This rafter length calculator turns a building’s run and a roof pitch into the board length you actually cut: the hypotenuse of the run–rise triangle, extended for the eave overhang and adjusted for the ridge board. Pitch in, cut length out — with the rise and roof angle shown for the layout work that follows.

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The Triangle Every Roof Sits On

A common rafter is a hypotenuse with a job. The RUN is the horizontal distance from the outside of the wall plate to the ridge centerline (half the building span for a centered ridge); the RISE is run × pitch/12; the rafter’s theoretical “line length” is √(run² + rise²). Defaults — 12 ft run at 6/12: rise 6.00 ft, line length 13.42 ft, roof angle 26.6°. Real boards need two corrections, both in the calculator: the overhang travels down the same slope (12 in of horizontal overhang adds 6 in of rise at 6/12 and stretches the board to 14.53 ft), and each rafter stops at the ridge BOARD’s face, not the centerline — half the ridge thickness measured along the slope (0.84 in here) comes off, landing the cut length at 14.46 ft (14 ft 5.6 in). Buy 16 ft stock; the birdsmouth seat cut and plumb cut consume layout room, not length, but offcuts and a mis-cut allowance are why carpenters round UP a size, never down.

The same triangle drives the whole layout: the unit length (rafter length per foot of run — 1.1180 ft at 6/12, a number worth memorizing alongside 1.4142 for 12/12) scales any run instantly; the angle feeds the speed square for plumb and seat cuts; and hip rafters reuse the method on the diagonal plan run (run × √2) at the same pitch. Gable stud lengths, lookouts and collar ties all inherit the geometry — which is why one verified triangle at the start beats a tape measure on a ladder at the end.

Worked Examples (Verified)

  • Defaults (12 ft run, 6/12, 12-in overhang): cut length 14.46 ft; angle 26.6°.
  • Steep cottage: run 10 ft, 12/12 pitch, no overhang: rise 10, length 14.14 ft, angle 45.0° — the √2 case.
  • Low-slope shed: run 8 ft, 2/12, 12-in overhang: rise 1.33 ft; full length √(9² + 1.5²) = 9.12 ft — shallow pitches still gain real length from overhangs.

Rafter Discipline

  • Measure the run to the ridge CENTERLINE, then deduct the half-ridge as shown — skipping the deduction leaves every pair of rafters 1.5 in too long at the peak.
  • Overhang is specified horizontally (plan dimension) on most drawings; the board grows along the slope — the calculator’s rise-growth line is that conversion made visible.
  • The seat cut (birdsmouth) must leave at least ⅔ of the rafter depth uncut over the plate in many jurisdictions — length is computed here; notch limits are code, checked separately.
  • Crown every board UP and cut the pattern rafter first, test-fit it, then batch-cut — the calculator gives the number, the pattern rafter proves it against the real building.
  • Spans have structural limits independent of length: a 14-ft rafter that CUTS fine may still fail span tables for its species/grade/spacing and snow load — length ≠ permission.

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Frequently Asked Questions

How do you calculate rafter length?

Rise = run × pitch/12; length = √(run² + rise²), then extend along the slope for the overhang and deduct half the ridge thickness measured along the slope. Defaults: run 12 ft at 6/12 → rise 6 ft, line 13.42 ft; with a 12-in overhang and 1.5-in ridge: cut length 14.46 ft. This calculator chains all three steps and reports feet/inches.

What is the run of a rafter?

The horizontal distance from the outer face of the supporting wall (the plate line) to the ridge centerline — for a symmetric gable, half the building’s span. It is NOT the rafter’s length and not the full span; confusing span for run doubles the rise error. All pitch math (rise per 12) keys off the run.

How much length does an overhang add?

Overhang (horizontal) ÷ cos(angle) — at 6/12 (26.6°) each horizontal foot adds 1.118 ft of board; at 12/12, 1.414 ft. The rise grows too (overhang × pitch/12), which the calculator shows because fascia height depends on it. Deep overhangs on steep roofs are the silent stock-length killers: 24 in at 9/12 adds 2.5 ft of board.

Why deduct for the ridge board?

The theoretical line length runs to the ridge CENTERLINE, but lumber stops at the board’s face. Two opposing rafters each give up half the ridge thickness — measured along the slope: (thickness/2)/cos(angle). For a 1.5-in ridge at 6/12 that’s 0.84 in per rafter. Skip it and the ridge rides high or the tails fall short; the deduction is small and utterly unforgiving.

What about hip and valley rafters?

Same formula, diagonal run: a hip rafter’s plan run is the common run × √2 (it travels the 45° plan diagonal), at the same pitch — so its unit length uses √(2 + slope²)… precisely, √((run√2)² + rise²) with the rise from the common run. Hip LENGTH for the defaults ≈ √(16.97² + 6²) = 18.0 ft before overhang/ridge adjustments. Jack rafters then step down the hip at the layout spacing.

What size lumber do I buy for the cut length?

The next stock length above the cut length, with the pattern-rafter workflow in mind: 14.46 ft → 16 ft boards. Steep pitches with deep overhangs push common shed roofs from 12 ft stock to 14s and 16s — running this calculator BEFORE the lumber order is the entire point; a yard’s price steps make short boards bought twice the most expensive boards on the job.

Does rafter length account for the birdsmouth?

The birdsmouth (seat + heel cuts where the rafter sits on the plate) is laid out FROM the line length — the plumb line of the birdsmouth sits exactly at the run point, so the theoretical length already contains it. What the notch affects is strength (depth remaining above the seat), not the cut length. Layout: measure along the top edge from the ridge plumb cut to the birdsmouth plumb line = line length minus ridge deduction.

Is this rafter length calculator free?

Yes — free, instant, no sign-up. Enter run, pitch, overhang and ridge thickness for the cut length in feet and feet/inches, plus rise and roof angle.