Roof Pitch Calculator: Angle, Slope % and Rafter Length
Enter the rise and run of the roof, or the angle and run. You get the pitch, angle, slope %, rafter length and how much bigger the sloped area is than the plan area.
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Roof angle
18.4°
- Pitch
- 4 : 12
- Slope
- 33.3%
- Rafter length
- 3.162 m
- Area factor
- 1.0541
Sloped roof area = plan area × area factor (rafter ÷ run). Overhang not included.
Show working
- θ = atan(1 ÷ 3) = 18.43°
- 12 × 1 ÷ 3 = 4 : 12
- √(1² + 3²) = 3.162 m
How to use
- Choose whether you know rise and run, or the angle.
- Enter rise (height) and run (horizontal distance, usually half the span).
- Read the angle, pitch, slope and rafter length.
Good to know
Angle = arctan(rise ÷ run). Pitch = 12 × rise ÷ run, written as “x : 12” (the inches of rise per foot of run). Slope % = rise ÷ run × 100. Rafter length = √(rise² + run²), not counting overhang.
The roof area factor (rafter ÷ run) converts plan area to sloped area: a 4:12 roof has 1.054 times more sheet or tile area than its floor plan. Add overhangs and laps on top.
Typical minimum slopes: metal sheets about 5–10° depending on profile, clay or concrete tiles about 20–30°, and flat RCC roofs need a small fall of about 1:100 for drainage. Follow the sheet or tile maker’s minimum pitch.
Questions
What does 4:12 pitch mean?
The roof rises 4 units for every 12 units across, about 18.4° or 33% slope.
How do I find the run?
For a gable roof, run is half the building span, measured to the centre of the ridge.
How do I convert degrees to pitch?
Pitch = 12 × tan(angle). 30° gives 6.93 : 12.
What slope do metal sheets need?
Usually at least about 5–10°, depending on the profile and the maker’s instructions.
Does rafter length include the overhang?
No. Add the overhang along the slope (overhang ÷ cos angle).