How this calculator works
The calculator takes the house footprint plus overhang on all sides, then multiplies by a pitch factor to get the true sloped area. A roofing “square” is 100 sq ft; shingles are usually sold three bundles per square.
Plan area = (Length + 2×Overhang) × (Width + 2×Overhang)
Pitch multiplier = √(1 + (Pitch ÷ 12)²)
Roof area = Plan area × Pitch multiplier
Squares = Roof area × (1 + Waste%/100) ÷ 100
Bundles = round up( Squares × Bundles per square )
Assumptions and tips
- Works for simple gable and hip roofs where all faces share one pitch. Dormers, valleys and multiple pitches need a measured takeoff.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| House length | 40 ft |
| House width | 30 ft |
| Overhang (eaves and rakes) | 12 in |
| Roof pitch (rise per 12 in) | 6 /12 |
| Roof type | Gable (≈10% waste) |
| Bundles per square | 3 |
| Underlayment roll coverage | 1,000 sq ft |
| Result | Value |
|---|---|
| Roofing squares | 16.5 squares |
| Shingle bundles | 50 bundles |
| Roof area (no waste) | 1,503 sq ft |
| Pitch multiplier | 1.118 |
| Underlayment rolls | 2 rolls |
Step by step:
- Plan area = (40 + 2) × (30 + 2) = 1,344 sq ft
- Pitch multiplier = √(1 + (6/12)²) = 1.118
- Roof area = 1,502.64 sq ft; + 10% waste ÷ 100 = 16.53 squares
- Bundles = 16.53 × 3 → 50
Pitch multipliers
Multiply the flat (plan) area by these factors to get the sloped roof area.
| Pitch | Pitch multiplier |
|---|---|
| 3/12 | 1.031 |
| 4/12 | 1.054 |
| 5/12 | 1.083 |
| 6/12 | 1.118 |
| 7/12 | 1.158 |
| 8/12 | 1.202 |
| 9/12 | 1.25 |
| 10/12 | 1.302 |
| 12/12 | 1.414 |
Common mistakes to avoid
- Measuring the house walls and forgetting the overhang.
- Assuming 3 bundles per square for every product.
Frequently asked questions
How many bundles of shingles are in a square?
Most standard asphalt shingles are three bundles per square (100 sq ft), but some products use four or five — check the package coverage.
What is a 6/12 pitch?
The roof rises 6 inches for every 12 inches of horizontal run, about 26.6°.