BuildCalcs Angle, rise and run
If rise and horizontal run are known, pitch angle = arctan(rise ÷ run). For a 1.0 m rise over a 2.0 m horizontal run, rise/run = 0.5 and the angle is approximately 26.6°.
Pitch reference table
| Pitch | Rise ÷ run | Slope length factor |
|---|---|---|
| 10° | 0.176 | 1.015 |
| 15° | 0.268 | 1.035 |
| 20° | 0.364 | 1.064 |
| 25° | 0.466 | 1.103 |
| 30° | 0.577 | 1.155 |
| 35° | 0.700 | 1.221 |
| 40° | 0.839 | 1.305 |
| 45° | 1.000 | 1.414 |
| 50° | 1.192 | 1.556 |
| 55° | 1.428 | 1.743 |
| 60° | 1.732 | 2.000 |
The slope length factor is 1 ÷ cos(angle). Multiply a horizontal run by this factor to obtain the simple geometric slope length before allowing for construction details such as overhangs or connection geometry.
Worked geometry example
At 30°, the factor is about 1.155. A horizontal run of 3.0 m therefore corresponds to a simple slope length of about 3.464 m. This is a geometry result, not a cut length for a specific structural detail.
Pitch versus rafter length
Pitch tells you the angle. Rafter length requires a run as well. A 30° roof over a 2 m run and a 30° roof over a 4 m run share the same pitch but not the same rafter length.
Why BuildCalcs separates geometry from design
Calculators can solve dimensions from the inputs you provide, but they do not determine structural adequacy, member sizes, loading or compliance. Those questions require the appropriate project design and professional or regulatory checks.
Measurement checks
- Be clear whether a dimension is horizontal run or sloping length.
- Do not confuse full building span with half-span/run on a symmetrical roof.
- Keep units consistent.
- Use the calculator for geometry, not structural design.
Enter your actual measurements rather than relying on a generic example.
Open the Roof Pitch Calculator →