Roof pitch calculator — degrees, percent and roof area
Work out the roof pitch in degrees and percent — from a measured rise and run, from the angle, or from an x:12 pitch in US units — and get the true roof area to order roofing by from the building’s dimensions.
Height difference from eaves to ridge.
Horizontal distance under the same roof plane.
Roof area
Projection per side, typically 0.3–0.5 m.
The roof area covers the roof plane itself and ignores dormers, valleys and hips. Add 10–15% waste when ordering roofing, more for tiles on complex roofs.
The calculation is an estimate and does not replace engineering design or the supplier’s instructions. Always check against project documents, product data sheets and applicable codes.
Degrees, percent or x:12 — the same slope three ways
The same roof slope can be stated in several conventions. Degrees is the angle from horizontal. Percent is the rise divided by the horizontal run times 100 — 45° equals 100%. US practice states pitch as inches of rise per 12 inches of run, so a 6:12 roof rises 6 inches per foot, which is 50% or about 26.6°. In US units the calculator takes pitch directly as an x:12 input and reports it in the results; in metric it shows the ratio as 1:N instead, the form metal- and membrane-roofing manufacturers tend to use for low-slope limits.
The covering sets the limit for how flat a roof can be. Typical rules of thumb: concrete and clay tiles need at least about 14° (3:12), profiled metal with sealed laps about 5.7° (1:10), standing-seam metal down to roughly 3.6°, and membranes lower still. Near the minimum, the underlayment requirements get stricter — always follow the manufacturer’s installation instructions.
From building footprint to true roof area
To order roofing you need the area of the sloped roof plane, not the footprint. The calculator takes the building’s length and width, adds the overhang on every side, and divides the horizontal area by the cosine of the pitch — at 27° the roof is about 12% larger than its footprint, at 45° over 41% larger. For a gable roof the total splits over two planes; a shed roof is a single plane.
The result covers plain roof planes and ignores dormers, valleys and hips, so add 10–15% waste when ordering — more for tiles on a complex roof. To check an existing roof, hold a level horizontally against the slope and measure the drop at its free end: 250 mm over a 1-meter level is 25% (about 14°), and 6 inches over a 3-foot level is a 2:12 pitch.
Frequently asked questions
How do I convert roof pitch from x:12 to degrees?
How do I convert roof pitch from degrees to percent?
What is the minimum pitch for different roof coverings?
How do I measure the pitch of an existing roof?
How much larger is the true roof area than the building footprint?
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