How to use the roofing calculator
- Choose Building footprint (enter length, width, overhang and pitch) or Roof planes (add up planes you've measured directly on the roof, along the slope).
- In footprint mode, pick a roof style (gable or hip) -- this only changes the estimated ridge length, not the total roof area.
- Enter your pitch as x/12, degrees or percent -- whichever your plans give you.
- Pick a roof complexity to set the waste allowance, and optionally a price per square for a cost estimate.
- Read the roof area, roofing squares, shingle bundles, underlayment rolls, drip edge and ridge cap below.
No signup, no email and no address lookup -- everything runs in your browser.
How much roofing material do I need? The formula
Roof area = footprint area (including overhang) × pitch multiplier. Squares = roof area ÷ 100. Bundles = squares (with waste) × 3.
Worked example: a 40 x 30 ft building with a 12 in overhang on every side and a 6/12 pitch. Extended footprint: 42 × 32 ft = 1,344 sq ft. Pitch multiplier at 6/12: 1.118. Roof area: 1,344 × 1.118 = 1,503 sq ft (15.03 squares). With a 10% waste allowance (simple roof), that's 16.53 squares to buy, or 50 bundles at 3 per square. At $450.00 per square, materials come to roughly $7,437.87.
Roof pitch: x/12, degrees and percent
Pitch is the roof's steepness, written as inches of rise per 12 inches of horizontal run ("x/12"). The same slope can be written three ways -- as a ratio (6/12), an angle in degrees (26.6° for 6/12), or a percent slope (50% for 6/12) -- and the calculator above accepts any of the three.
The pitch multiplier is how much longer the sloped roof surface is than its flat, plan-view footprint: multiplier = √(1 + (rise÷run)²). At 6/12, that's √(1 + 0.5²) = 1.118 -- a 1,000 sq ft footprint becomes a 1,118 sq ft roof.
Roof pitch multiplier table
| Pitch (x/12) | Degrees | Percent slope | Area multiplier |
|---|---|---|---|
| 2/12 | 9.5° | 17% | 1.0138x |
| 3/12 | 14° | 25% | 1.0308x |
| 4/12 | 18.4° | 33% | 1.0541x |
| 5/12 | 22.6° | 42% | 1.0833x |
| 6/12 | 26.6° | 50% | 1.118x |
| 7/12 | 30.3° | 58% | 1.1577x |
| 8/12 | 33.7° | 67% | 1.2019x |
| 9/12 | 36.9° | 75% | 1.25x |
| 10/12 | 39.8° | 83% | 1.3017x |
| 11/12 | 42.5° | 92% | 1.3566x |
| 12/12 | 45° | 100% | 1.4142x |
| 14/12 | 49.4° | 117% | 1.5366x |
| 16/12 | 53.1° | 133% | 1.6667x |
| 18/12 | 56.3° | 150% | 1.8028x |
Every row is computed directly (multiplier = √(1 + (rise÷12)²)) -- this is pure geometry, not a looked-up table.
Rafter length from span and pitch
Rafter length (ridge to wall plate, no overhang) = half the building's span × the pitch multiplier. For a 24 ft span at a 6/12 pitch: run = 24 ÷ 2 = 12 ft, rise = 12 × (6÷12) = 6 ft, rafter length = 12 × 1.118 = 13.42 ft. Add a 12 in overhang along the same slope and the full rafter (including the tail) is 14.53 ft. This assumes a simple symmetric gable and ignores ridge-board thickness and birdsmouth cuts.
Shingle bundles, underlayment, drip edge and ridge cap
- Shingle bundles: 3 bundles per square is standard for 3-tab and architectural/laminate shingles (check the wrapper for heavier designer lines, which can run 4-5 per square).
- Waste allowance: 10% for a simple gable roof, 15% for a moderate roof (hip, or 1-2 valleys), 20% for a complex roof with multiple valleys, dormers or skylights.
- Underlayment: one roll of synthetic underlayment covers about 10 squares nominally -- order rounded up, since seam overlap uses some of that coverage.
- Drip edge: sold in 10 ft sticks, run along the whole eave and rake perimeter.
- Ridge cap: roughly 20 linear ft per bundle for Owens Corning DuraRidge (the most conservative common brand); GAF Seal-A-Ridge covers about 25 ft/bundle and TAMKO about 33 ft/bundle, so check your product's own coverage.
All figures above are widely published estimates, not code requirements -- always confirm exact coverage on the product you buy.
