Roof pitch & roof area calculator

What a roof pitch calculator does

A roof pitch calculator converts the steepness of a roof between rise per 12 (such as 6/12), degrees and percent slope, then uses that angle to turn plan measurements into true sloping roof area. Enter the pitch once in whichever format you have, add each roof plane's eave length and horizontal run, and it returns the angle, the sloping area, the area with waste and a count of full-length covering strips.

The key idea is the slope factor. A sloping roof is always larger than its footprint, and the difference depends only on the pitch: a 6/12 roof has 1.118 times the area of its plan projection, a 12/12 roof 1.414 times. Measure the roof in plan, from drawings or from the ground, where it is safe and easy, and let the factor do the rest.

Use it to price roofing, order metal panels or sheets, check a pitch given in degrees on a drawing against a framer's x/12 figure, or sanity-check a roofer's square count before you sign a quote.

How to use the roof pitch & roof area calculator

1. Choose Imperial or Metric. Imperial defaults to rise per 12 with lengths in feet; metric defaults to degrees with lengths in metres.
2. Pick a Roof form: Manual planes to enter each plane yourself, or Gable, Hip or Shed / mono-pitch to derive every plane from the building length, width and overhang.
3. Set Pitch format to Degrees, Rise per 12 or Percent, then enter the pitch value. For a 6/12 roof choose Rise per 12 and enter 6.
4. Enter the effective covering width after side laps, the underlay roll coverage, the roof area per fastener from the manufacturer's pattern, a waste allowance and, if you want a cost figure, a price per square foot or square metre.
5. In Manual planes, add one row per rectangular roof plane with its eave length and horizontal run measured in plan, from the eave line to the ridge. Use Add roof plane for more rows; presets ignore the rows.
6. Read the angle, rise per 12, slope percent, sloping area, area with waste, strip count, underlay rolls and fastener count, plus rafter, ridge, hip, eave/fascia and rake lengths for presets. The table lists slope length, area and full-length strips per plane, and a section diagram shows the first plane. Export PDF, CSV or SVG, or copy the results.

Worked example

Imperial: A 20 ft eave and 12 ft horizontal run at a 6:12 pitch form a 268.33 ft² sloping plane.

Metric: A 6 m eave and 4 m horizontal run at 30° form a 27.71 m² sloping plane.

Roof pitch to degrees chart

Roof pitch in the US is written as inches of rise per 12 inches of horizontal run. The chart converts each whole pitch from 1/12 to 12/12 into degrees and percent slope, with the slope factor you multiply plan area by and the length along the slope for every 12 in of run.

The sloping length is the rafter length before overhangs and cuts. Add the overhang, and allow for the ridge board and plumb cuts when you cut an actual rafter.

How to convert roof pitch to degrees and percent

Pitch, angle and percent are three ways of stating the same rise over run. Degrees = atan(rise ÷ run). Percent = rise ÷ run × 100. Rise per 12 = 12 × tan(angle). The slope factor is 1 ÷ cos(angle), which is the same as √(rise² + run²) ÷ run.

So a 7/12 roof is atan(7 ÷ 12) = 30.26°, and a 30° roof is 12 × tan(30°) = 6.93, just under 7/12. Metric drawings and many product data sheets give roof angles in degrees, while US framers and roofers work in x/12. The chart below converts common angles.

How to calculate roof area from a floor plan

Measure each roof plane in plan: the eave length (including rake overhangs) and the horizontal run from the eave line to the ridge (including the eave overhang). Multiply length × run × slope factor. You never need to measure up the slope.

Worked example: a 40 × 28 ft house with a simple gable roof at 8/12 and 1 ft overhangs all round. Each plane has an eave length of 42 ft and a run of 15 ft (half of 28 ft, plus the 1 ft overhang). The slope length is 18.03 ft, so each plane is 757.17 ft² and the roof is 1,514.33 ft², or 15.14 squares (one roofing square is 100 ft² of roof surface). With 10% waste the order area is 1,665.76 ft². With 3 ft cover panels, each 42 ft eave takes 14 full-length strips, 28 in total.

Leaving out the overhangs gives 1,346.07 ft², which understates this roof by 168.26 ft², or 12.5%. Overhangs are the most common reason a desk estimate comes in short.

For a hip roof, or any roof where every plane has the same pitch, there is a shortcut: total sloping area = plan area of the roof (including overhangs) × slope factor, whatever the shape of the individual planes. The 42 × 30 ft plan above gives 1,260 × 1.202 = 1,514.33 ft² whether it is framed as a gable or a hip. In the calculator, enter one row with the overall length as the eave length and the overall depth as the run. The area is correct; ignore the strip count for that row, because hip planes are triangles and trapezoids.

Minimum roof slope for common roof coverings

The pitch you can use depends on the covering. The limits below are from the 2021 International Residential Code (IRC). Confirm them against the edition your jurisdiction has adopted and the manufacturer's installation instructions, which can be stricter.

• Asphalt shingles: 2/12 minimum (2021 IRC R905.2.2). From 2/12 up to 4/12, double underlayment is required.
• Built-up roofing: a design slope of at least 1/4 in per foot (about 2%), or 1/8 in per foot for coal-tar built-up roofs (2021 IRC R905.9.1).
• Metal panels, tile, slate and wood shakes: each has its own minimum slope in IRC Section R905, and products and lap details vary. Check the relevant section and the product instructions.
• Pitch versus slope: strictly, pitch is rise divided by the full span, so a 6/12 slope on a symmetrical gable is a 1/4 pitch and 12/12 is a 1/2 pitch. In everyday use "pitch" means rise per 12, and that is what the calculator uses.

Who it is for and when to use it

Method, formulas & assumptions

Angle = atan(rise/run); slope factor = 1/cos(angle). Plane area = eave length × horizontal run × slope factor (halved for triangular hip ends). Gable, hip and shed presets derive the planes from building length, width and overhang: rafter = run × slope factor, hip rafter = √(run² + run² + rise²). Cover strips = ceil(eave length ÷ effective cover width); underlay rolls = ceil(area with waste ÷ roll coverage); fasteners = ceil(area with waste ÷ area per fastener).

Imperial inputs are converted with exact factors (1 ft = 0.3048 m, 1 lb = 0.45359237 kg, 1 US gal = 3.785411784 L) before the calculation runs in SI, then results are converted back.

Common mistakes to avoid

• Entering the sloping length as the run. The run is the horizontal distance from eave to ridge; entering the slope length applies the slope factor twice.
• Forgetting eave and rake overhangs. In the worked example above they add 12.5% to the roof area.
• Using the full width of a metal panel instead of its effective cover width after side laps, which undercounts strips.
• Mixing up pitch formats. 30 entered as rise per 12 is a 68.2° roof, not a 30° one.
• Expecting the waste allowance to cover hips, valleys and penetrations. Complex roofs need larger allowances and separate ridge, hip and valley materials.
• Counting a roofing square as 100 ft² of footprint. It is 100 ft² of sloping roof surface.

Scope & limits

In Manual planes each table row is one rectangular roof plane; presets are symmetrical and ignore dormers, valleys and penetrations. Enter effective cover width after laps. Trims, flashings and product-specific cutting plans need separate allowances.

Frequently asked questions

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