📐 Roof Pitch Calculator
Enter the roof’s rise and run and get the pitch as x-in-12, the angle in degrees, the slope percentage and the rafter length.
Pitch (rise-in-12)
4:12
Angle
18.4°
Slope
33.3%
Rafter length multiplier
×1.0541
per unit of horizontal run
Rafter length (½ span)
—
enter a span
Pitch = rise ÷ run × 12; angle = arctan(rise ÷ run); rafter = √(rise² + run²) per unit run. Add a span for the actual rafter length. 🔒 In your browser.
How the roof pitch calculator works
Roof pitch is the ratio of vertical rise to horizontal run. Expressed the US way it is the rise per 12 units of run (so a 4/12 pitch rises 4 for every 12 across). The angle in degrees is arctan(rise ÷ run), the slope percentage is rise ÷ run × 100, and the rafter length per unit of run is √(rise² + run²) ÷ run. Enter a total span and the tool returns the actual rafter length from ridge to eave for one side (half the span × the multiplier).
Common pitches: 4/12 ≈ 18.4°, 6/12 ≈ 26.6°, 9/12 ≈ 36.9°, 12/12 = 45°. Low-slope roofs (below about 3/12) usually need membrane rather than shingles, and pitches above 9/12 are “steep-slope” and cost more to work on. Always confirm structural details with a builder or the shingle manufacturer’s spec.
Frequently asked questions
How do I calculate roof pitch?
Divide the vertical rise by the horizontal run, then multiply by 12 to express it as x-in-12. A roof that rises 4 units over 12 units of run is a 4/12 pitch; its angle is arctan(4 ÷ 12) ≈ 18.4°.
What does 4/12 pitch mean in degrees?
A 4/12 pitch rises 4 inches for every 12 inches of horizontal run, which is an angle of about 18.4°. In general, angle = arctan(rise ÷ run).
What is the difference between pitch, angle and slope?
Pitch is the rise-in-12 ratio (e.g. 6/12), angle is that expressed in degrees (26.6°), and slope percentage is rise ÷ run × 100 (50%). They all describe the same steepness in different units.
How do I find the rafter length?
The rafter length per unit of run is √(rise² + run²) ÷ run. Multiply that by the horizontal run (half the span for a symmetric ridge) to get the ridge-to-eave rafter length. Add for overhang and ridge board separately.
What is considered a steep roof?
Roofs steeper than about 9/12 (36.9°) are generally called steep-slope and are harder and costlier to work on. Below roughly 3/12 (14°) a roof is low-slope and usually needs a membrane rather than standard shingles.