Roofing Calculator: Squares, Shingle Bundles and Cost
Estimate a roof from the building size, overhang and pitch. Get roof area, squares with waste, shingle bundles, underlayment rolls, nails and cost, in feet or metres.
Added to every side of the footprint.
Enter 0 for a flat roof.
One square covers 100 ft².
Enter the building length and width, and the roof pitch.
About the roofing calculator
The Roofing Calculator turns the footprint of a building into a roof area and a materials list. Enter the length and width of the building, the roof pitch, the overhang and how complicated the roof is. The calculator returns the sloped roof area in square feet and square metres, the number of roofing squares including waste, the bundles of shingles, the rolls of underlayment, the roofing nails and, if you add a price per square, the cost. Length and width each have their own unit, so you can type feet for one and inches for the other, or work entirely in metres.
A roof is always bigger than the building underneath it, because it slopes and because it usually hangs over the walls. This calculator handles both effects, so you do not have to convert pitch to a multiplier by hand.
How to estimate a roof
Steps
- Enter the building length and width at the walls and pick a unit for each: feet, inches, yards, metres, centimetres or millimetres.
- Enter the pitch. Choose rise per 12, such as 6 for a 6:12 roof, or choose degrees if your plans show an angle. A flat roof has a pitch of 0.
- Enter the overhang per side. It defaults to 0 and can use any length unit. The same overhang is added to all four sides.
- Choose the roof complexity. A simple roof adds 10 percent waste. A complex roof adds 15 percent.
- Optionally enter a price per roofing square to get a cost.
- Read the area, the squares, bundles, underlayment rolls, nails and cost.
Formulas
Area and squares
The overhang makes the footprint larger on every side:
The slope factor converts a flat footprint into a sloped surface. With a rise of inches per 12 inches of run:
If the pitch is given as an angle , the same factor is . The two forms agree because rise per 12 equals . Roof area and squares then follow:
One roofing square is 100 square feet, and is the waste allowance, 0.10 or 0.15.
Materials
All material counts use the area including waste, rounded up to whole units:
- Bundles = squares x 3, because three bundles of shingles cover one square.
- Underlayment rolls = squares / 4, because one roll covers about four squares.
- Nails = squares x 320, about 320 nails per square.
- Cost = squares x price per square.
Common pitches and slope factors
| Pitch | Angle | Slope factor | | --- | --- | --- | | 4:12 | 18.4 degrees | 1.054 | | 6:12 | 26.6 degrees | 1.118 | | 8:12 | 33.7 degrees | 1.202 | | 12:12 | 45.0 degrees | 1.414 |
A flat roof has a slope factor of exactly 1, so its area equals its footprint.
Worked examples
Simple roof. A building 40 ft by 30 ft with a 6:12 pitch and a 1 ft overhang has an effective footprint of 42 ft x 32 ft, which is 1,344 square feet. The slope factor is 1.1180, so the roof area is 1,502.6 square feet, or 139.6 square metres. With 10 percent waste that is 16.53 squares. You need 50 bundles, 5 rolls of underlayment and 5,290 nails. At 450 per square the cost is 7,438.06.
Pitch in degrees. A 30 degree roof has a slope factor of 1.1547, which is the same as a rise of 6.93 per 12. A 36 ft by 24 ft building with a 1.5 ft overhang has a footprint of 1,053 square feet and a roof area of 1,215.9 square feet. With 10 percent waste that is 13.37 squares, 41 bundles, 4 rolls and 4,280 nails.
Complex roof. A 50 ft by 28 ft building with a 1 ft overhang and an 8:12 pitch has a footprint of 1,560 square feet and a roof area of 1,874.9 square feet. With 15 percent waste that is 21.56 squares, 65 bundles, 6 rolls and 6,900 nails. At 400 per square the cost is 8,624.48.
What the calculator assumes
The pitch is applied to the whole footprint, as on a simple gable or hip roof. Valleys, dormers, chimneys, skylights and separate roof sections are not modelled. If your roof has any of these, choose the complex setting, which adds the extra 5 percent for cuts and offcuts. Ridge cap, starter strips, flashing, drip edge and vents are separate products and are not included. Check the coverage printed on your product, since some heavy shingles come four bundles to the square.
Converting units
To compare the area with other figures, the Area Converter switches between square feet, square metres and other units. If you need to adjust a waste allowance by a percentage of your own, the Percentage Calculator is quick. When a supplier quotes shingles by the bundle and another by the square, divide the square price by three to compare them.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device.
Related tools
If you mainly want the shingle count, the Roof Shingle Calculator explains the bundle arithmetic in detail. For the walls below the roof, the Paint Calculator estimates coverage, and the Angle Converter changes pitch angles between degrees and other units.
Frequently asked questions
What is a roofing square?
A roofing square is 100 square feet of roof surface, about 9.29 square metres. Roofing materials and labor are commonly priced per square, which is why the calculator reports squares as well as square feet.
How do I enter the roof pitch?
Choose rise per 12 if your plans say something like 6:12, and type the rise, which is 6. Choose degrees if you have an angle. Both are converted to the same slope factor, so a 30 degree roof and a 6.93:12 roof give identical results. Enter 0 for a flat roof.
Why is the roof area larger than the floor area of the building?
Two things add area: the overhang, which extends the roof past the walls on every side, and the slope, which makes the surface longer than its flat footprint. A 6:12 roof is 11.8 percent larger than its footprint, and a 12:12 roof is 41.4 percent larger.
What is the difference between a simple and a complex roof?
A simple roof, such as a plain gable or hip, adds 10 percent waste. A complex roof adds 15 percent, which covers extra cuts around valleys, dormers, chimneys and several roof sections. The calculator does not model those features, so use the complex setting when your roof has them.
How many underlayment rolls and nails does the calculator allow?
It assumes one roll of underlayment covers 4 squares and about 320 nails go into each square. Both use the area including waste and round up to a whole number. Check the coverage on your product, because rolls vary by brand and type.
Does the result include ridge cap, flashing and vents?
No. The bundle count covers the main roof surface. Ridge cap, starter strips, drip edge, flashing and vents are separate products, so add them to your order based on the length of ridge, hip and eaves.
Is my data sent anywhere?
No. The roof area and materials sums run in your browser, so your building dimensions, pitch and prices are never uploaded or stored on a server. They stay on this device and are not shared with anyone.