Rafter Length Calculator
Calculate rafter length from span and pitch. Enter horizontal run (wall to ridge) and pitch for true rafter length, with overhang and hip/valley. Free, no signup.
Rafter / Hip & Valley
Calculate rafter and hip/valley lengths from pitch and span
Plan-view distance from wall to ridge (one rafter)
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Rafter length - did you know?
The rafter length formula
Rafter = run ÷ cos(pitch°). Run is wall-to-ridge horizontal distance. At 35° with 4m run: 4 ÷ cos(35) = 4.88m.
Span vs run - critical difference
Span is total building width (wall to wall). Run is half the span. Rafter length uses run. 8m wide building = 4m run.
Adding overhang
Total = (run + overhang) ÷ cos(pitch°). 4m run + 0.3m overhang at 35° = 5.25m. Account for bird's mouth when ordering.
Hip and valley lengths
Hip = hip_run × sqrt((1/cos(pitch°))² + 1). At 35° with 4m hip run: 6.31m. Significantly longer than common rafters.
Formulas used
Rafter length
rafter = run ÷ cos(pitch°)
With overhang
total = (run + overhang) ÷ cos(pitch°)
Hip/valley
hip = hip_run × sqrt((1/cos(pitch°))² + 1)
Run from span
run = span ÷ 2 (gable)
Frequently asked questions
How do I calculate rafter length?
Divide run by cos(pitch°). 4m run at 35° = 4.88m. Add overhang ÷ cos for total length.
Difference between span and run?
Span = total building width. Run = half span = wall-to-ridge distance. Calculations use run.
How do I calculate hip rafter length?
Hip = hip_run × sqrt((1/cos(pitch°))² + 1). At 35°, 4m hip run = 6.31m.
Best free rafter length calculator?
One that takes span + pitch, includes overhang and hip/valley. This tool does all, free.