Roof style: gable vs. hip
A gable roof has two sloped sides meeting at a ridge that runs the building's full length, with vertical triangular walls at each end. A hip roof slopes on all four sides, so its ridge is shorter -- for a rectangular building, roughly the difference between the long and short sides (a square hip roof comes to a point, with no ridge at all). Both styles cover the same footprint with the same total roof area at the same pitch; only the ridge length (and therefore ridge cap) estimate changes here. Hip roofs generally need more waste for their extra hip cuts, which the complexity dropdown accounts for.
Other calculators
- Just need the footprint area, not the roof? Try the square footage calculator.
- Pouring concrete for a foundation or footing? Try the concrete calculator.
Frequently asked questions
How do I calculate how many roofing squares I need?
Find the roof's actual (sloped) area, then divide by 100 -- one roofing square always equals 100 sq ft, whatever the material. For a 40 x 30 ft building with a 12 in overhang and a 6/12 pitch, the roof surface works out to 1,503 sq ft, or 15.03 squares. The calculator above does this automatically for a footprint or for planes you measure directly.
How many bundles of shingles do I need?
Standard 3-tab and architectural (laminate) shingles are packaged 3 bundles per square, so multiply your squares (including waste) by 3. The 15.03-square example above, with a 10% waste allowance for a simple roof, needs 16.53 squares to buy, or 50 bundles. Some heavier "designer" laminate shingles use 4-5 bundles per square instead -- check the coverage printed on the wrapper.
What is roof pitch and how is it measured?
Roof pitch is how steep a roof is, expressed as inches of rise for every 12 inches of horizontal run -- written "x/12". A 6/12 pitch rises 6 in for every 12 in of run, which is 26.6 degrees, a 50% slope, and makes the roof surface 1.118x longer than its flat footprint. The reference table below lists this for every common pitch.
How do I convert roof pitch to degrees or percent?
Degrees = arctangent(rise / run); percent = (rise / run) x 100. A 30 degree roof angle works out to a 6.93/12 pitch (a 58% slope). The calculator above accepts pitch as x/12, degrees or percent and converts automatically -- pick whichever your plans give you.
How much extra shingle should I buy for waste?
Add 10% for a simple gable roof with no valleys, 15% for a moderate roof (a hip roof, or one to two valleys), and 20% for a complex roof with multiple valleys, dormers or skylights. On the 15.03-square example, that's the difference between 16.53 squares to buy at 10% waste and 18.03 squares at 20%.
How many rolls of underlayment do I need?
A roll of synthetic roofing underlayment covers about 10 squares (1,000 sq ft) nominally -- always round up, since overlap at the seams uses up some of that coverage. The 15.03-square example above needs 2 rolls once waste is included.
How much drip edge and ridge cap do I need?
Drip edge is sold in 10 ft sticks and runs around the whole eave and rake perimeter; ridge cap shingles run along the ridge line and cover about 20 linear ft per bundle for the most conservative common brand (Owens Corning DuraRidge; GAF's Seal-A-Ridge covers about 25 ft/bundle and TAMKO's about 33 ft/bundle, so check your own brand). For the example above, that perimeter and ridge length works out to roughly 15 drip-edge pieces and 3 ridge-cap bundles -- both are approximate, since real eave/ridge lengths depend on the exact roof shape.
What's the difference between a hip roof and a gable roof for materials?
They cover the same footprint with the same total roof area and the same pitch multiplier -- a hip roof just has four sloped sides meeting at a shorter ridge instead of two sloped sides and two vertical gable ends. For the 40 x 30 ft example, the gable version has a 42 ft ridge, while the equivalent hip roof's ridge is only about 10 ft, so a hip roof needs less ridge cap but (in practice, not modeled here) more hip flashing and cutting waste -- which is why "hip" is a step up in the complexity/waste dropdown.
How do I find rafter length from the span and pitch?
Rafter length (to the ridge) = half the building's span x the pitch multiplier. A 24 ft span with a 6/12 pitch has a 12 ft run and rises 6 ft, giving a rafter length of 13.42 ft; add a 12 in overhang and the rafter (including the tail) is 14.53 ft. This ignores ridge-board thickness and birdsmouth cuts, which a framer accounts for on site.
Do I need to give my address or sign up to use this calculator, and is my data saved anywhere?
No. Enter your roof dimensions and pitch directly -- there is no signup, no email and no address lookup. Your measurements are saved only in your own browser (so they are there next time you open this page on the same device); nothing is sent to our servers.