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Rafter Length Calculator

Construction

Calculate the exact rafter length you need, including overhang, from your roof's horizontal run and pitch. Get instant results for framing any roof.

260
036

Total Rafter Length

14.53
Rise
6

This calculator computes your Total Rafter Length, Rise from the values you enter.

Inputs
Horizontal RunRoof Pitch (rise per 12 in run)Overhang
Outputs
Total Rafter LengthRise

What is a Rafter Length?

A Rafter Length Calculator determines the total length of lumber needed to cut a common roof rafter, including the sloped distance from ridge to wall plate plus any eave overhang, based on your roof's horizontal run and pitch. This calculation is fundamental to stick-frame roof construction, where each rafter must be cut to the correct length and angle to properly support the roof surface.

Getting rafter length right avoids wasted lumber from over-cutting or, worse, discovering a rafter is too short after cutting. Pair this calculator with the Roof Pitch Calculator if you need to derive your pitch from existing measurements, or the Birdsmouth Cut Calculator for the notch where the rafter meets the wall plate.

How to use this Rafter Length calculator

  1. Enter your Horizontal Run in feet — typically half your building's width for a simple gable roof.
  2. Select your Roof Pitch (rise per 12 in run) from the dropdown.
  3. Enter your desired Overhang in inches — the eave extension beyond the wall line.
  4. Review the Total Rafter Length result for your minimum stock length to purchase and cut.
  5. Review the Rise result to cross-check your roof's overall height calculation.

Formula & Methodology

The calculator applies the Pythagorean theorem to find core rafter length, then adds overhang:

Rise = Run × (Pitch Rise ÷ 12)

Core Rafter Length = √(Run² + Rise²)

Hypotenuse Factor = √(1 + (Pitch Rise ÷ 12)²)

Overhang Length = (Overhang ÷ 12) × Hypotenuse Factor

Total Rafter Length = Core Rafter Length + Overhang Length

Worked example: For a 12 ft run with a 6/12 pitch and 12 in overhang:

Rise = 12 × (6 ÷ 12) = 6 ft

Core Rafter Length = √(12² + 6²) = √(144 + 36) = √180 ≈ 13.42 ft

Hypotenuse Factor = √(1 + 0.5²) = √1.25 ≈ 1.118

Overhang Length = (12 ÷ 12) × 1.118 ≈ 1.118 ft

Total Rafter Length = 13.42 + 1.118 ≈ 14.54 ft

Frequently Asked Questions

Rafter length is calculated using the Pythagorean theorem, treating the rafter as the hypotenuse of a right triangle formed by the horizontal run and the vertical rise (run multiplied by the pitch factor). This calculator adds the overhang length automatically for the total cut length.
Run is the horizontal distance the rafter spans (typically half the building width for a simple gable roof), while rafter length is the actual sloped distance along the rafter itself, which is always longer than the run since it accounts for the rise created by the roof's pitch.
The overhang is the portion of the rafter that extends beyond the wall line to form the eave, protecting the wall from rain runoff — this extra length needs to be added to the core rafter length (from ridge to wall) for the total board length you'll need to cut.
A steeper pitch increases the rise for the same horizontal run, which increases the total rafter length needed via the Pythagorean relationship — a 12/12 pitch rafter is noticeably longer than a 4/12 pitch rafter covering the same horizontal run.
A common rafter runs perpendicular from the ridge board to the wall plate at a consistent pitch, which is what this calculator computes — hip and valley rafters run at a 45-degree angle in plan view and require a different (longer) length calculation not covered by this tool.
For a simple gable roof, the run is typically half of the building's width (measured from the outside of the exterior walls to the centerline where the ridge sits), though you should account for ridge board thickness for very precise measurements.
Many carpenters buy rafter stock slightly longer than the calculated minimum to allow for the ridge cut angle and any layout adjustments — check your specific rafter tail detail and ridge connection method before cutting to final length.
This calculator computes individual rafter length for stick-framed roofs; the [Roof Pitch Calculator](/roof-pitch-calculator/) helps you determine or verify pitch from rise and run measurements, and the [Roof Truss Calculator](/roof-truss-calculator/) estimates how many prefabricated trusses you'd need if using engineered trusses instead of stick framing.
Also known as
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