Calculated result
Adjust the editable example, then select Calculate.
Method and assumptions
In pitch mode, rise = run × pitch ÷ 12. In rise mode, pitch = 12 × rise ÷ run. Basic slope length = √(run² + rise²), and angle = arctan(rise ÷ run). Horizontal overhang is converted separately to slope length using the same rise/run ratio.
Worked example
A 12 ft horizontal run at 6:12 pitch rises 6 ft. Its basic slope length is √180 ≈ 13.416 ft; the angle is about 26.565°.
Common mistake
A horizontal overhang is not already a slope length. Do not use the geometric length as a construction-ready cut length.
Planning limits
This tool does not assess spans, lumber size, loads, uplift, connections, birdsmouth cuts, bearing, or code compliance. Structural rafter design requires separate verification.
Privacy
Measurements and results stay in this browser tab. The calculator does not send input values, store them, or place them in the URL. No account or upload is required.
Quick question
Is the calculated length a cut length?
No. It is a theoretical geometric slope length. Real cuts, bearings, overhang details, and structural design are outside this calculator.