Measuring a roof directly means climbing on it. This calculator uses a safer shortcut: the area of the house's footprint (including the overhang) multiplied by a factor for the roof's slope. The result is converted to roofing squares and bundles of shingles so you can compare quotes or order materials.
How to use the roofing calculator
- Measure the footprint — the outside length and width of the house plus the eave overhang on each side. A house 38 × 28 ft with 1 ft overhangs has a 40 × 30 ft roof footprint.
- Enter the pitch as inches of rise per 12 inches of run, for example 6 for a 6/12 roof.
- Set the waste allowance. Simple gable roofs need about 10%; roofs with hips, valleys and dormers about 15%.
- Check bundles per square on the shingle wrapper.
Roof area formula
Roof area = footprint length × footprint width × pitch multiplier
Squares = roof area × (1 + waste% ÷ 100) ÷ 100
Bundles = ⌈ squares × bundles per square ⌉
The multiplier comes from the Pythagorean theorem: a rafter rising 6 in over 12 in of run is √(12² + 6²) ÷ 12 = 1.118 times longer than its horizontal distance.
Worked example
A ranch house has a 40 × 30 ft roof footprint and a 6/12 pitch.
- Footprint: 40 × 30 = 1,200 sq ft
- Multiplier for 6/12: √(1 + 0.5²) = 1.118
- Roof area: 1,200 × 1.118 = 1,342 sq ft
- With 10% waste: 1,342 × 1.10 ÷ 100 = 14.76 squares
- Bundles: 14.76 × 3 = 44.3 → 45 bundles
Pitch multipliers
| Pitch | Multiplier | Pitch | Multiplier |
|---|---|---|---|
| 2/12 | 1.0138 | 8/12 | 1.2019 |
| 3/12 | 1.0308 | 9/12 | 1.2500 |
| 4/12 | 1.0541 | 10/12 | 1.3017 |
| 5/12 | 1.0833 | 11/12 | 1.3566 |
| 6/12 | 1.1180 | 12/12 | 1.4142 |
| 7/12 | 1.1577 |
Shingles for a 1,500 sq ft footprint
10% waste, 3 bundles per square.
| Pitch | Roof area | Squares (no waste) | Bundles |
|---|---|---|---|
| 4/12 | 1,581 sq ft | 15.81 | 53 |
| 6/12 | 1,677 sq ft | 16.77 | 56 |
| 8/12 | 1,803 sq ft | 18.03 | 60 |
| 10/12 | 1,953 sq ft | 19.53 | 65 |
| 12/12 | 2,121 sq ft | 21.21 | 71 |
What this estimate leaves out
- Starter strip and hip/ridge caps are separate products, ordered by linear feet of eave and ridge.
- Underlayment, drip edge and flashing are ordered separately.
- Different pitches on one roof — calculate each section separately and add.
- Safety first. Roof work is dangerous; this calculator is for planning, not a substitute for a professional inspection.
Need the footprint of an odd-shaped house? Break it into rectangles with the Square Footage Calculator.
Frequently asked questions
What is a roofing square?
A square is 100 square feet of roof surface. A 2,000 sq ft roof is 20 squares. Shingles and labor are commonly priced per square.
How many bundles of shingles are in a square?
Most standard asphalt shingles come three bundles to a square, but some heavier products use four or more. The wrapper states the coverage — enter the number from your product.
How do I find my roof pitch?
Pitch is the rise in inches over 12 inches of horizontal run. Hold a level horizontally against a rafter or the roof surface, measure 12 inches along it, then measure straight up to the roof. A 6 in rise means a 6/12 pitch.
Why is the roof area bigger than the house's footprint?
Because the roof slopes. The steeper the pitch, the more surface it has over the same footprint. A 6/12 roof has about 12% more area than its footprint; a 12/12 roof about 41% more.
Does this work for hip roofs?
Yes, as an estimate. For a roof with the same pitch on every face, the footprint-times-pitch-multiplier method gives the same area for gable and hip roofs. Use a higher waste allowance for hips and valleys because of the extra cuts, and buy hip-and-ridge shingles separately.