How to use the roofing calculator
You don't need to climb on the roof. Measure the house's footprint from the ground, then add the overhangs: if the eaves stick out 12 in on each side, add 2 ft to each dimension. Choose the pitch and whether the roof is a gable (two slopes meeting at a ridge, flat triangle ends) or a hip (four slopes).
Not sure of the pitch? From the attic, hold a level against a rafter, measure 12 in along the level, and measure straight up to the rafter. That number is the rise: 6 in means 6/12.
How roof area is calculated
A sloped roof covers more area than the footprint under it. The pitch multiplier converts one to the other:
- Pitch multiplier = √(1 + (rise ÷ 12)²)
- Roof area = footprint length × width × multiplier. This works for gable and hip roofs alike, as long as every plane has the same pitch.
- Squares = roof area ÷ 100
- Bundles = squares × (1 + waste) × bundles per square, rounded up
Pitch multiplier chart
| Pitch | Angle | Multiplier | 1,000 sq ft footprint becomes |
|---|---|---|---|
| 2/12 | 9.5° | 1.014 | 1,014 sq ft |
| 3/12 | 14° | 1.031 | 1,031 sq ft |
| 4/12 | 18.4° | 1.054 | 1,054 sq ft |
| 5/12 | 22.6° | 1.083 | 1,083 sq ft |
| 6/12 | 26.6° | 1.118 | 1,118 sq ft |
| 7/12 | 30.3° | 1.158 | 1,158 sq ft |
| 8/12 | 33.7° | 1.202 | 1,202 sq ft |
| 9/12 | 36.9° | 1.25 | 1,250 sq ft |
| 10/12 | 39.8° | 1.302 | 1,302 sq ft |
| 12/12 | 45° | 1.414 | 1,414 sq ft |
Edges and trim
- Starter strip runs along the eaves, and many installers also run it up the rakes of a gable.
- Drip edge covers eaves and rakes, sold in 10 ft pieces.
- Ridge cap covers the ridge and, on a hip roof, every hip. Hip length per foot of run is √(2 + (rise ÷ 12)²).
- Underlayment: synthetic rolls often cover 10 squares; felt rolls cover far less. Check the roll's label.
Worked example: a hip roof
A 48 × 26 ft house with 12 in overhangs is a 50 × 28 ft footprint. The roof is a 5/12 hip.
- Multiplier: √(1 + (5 ÷ 12)²) = 1.083.
- Roof area: 50 × 28 × 1.083 = 1,517 sq ft = 15.17 squares.
- With 15% waste for a hip roof: 17.44 squares × 3 = 53 bundles.
- Ridge: 50 − 28 = 22 ft. Each hip: 14 ft run × √(2 + (5 ÷ 12)²) = 20.7 ft, four of them = 83 ft. Ridge cap: 105 ft.
- Eaves all the way around: 2 × (50 + 28) = 156 ft of starter and drip edge (16 pieces).
Questions people ask
How many bundles of shingles do I need for 1,000 sq ft of roof?
1,000 sq ft is 10 squares. With 10% waste, 11 squares × 3 = 33 bundles of standard 3-tab or architectural shingles.
Does every shingle come 3 bundles to a square?
Most do, but not all. Some heavier designer shingles come 4 or even 5 bundles to a square. The wrapper states the coverage per bundle; trust it over any rule of thumb.
How much waste should I add?
About 10% for a simple gable roof and 15% for a hip roof or a cut-up roof with valleys and dormers. Ridge cap is usually bought separately from field shingles.