How much roofing should I buy?
Type the length and width of the roof seen from above and its pitch. The roofing calculator shows the sloped roof area, the roofing squares with an extra share for waste, and the bundles of shingles.
- Roofing squares
- 14.76
The roof is 1,341.64 ft² at this pitch, 14.76 squares with the extra; bundles to buy: 45.
- Roof area with the extra, m²
- 137.11
- Bundles of shingles
- 45
- Roof area, ft²
- 1,341.64
- Roof area, m²
- 124.64
- Slope factor
- 1.118
Roofing squares: 14.76. The roof is 1,341.64 ft² at this pitch, 14.76 squares with the extra; bundles to buy: 45.
How to calculate
Computes a roof’s sloped area from its footprint and pitch, the roofing squares (100 ft² each) with an extra share for waste, and the bundles of shingles to buy.
Example with the default inputs (Footprint length 40 ft, Footprint width 30 ft, Pitch (rise in 12) 6, Extra for waste 10%, Bundles per square 3): The roof is 1,341.64 ft² at this pitch, 14.76 squares with the extra; bundles to buy: 45.
Method: Roof area = footprint length × width × √(144 + pitch²) ÷ 12; squares = roof area in ft² × (1 + extra ÷ 100) ÷ 100; bundles = squares × bundles per square, rounded up.
- Every plane of the roof has the same pitch, so the sloped area is the footprint times one slope factor, for a gable or a hip roof.
- The footprint is measured level from above and includes the overhangs.
- Extra for waste is 10% by default, for cuts at hips, valleys and rakes and for the starter course and ridge caps.
- A square is 100 ft² of roof; the bundles per square come from the shingle maker (3 by default).
Worked examples
Each example is checked against the calculator on every build.
- Footprint length 40 ft, Footprint width 30 ft, Pitch (rise in 12) 6, Extra for waste 10%, Bundles per square 3 gives Slope factor 1.118034, Roof area, ft² 1,341.640786, Roofing squares 14.758049, Bundles per square 45.
- Footprint length 36 ft, Footprint width 24 ft, Pitch (rise in 12) 5, Extra for waste 0%, Bundles per square 3 gives Slope factor 1.083333, Roof area, ft² 936, Roofing squares 9.36, Bundles per square 29.
- Footprint length 40 ft, Footprint width 20 ft, Pitch (rise in 12) 9, Extra for waste 0%, Bundles per square 3 gives Slope factor 1.25, Roof area, ft² 1,000, Roofing squares 10, Bundles per square 30.
- Footprint length 32.81 ft, Footprint width 26.25 ft, Pitch (rise in 12) 6, Extra for waste 10%, Bundles per square 3 gives Roof area, m² 89.442719, Roofing squares 10.590288, Bundles per square 32.Source: NIST Handbook 44, Appendix C (1 ft² = 0.09290304 m²)
How it works
The pitch is the rise p for every 12 units of run. By the Pythagorean theorem, 12 units of run have √(12² + p²) units of roof along the slope, so:
- Slope factor = √(144 + p²) ÷ 12
- Roof area = footprint length × footprint width × slope factor
- Squares = roof area in ft² × (1 + extra ÷ 100) ÷ 100
- Bundles = squares × bundles per square, rounded up to a whole bundle
The footprint is the roof seen from above, including the overhangs. When all planes have the same pitch, the same factor applies to a gable, a hip or any other roof shape.
Exact arithmetic. Each length is read as the exact decimal you typed, in the unit you typed it in (12 ft 6 in is exactly 150 in); the page stores it as a number in metres and reads it back as the decimal with the fewest significant digits, in one of the box's units, that gives the same stored number; a feet-and-inches pair counts by its value in inches (12 ft 6 in counts as 150, and 426.72 cm, exactly 14 ft, stays 426.72 cm), and on a tie the unit listed first in the box's unit menu wins. The pitch is the exact decimal typed. The slope factor is exact when 144 + p² is the square of a fraction (p = 0, 3.5, 5, 9, 16 and others: 12/12, 12.5/12, 13/12, 15/12, 20/12); otherwise it is a square root, and the calculator uses the nearest floating-point value. So a 9/12 roof of 1,000 ft² is exactly 10 squares and 30 bundles, not 31.
Rounding. The squares and areas show with at most 2 decimals and the slope factor with 4, rounded half up from the exact value, with trailing zeros left off. Bundles are rounded up.
Assumptions
- Every roof plane has the same pitch; the footprint is measured level and includes the overhangs.
- The extra for waste is 10% by default, from 0% to 100%.
- A square is 100 ft² of roof. Bundles per square are 3 by default, from 1 to 10.
- Lengths are more than 0 and at most 1,000 m; the pitch is from 0 to 24.
- No material prices: the calculator counts squares and bundles only.
Worked examples by hand
A 40 × 30 ft footprint at 6/12, 10% extra, 3 bundles per square. Slope factor √(144 + 36) ÷ 12 = √180 ÷ 12 = 1.1180. Roof area 1,200 × 1.1180 = 1,341.64 ft². Squares 1,341.64 × 1.1 ÷ 100 = 14.76; bundles 14.76 × 3 = 44.27, so 45 bundles.
A 36 × 24 ft footprint at 5/12, no extra. Slope factor √169 ÷ 12 = 13/12. Roof area 864 × 13 ÷ 12 = 936 ft² = 9.36 squares; 9.36 × 3 = 28.08, so 29 bundles.
A 40 × 20 ft footprint at 9/12, no extra. Slope factor √225 ÷ 12 = 15/12 = 1.25. Roof area 800 × 1.25 = 1,000 ft² = 10 squares, exactly 30 bundles.
A 10 × 8 m footprint at 6/12, 10% extra. Roof area 80 × 1.1180 = 89.44 m² = 962.75 ft²; squares 962.75 × 1.1 ÷ 100 = 10.59; bundles 31.77, so 32.
Other questions people ask
How do I calculate roofing squares?
Find the sloped roof area in square feet and divide by 100: a roofing square is 100 ft² of roof. A 40 × 30 ft footprint at a 6/12 pitch has 1,200 × 1.1180 = 1,341.6 ft² of roof, which is 13.42 squares, or 14.76 squares with 10% extra.
How does the pitch change the roof area?
A sloped roof is larger than its footprint. For a pitch of p inches of rise per 12 inches of run, each foot of run has √(144 + p²) ÷ 12 feet of roof along the slope: 1.0541 at 4/12, 1.0833 at 5/12, 1.1180 at 6/12, 1.2019 at 8/12 and 1.4142 at 12/12. Multiply the footprint by this slope factor.
How many bundles of shingles are in a square?
Many asphalt shingles are packed three bundles to a square, but some heavy or designer shingles take four or five. Check the coverage printed on the bundle and set “Bundles per square” to match.
How much extra roofing should I add for waste?
The calculator adds 10% by default for the shingles cut at hips, valleys and rakes and for the starter course and ridge caps. A plain gable roof may need less; a roof with many hips, valleys and dormers may need 15% or more.
Why measure the footprint instead of each roof plane?
When every plane has the same pitch, the sloped area of the whole roof is its footprint times one slope factor, whether it is a gable or a hip roof. The footprint is easy to measure from the ground or from a plan: the length and width of the building plus the overhangs.
What is the lowest pitch for asphalt shingles?
The International Residential Code (IRC R905.2.2) allows asphalt shingles on slopes of 2/12 or more, with extra underlayment from 2/12 up to 4/12. The calculator works at any pitch from 0 to 24, but below 2/12 a roof needs a low-slope material instead of shingles.