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Rafter Length

Span and pitch in — the exact board to order out.

Every rafter is the same right triangle: run × √(1 + (pitch/12)²). This calculator does the classic rafter-table math — deducting half the ridge board before the multiplier, stretching the overhang along the slope, and switching to the hip multiplier √(2 + (pitch/12)²) for hips — then rounds up to the lumber yard's 2-foot increments.

Why people use this

Framers use it to order and cut rafters without climbing back up to re-measure: span, pitch, and ridge thickness in, exact cut length and plumb angle out — the same multipliers printed on a framing square. Homeowners pricing an addition use it to sanity-check the lumber list before the quote.

How it works

1. Enter building span (wall to wall) and pitch — the run is half the span.
2. Pick ridge thickness; half of it is deducted from the run, measured horizontally.
3. Read the rafter length, tail length, total to cut, and plumb cut angle.
4. Order the shown stock length — next 2-foot increment up.

Common questions

How long is a common rafter for a 24-ft span at 6/12 pitch?

Run is 12 ft (144"), rise 6 ft. The 6/12 multiplier is √(1 + 0.5²) = 1.1180, so the rafter is 144 × 1.1180 = 161" ≈ 13' 5" before the ridge deduction; with a standard 1½" ridge board it is 13' 4". A 16-ft 2× covers it with the tail.

Why is a hip rafter longer at the same pitch?

The hip runs on the plan diagonal, so its horizontal run is the common run × √2 while the rise is unchanged. Its multiplier is √(2 + (pitch/12)²) — exactly 1.5 at 6/12 — which is why hips also sit at a shallower effective slope and need a different (compound) plumb cut.

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