Roofing Calculator
Find the roof surface area, roofing squares and shingle bundles for a gable or hip roof from the building size, overhang and pitch.
Cost (optional)
A 6/12 pitch adds 11.8% to the flat footprint. Asphalt shingles are allowed at this pitch without the low slope underlayment rule.
Gable roof, 40 × 30 ft building with 12 in overhang.
Shingle bundles to order
Covers 16.67 squares, enough for 16.53 squares including 10% waste. Bundles are rounded up to whole packs.
Your calculation
The same steps the calculator runs, with your numbers.
- Convert the overhang: 12 in ÷ 12 = 1 ft.
- Add the overhang to both sides: 40 + 2 × 1 = 42 ft and 30 + 2 × 1 = 32 ft.
- Footprint: 42 × 32 = 1,344 ft².
- Pitch factor: √(1 + (6 ÷ 12)²) = 1.1180.
- Roof area: 1,344 × 1.1180 = 1,502.6 ft².
- Squares: 1,502.6 ÷ 100 = 15.03.
- Add 10% waste: 15.03 × 1.1 = 16.53 squares.
- Bundles: 16.53 × 3 = 49.59, rounded up to 50 bundles.
The same roof at other pitches
Same building, overhang, waste allowance and bundle size, with only the pitch changed.
| Pitch | Roof area (ft²) | Squares | Bundles |
|---|---|---|---|
| 3/12 (14°) | 1,385 | 13.85 | 46 |
| 4/12 (18.4°) | 1,417 | 14.17 | 47 |
| 8/12 (33.7°) | 1,615 | 16.15 | 54 |
| 10/12 (39.8°) | 1,749 | 17.49 | 58 |
| 12/12 (45°) | 1,901 | 19.01 | 63 |
An estimate for a simple gable or hip roof with one pitch. Valleys, dormers and extra roof planes add area and waste, so confirm measurements on site before ordering.
How the Roofing Calculator Works
This roofing calculator turns a house's plan dimensions into the sloped area that shingles actually cover. It adds the overhang to every side, multiplies length by width to get the flat footprint, then multiplies by a pitch factor that grows as the roof gets steeper. The result is divided into roofing squares and shingle bundles, with a waste allowance added before rounding.
Measuring the footprint from the ground is safer than climbing onto the roof, and it gives the same answer for any simple roof where every plane has the same pitch. A roof's slope is written as rise over a run of 12 inches, so a 6/12 roof rises 6 inches for every foot it runs horizontally.
Roof area formula
Squares and bundles
The pitch factor is the length of a rafter divided by its horizontal run. A rafter that runs 12 inches and rises 6 inches is √(12² + 6²) = 13.42 inches long, and 13.42 ÷ 12 = 1.118. Every square foot of footprint under a 6/12 roof therefore carries 1.118 square feet of roofing.
A gable roof and a hip roof over the same walls have the same surface area when all planes share one pitch, because each plane's area is its flat projection times the same factor. The two shapes differ in their edges: a hip roof has a shorter ridge plus four hips, and no rakes.
Roof Pitch Chart: Angle, Pitch Factor and Slope
Use this chart to convert a roof pitch into degrees, percent slope and the multiplier that turns footprint into roof area. The last column applies the asphalt shingle slope rule in the International Residential Code.
| Pitch | Angle | Pitch factor | Percent slope | Asphalt shingles |
|---|---|---|---|---|
| 1/12 | 4.8° | 1.003 | 8.3% | Not permitted |
| 2/12 | 9.5° | 1.014 | 16.7% | Double underlayment |
| 3/12 | 14.0° | 1.031 | 25.0% | Double underlayment |
| 4/12 | 18.4° | 1.054 | 33.3% | Permitted |
| 5/12 | 22.6° | 1.083 | 41.7% | Permitted |
| 6/12 | 26.6° | 1.118 | 50.0% | Permitted |
| 7/12 | 30.3° | 1.158 | 58.3% | Permitted |
| 8/12 | 33.7° | 1.202 | 66.7% | Permitted |
| 9/12 | 36.9° | 1.250 | 75.0% | Permitted |
| 10/12 | 39.8° | 1.302 | 83.3% | Permitted |
| 11/12 | 42.5° | 1.357 | 91.7% | Permitted |
| 12/12 | 45.0° | 1.414 | 100.0% | Permitted |
| 14/12 | 49.4° | 1.537 | 116.7% | Permitted |
| 16/12 | 53.1° | 1.667 | 133.3% | Permitted |
| 18/12 | 56.3° | 1.803 | 150.0% | Permitted |
The angle is arctan(rise ÷ 12) and the percent slope is rise ÷ 12 × 100. A 12/12 roof is a 45 degree roof with a 100 percent slope, and it carries 41.4 percent more roofing than its footprint.
