Rafter Length Calculator

Calculate common, hip, and jack rafter lengths from span and pitch. Get total length, plumb cut angle, seat cut, and birdsmouth dimensions for accurate roof framing.

Total width from outside of wall to outside of wall.

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Enter pitch as rise over run (e.g. 6 : 12).

Horizontal projection beyond the outside wall.

Actual ridge board thickness. 0 if rafters meet at a point.

Horizontal length of the seat cut on top of the wall.

Results

Rafter lengths

Common / hip centerline

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Total length including overhang

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Angles & cuts

Plumb cut angle (at ridge)

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Seat cut angle (at birdsmouth)

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Birdsmouth dimensions

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How this rafter length calculator works

This tool uses basic right‑triangle geometry and standard carpentry conventions to calculate rafter lengths and cut angles. You can work in imperial (feet and inches) or metric (meters and millimeters), and choose between common, hip/valley, and jack rafters.

Key definitions

  • Span – total building width from outside of one bearing wall to outside of the opposite wall.
  • Run – horizontal distance from the outside of the wall to the centerline of the ridge. For a symmetrical roof, run = span ÷ 2.
  • Pitch – roof slope expressed as rise over run, typically rise per 12 units of run (e.g. 6:12).
  • Overhang – horizontal projection of the rafter beyond the outside of the wall.
  • Ridge thickness – actual thickness of the ridge board measured horizontally.
  • Birdsmouth – the notch cut where the rafter sits on the wall plate, made of a plumb cut and a seat cut.

Formulas used

1. Pitch factor (common rafter)

Let the pitch be rise : run (usually run = 12).

Pitch factor (per unit of run):

$$F_c = \frac{\sqrt{\text{rise}^2 + \text{run}^2}}{\text{run}}$$

2. Common rafter length (wall to ridge centerline)

Run:

$$\text{run} = \frac{\text{span}}{2} - \frac{\text{ridge thickness}}{2}$$

Rafter length:

$$L_c = \text{run} \times F_c$$

3. Overhang length along rafter

Horizontal overhang = \(h\). Length along rafter:

$$L_o = h \times F_c$$

4. Total rafter length including overhang

$$L_{\text{total}} = L_c + L_o$$

5. Plumb and seat cut angles

Plumb cut angle (from horizontal):

$$\theta_p = \arctan\left(\frac{\text{rise}}{\text{run}}\right)$$

Seat cut angle (from horizontal):

$$\theta_s = 90^\circ - \theta_p$$

Hip and jack rafters

Hip and valley rafters run diagonally across the plan, so their length factor is larger than a common rafter. For a square plan with equal pitches:

  • Hip/valley factor ≈ common factor × √2
  • Jack rafters are shortened along the plate; this calculator uses the jack offset you enter to reduce the length from the hip/valley.

For most residential work, this approximation is within practical tolerance. For complex roofs or unequal pitches, consult structural drawings or a framing square table.

Practical tips for framing rafters

  • Always measure the actual building span on site, not just from plans.
  • Confirm lumber sizes and ridge thickness; nominal sizes (e.g. 2×10) are smaller in reality.
  • Keep the birdsmouth depth within code limits (typically no more than one‑third of the rafter depth).
  • Cut and test one rafter first, then use it as a pattern for the rest.

FAQ

How do you calculate the length of a common rafter?

Divide the span by 2 to get the run, subtract half the ridge thickness, then multiply by the pitch factor. The pitch factor is based on the roof pitch (rise over run). This calculator does all of that automatically when you enter span, pitch, and ridge thickness.

What is the difference between span and run?

Span is the full width of the building between outside faces of the bearing walls. Run is half that distance, from the outside of the wall to the centerline of the ridge. Rafter length is always calculated from the run, not the full span.

Does the rafter length include the overhang?

Structurally, the rafter length is measured from the wall to the ridge. The overhang is extra length beyond the wall. This tool shows both the structural rafter length and the total length including the overhang you enter.

Is this calculator suitable for building code compliance?

The geometry is accurate, but building codes also specify minimum sizes, spacing, and allowable spans based on loads and species. Use this calculator for layout and estimating, and always verify final sizes and details against your local building code and engineering requirements.