How roof area is calculated
Footprint = (length + 2 × overhang) × (width + 2 × overhang)
Pitch factor = √(1 + (rise ÷ 12)²)
Roof area = footprint × pitch factor · Squares = roof area ÷ 100
A sloped roof has more surface than the ground it covers. The pitch factor converts the flat footprint into the sloped area, and it works for simple gable and hip roofs alike as long as all planes have the same pitch. Roofers count area in squares: one square is 100 sq ft of roof. Asphalt shingles are usually packed three bundles to the square, but check the wrapper because some heavy designer shingles need four or five.
Pitch factors
| Pitch | Angle | Pitch factor |
|---|---|---|
| 3/12 | 14.0° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 5/12 | 22.6° | 1.083 |
| 6/12 | 26.6° | 1.118 |
| 7/12 | 30.3° | 1.158 |
| 8/12 | 33.7° | 1.202 |
| 9/12 | 36.9° | 1.250 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45.0° | 1.414 |
Worked example: 40 × 30 ft house, 6/12 gable roof
- With a 12 in (1 ft) overhang all round, the footprint is 42 × 32 = 1,344 sq ft.
- Roof area: 1,344 × 1.118 = 1,502.6 sq ft, or 15.03 squares.
- With 10% waste for a gable roof: 16.53 squares.
- Bundles at 3 per square: 16.53 × 3 = 49.6, so 50 bundles.
The same house with a hip roof and 15% waste needs 17.28 squares, which is 52 bundles.
How to find your roof pitch
Pitch is the rise in inches over a horizontal run of 12 inches. From the attic or at the gable end, hold a level horizontally against the underside of a rafter, measure 12 in along the level, and then measure straight up to the rafter. A rise of 6 in is a 6/12 pitch. Apps that measure angle can also help: a 26.6° slope is 6/12. Never climb onto a roof without proper fall protection.
Other materials to plan
- Underlayment: roughly the roof area plus overlaps; synthetic rolls state their coverage in squares.
- Starter strip along the eaves and rakes, and ridge cap shingles along ridges and hips, sold by the linear foot.
- Drip edge, flashing and ice-and-water membrane where your climate and code require it.
- Roofing nails: follow the shingle manufacturer's nailing pattern, commonly 4 to 6 nails per shingle.
Limits of the estimate
Dormers, valleys, skylights and roofs with sections at different pitches change the area. For those, calculate each plane separately or use the estimate as a starting point and have a roofer measure. Some regions also allow or require a second layer, or a full tear-off, which affects disposal costs but not the shingle count.
Step by step: measuring a roof safely
- Measure the outside walls of the house from the ground, not on the roof.
- Measure how far the eaves and gable overhangs stick out.
- Find the pitch from the attic, from building plans or with an angle app at the gable end.
- Note dormers, valleys, skylights and sections at different pitches.
- Choose the waste allowance for the roof shape and round the bundles up.
Metric example
A house 12 × 9 m with a 30 cm overhang has a footprint of 12.6 × 9.6 = 120.96 m². With a 6/12 pitch (about 26.6°) the roof area is 120.96 × 1.118 = 135.2 m², which is 1,455 sq ft or 14.55 squares. With 10% waste that is 16.01 squares, or 49 bundles.
Shingles, metal or tile?
The square is used for every roofing material, but the way it is bought differs. Asphalt shingles come in bundles; metal roofing is ordered as panels cut to the length of each roof plane; clay and concrete tiles are counted per square from the manufacturer's coverage table. Minimum pitches also differ: many asphalt shingles need at least 2/12 with extra underlayment, and tile roofs usually need a steeper slope. Check the product's installation instructions before you order.
Common mistakes
- Using the floor area of the house. A two-storey home has the same roof footprint as its ground floor, not its total living area.
- Forgetting the overhang, which adds about 10% on a typical house.
- Ignoring waste. Cuts at hips and valleys use up more shingles than on a plain gable.
Frequently asked questions
How many bundles of shingles do I need?
Find the roof area in squares (100 sq ft each), add 10–15% waste and multiply by 3 bundles per square. A 1,500 sq ft roof is 15 squares, 16.5 with 10% waste, so 50 bundles.
What is a square in roofing?
A roofing square is 100 square feet of roof surface. Most asphalt shingles come three bundles to a square.
How do I calculate roof area from the house size?
Multiply the footprint including overhangs by the pitch factor. A 42 × 32 ft footprint with a 6/12 pitch (factor 1.118) is about 1,503 sq ft of roof.
What is a 6/12 roof pitch in degrees?
About 26.6°. The roof rises 6 inches for every 12 inches of horizontal run, and its pitch factor is 1.118.
How many squares is a 2,000 sq ft roof?
20 squares, because one roofing square is 100 sq ft. With 10% waste that is 22 squares, or 66 bundles at three bundles per square.
Sources
- Asphalt Roofing Manufacturers Association (ARMA), Residential Asphalt Roofing Manual (roofing squares, slope, application).
- Shingle manufacturers' installation instructions (bundles per square, nailing patterns).
Last reviewed 20 October 2026. Results are estimates; see our disclaimer.