Roofing Squares by Roof Footprint and Pitch
Squares of roof surface before waste for common footprints, measured over the walls plus the overhang. The figure after the slash is the bundle count with 10 percent waste for a shingle packed 3 bundles per square.
| Roof footprint | 4/12 | 6/12 | 8/12 | 12/12 |
|---|---|---|---|---|
| 1,000 ft² | 10.54 / 35 | 11.18 / 37 | 12.02 / 40 | 14.14 / 47 |
| 1,500 ft² | 15.81 / 53 | 16.77 / 56 | 18.03 / 60 | 21.21 / 71 |
| 2,000 ft² | 21.08 / 70 | 22.36 / 74 | 24.04 / 80 | 28.28 / 94 |
| 2,500 ft² | 26.35 / 87 | 27.95 / 93 | 30.05 / 100 | 35.36 / 117 |
| 3,000 ft² | 31.62 / 105 | 33.54 / 111 | 36.06 / 119 | 42.43 / 141 |
A roof footprint is not the same as the living area. A single story house with 1,800 square feet of floor space and 12 inch overhangs has a larger roof footprint than its floor plan, while a two story house with the same floor space has roughly half the roof footprint.
Ridge, Hip, Rake and Drip Edge Lengths
Quantities of ridge cap shingles, drip edge and starter strips depend on edge lengths, so the calculator also measures the roof's edges. The eaves are the level bottom edges, the rakes are the sloped edges at a gable end, the ridge is the level top line and the hips are the sloped lines where two hip roof planes meet.
In both formulas the length and width already include the overhang, and the width is the shorter side. On a hip roof the eaves run around all four sides, so the drip edge equals the perimeter: 2 × (42 + 32) = 148 feet in the example. When the building is square, the ridge length is zero and the four hips meet at a point.
Section R905.2.8.5 of the International Residential Code requires a drip edge at the eaves and rake edges of shingle roofs, with adjacent pieces overlapped at least 2 inches. Add a little to the drip edge total for those overlaps.
Shingle Bundles per Square
A roofing square is 100 square feet of roof surface. Manufacturers pack shingles so that a whole number of bundles covers one square. GAF lists its Timberline HDZ architectural shingle at 3 bundles and 64 pieces per square, each shingle 13 1/4 by 39 3/8 inches with a 5 5/8 inch exposure.
The exposed part of each shingle is what counts toward coverage, because each course overlaps the one below. This is why 3 bundles cover slightly less than a full 100 square feet, and why a small waste allowance is still needed on the simplest roof. Some heavier shingles are packed in more bundles per square, so set the bundle count from the wrapper of the product you buy.
Minimum Slope, Underlayment and Waste
The 2021 International Residential Code, Section R905.2.2, permits asphalt shingles only on slopes of 2/12 or steeper and requires a double underlayment from 2/12 up to 4/12. The hero scale in the results marks these limits. Below 2/12 the calculator still reports the roof area, which suits a low slope covering, but it shows no shingle count.
The Owens Corning Roofer's Guide says most jobs need 2 to 10 percent extra shingles for trim waste, depending on the roof's complexity. Valleys, dormers, hips and penetrations such as vents and chimneys all add cut edges. The calculator's 10 percent default sits at the top of that range, which suits hip roofs and roofs with valleys.
How to Measure Your Roof
- Measure the walls. Take the outside length and width of the house at ground level.
- Measure the overhang. Measure from the wall to the edge of the fascia at an eave and at a gable end.
- Find the pitch. Owens Corning describes setting one end of a 12 inch carpenter's level on the roof, holding it level, and measuring from the other end straight down to the roof. A 6 inch gap means a 6/12 pitch.
- Enter the numbers. Pick gable or hip, set the waste allowance and bundle count, and read the results.
For an L shaped house, split the roof into rectangles, run each one, and add the bundle counts.
Common Mistakes
- Using the living area. A two story house has about half the roof of its floor area.
- Forgetting the overhang. 12 inches on each side adds 144 ft² to a 40 × 30 ft footprint.
- Skipping the pitch factor. A 6/12 roof is 11.8% larger than its plan.
- Reading 6/12 as 6 degrees. A 6/12 pitch is 26.6°.
- Rounding bundles down. 49.59 bundles means buying 50.
