How to use the roofing calculator
- Measure the outside length and width of the house (the footprint), and estimate how far the eaves stick out past the walls, usually 12–24 inches.
- Choose your roof pitch. A 6/12 pitch rises 6 inches for every 12 inches across. If you don't know it, 4/12 to 6/12 is common on ranch homes and 8/12 or steeper on two-story homes.
- Pick the waste allowance for your roof shape. Hips and valleys mean more cut shingles.
Roofing formula
plan area = (length + 2 × overhang) × (width + 2 × overhang)
roof area = plan area × pitch factor
squares = roof area ÷ 100 · bundles = squares × 3
For any roof with a single pitch, the roof area is the same whether it's a gable or a hip: the slope factor applies to the whole footprint. Hips mainly add waste.
Roof pitch multipliers
| Pitch | Multiplier | Angle |
|---|---|---|
| 2/12 | 1.014 | 9.5° |
| 3/12 | 1.031 | 14.0° |
| 4/12 | 1.054 | 18.4° |
| 5/12 | 1.083 | 22.6° |
| 6/12 | 1.118 | 26.6° |
| 7/12 | 1.158 | 30.3° |
| 8/12 | 1.202 | 33.7° |
| 9/12 | 1.250 | 36.9° |
| 10/12 | 1.302 | 39.8° |
| 12/12 | 1.414 | 45.0° |
What is a roofing square?
Roofers measure in squares: one square is 100 sq ft of roof. Most asphalt shingles are packed 3 bundles to a square, so a 25 square roof needs about 75 bundles plus waste. Starter strips and ridge caps are sold separately; budget roughly one bundle of ridge cap per 25–33 linear ft of ridge.
This is a planning estimate. Dormers, chimneys, skylights and multiple pitches change the numbers, so get a measured quote before ordering for a full re-roof.
Frequently asked questions
How many bundles of shingles do I need for a 2,000 sq ft house?
For a 2,000 sq ft single-story footprint (such as 40 × 50 ft) with 12 inch overhangs and a 6/12 pitch, the roof is about 2,450 sq ft, or 24.5 squares. With 10% waste that is about 27 squares, so 81 bundles.
How many square feet does a bundle of shingles cover?
About 33.3 sq ft for standard 3-bundle-per-square shingles.
How do I find my roof pitch?
Hold a level horizontally against the roof (or a rafter in the attic), measure 12 inches along it, and measure straight up from that point to the roof. The rise in inches is your pitch over 12.