simple gable, 15–20% for cut-up hip roofs.
How to estimate roofing
A roofing square is 100 square feet of roof surface — the unit shingles and underlayment are sold and estimated by. A roof's actual surface is larger than the footprint it sits on, because the slope stretches each plane; the steeper the pitch, the bigger the difference. So you size a roof by taking the footprint, scaling it up for pitch, converting to squares, and then ordering bundles with a waste allowance on top.
The pitch multiplier comes straight from geometry: a 12-inch run with a given rise forms the hypotenuse the shingles actually lie along. Waste is applied to the bundle order, not to the squares, since the finished roof surface itself doesn't grow.
Worked example: a 40 × 30 footprint at 6/12
For a 40 ft × 30 ft footprint, the ground area is 40 × 30 = 1,200 sq ft. A 6/12 pitch gives a multiplier of √(1 + (6 ÷ 12)²) = 1.118, so the roof surface is 1,200 × 1.118 = 1,342 sq ft. Dividing by 100 gives 13.4 squares. At 3 bundles per square that's 13.4 × 3 = 40.3 bundles, and adding 10% waste brings it to 45 bundles.
Roof pitch multiplier
| Pitch (rise/12) | Multiplier |
|---|---|
| 4/12 | 1.054 |
| 6/12 | 1.118 |
| 8/12 | 1.202 |
| 12/12 | 1.414 |
Footprint, not roof surface
Measure the building footprint — the ground area the roof covers — and let the pitch multiplier do the rest; don't try to measure the sloped planes directly. This assumes a simple gable. Complex roofs with many hips, valleys, and dormers need more waste (15–20%) and a per-plane takeoff. For other materials on the same project, see the full list of calculators.
Frequently asked questions
What is a roofing square?
A roofing square is 100 square feet of roof surface — the unit roofing materials are sold and estimated in. A 1,500 sq ft roof is 15 squares.
How many bundles of shingles per square?
Architectural (laminate) shingles come 3 bundles per square; heavier shingles can be 4–5. A 15-square roof needs about 45 bundles before waste.
How does roof pitch affect the area?
The pitch multiplier is √(1 + (rise ÷ 12)²). A 6/12 pitch multiplies the footprint by 1.118; a 12/12 pitch by 1.414.
How many shingles for a 1500 sq ft roof?
1,500 sq ft is 15 squares → 15 × 3 = 45 bundles before waste, or about 50 after adding 10%. That 1,500 is the sloped surface, not the footprint.
How much waste should I add?
About 10% for a simple gable; 15–20% for cut-up roofs with many hips and valleys. Waste is added to the bundle count.
Do I measure the footprint or the roof surface?
Measure the footprint, then multiply by the pitch factor. A 40 × 30 footprint is 1,200 sq ft on the ground but about 1,342 sq ft of roof at 6/12.
Bundle counts follow standard manufacturer data — GAF specs architectural shingles at 3 bundles per square. The 100 sq ft per square conversion is exact.
Last reviewed June 2026