Metric Roof Area and Pitch in Degrees
In metric mode the calculator works in meters, converts the overhang from centimeters by dividing by 100, and reports the area in square meters. NIST defines one square foot as exactly 0.09290304 square meters, so one roofing square of 100 square feet is 9.290304 square meters. The calculator divides the metric area by 9.290304 to count squares, so the bundle count matches the US result for the same roof.
Set the pitch format to degrees to type an angle directly. Switching the format converts the value already entered, so 6/12 becomes 26.6 degrees and back again.
Frequently Asked Questions
How many bundles of shingles do I need?
Divide the roof surface area in square feet by 100 to get roofing squares, add a waste allowance, then multiply by the bundles per square printed on the wrapper. Many architectural shingles, including GAF Timberline HDZ, are packed 3 bundles per square. A 40 by 30 foot house with 12 inch overhangs and a 6/12 pitch has 1,502.6 square feet of roof, or 15.03 squares. Adding 10 percent gives 16.53 squares, and 16.53 × 3 = 49.59, which rounds up to 50 bundles. Always round up, because shingles are sold in whole bundles.
How do I calculate roof square footage?
Measure the building's length and width, add the overhang on each side, and multiply to get the flat footprint. Then multiply by the pitch factor, √(1 + (rise ÷ 12)²), which accounts for the slope. For a 6/12 pitch the factor is 1.118. A 40 by 30 foot house with 12 inch overhangs has a footprint of 42 × 32 = 1,344 square feet, and 1,344 × 1.118 = 1,502.6 square feet of roof surface. The same method works for gable and hip roofs when every roof plane has the same pitch.
What is a square in roofing?
A roofing square is 100 square feet of roof surface, which is about 9.29 square meters. Shingles, underlayment and labor are usually quoted by the square, and the Owens Corning Roofer's Guide defines one square as 100 square feet. To convert, divide the roof area by 100, so a 1,502.6 square foot roof is 15.03 squares. A square is measured on the sloped surface, not on the floor plan, so a steep roof over the same house has more squares than a low slope roof.
How many squares is a 2,000 square foot house?
It depends on the roof footprint and the pitch, not the floor area alone. A single story house whose roof footprint, including the overhangs, is 2,000 square feet has 21.08 squares at a 4/12 pitch, 22.36 squares at 6/12, 24.04 squares at 8/12 and 28.28 squares at 12/12. At 6/12 with 10 percent waste, that is 24.60 squares, or 74 bundles of a shingle packed 3 bundles per square. A two story house with 2,000 square feet of floor space has a much smaller roof, so measure the roof rather than using the living area.
What is the minimum roof pitch for shingles?
The 2021 International Residential Code, Section R905.2.2, allows asphalt shingles only on roof slopes of 2 units vertical in 12 units horizontal (2/12, about 17 percent) or steeper. For slopes from 2/12 up to 4/12, the code requires a double layer of underlayment. The Owens Corning Roofer's Guide also says never to use shingles on slopes lower than 2 inches per foot. Below 2/12, use a low slope roof covering instead. This calculator still shows the roof area at any pitch, but it does not count shingle bundles below 2/12.
How much extra shingles should I buy for waste?
The Owens Corning Roofer's Guide says most roofing jobs need about 2 to 10 percent extra shingles for trim waste, depending on how complex the roof is. A plain gable roof sits near the low end. Valleys, dormers, hips and many penetrations push the figure toward the high end, because each cut edge wastes part of a shingle. This calculator defaults to 10 percent. For a 40 by 30 foot house with 12 inch overhangs at 6/12, the order drops from 50 bundles to 46 bundles with no allowance.
Sources & References
- International Code Council. 2021 International Residential Code, Chapter 9: Roof Assemblies, Sections R905.2.2 (asphalt shingle slope and double underlayment) and R905.2.8.5 (drip edge). codes.iccsafe.org
- Owens Corning. Roofer's Guide to Roof Installation (1 square = 100 ft², 2 to 10 percent trim waste, measuring slope with a 12 inch level, no shingles below 2 inches per foot). owenscorning.com
- GAF. Timberline HDZ shingles, product specifications (3 bundles and 64 pieces per square, 13 1/4 × 39 3/8 in, 5 5/8 in exposure). gaf.com
- NIST. Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically (square foot to square meter). nist.gov
Results are estimates based on the dimensions entered for a roof with one pitch. They are not a contractor's measurement or a code review. Every number on this page was recomputed with the same formulas the calculator uses. Methodology reviewed: 2026.