Rafter Calculator
Estimate the theoretical straight-line length and slope angle of a common rafter from rise and horizontal run, or from an X/12 roof slope. An optional horizontal eave overhang can be included in the same roof plane.
Enter your rafter geometry
How the Rafter Calculator Works
The calculator models one straight common-rafter line as right-triangle geometry.
Rise & run mode
The entered rise and horizontal run establish the slope ratio. The theoretical line to the ridge is the triangle’s hypotenuse.
Pitch mode
Enter X in an X/12 roof slope. The calculator converts it to a slope ratio of X ÷ 12 and applies that ratio to the horizontal run.
Horizontal overhang
The optional overhang is treated as additional horizontal projection. Its corresponding rise is generated from the same slope ratio before the total slope length is calculated.
Angle
The angle from horizontal is arctan(rise ÷ run). This is a geometric slope angle, not a complete saw-cut or framing layout instruction.
Rafter Length Is Different From Structural Rafter Span
The result above is a sloped geometric length. Structural rafter design is a different problem. The American Wood Council’s span resources calculate maximum horizontal spans using factors such as lumber species and grade, spacing, bending and shear properties, deflection, and loading conditions.
AWC also describes rafter span as the horizontal projected span, rather than simply the sloping board length. Do not use the number from this page as proof that a proposed rafter size or spacing is structurally adequate.
Rafter Calculator Examples
These examples use the same formulas as the calculator and keep geometric length separate from structural design.
6/12 slope with 12-in overhang
Measured 3-ft rise over 9-ft run
9/12 slope with 18-in overhang
What This Calculator Does Not Determine
Structural member size or allowable span
Species, grade, member size, spacing, loads, deflection limits, bearing, moisture/service conditions, and applicable code requirements can affect structural design.
Birdsmouth, ridge, fascia, or connection details
The result is not a cut-ready board dimension. Project-specific ridge conditions, bearing/notches, connections, and eave/fascia details require separate layout and verification.
Hip and valley rafter geometry
This page is intentionally limited to a straight common-rafter plane. Hip and valley layouts require additional plan geometry and are outside this calculator’s scope.
Roof-covering suitability
A mathematical slope does not establish whether a roofing product is permitted at that slope. Check the applicable code and the exact manufacturer’s installation requirements.
Roof and Framing Work Requires Separate Safety Planning
Under OSHA 29 CFR 1926.501(b)(13), workers engaged in qualifying residential construction 6 feet or more above lower levels must be protected by conventional fall protection or another measure permitted by the standard; OSHA also provides a specific infeasibility/greater-hazard exception requiring a site-specific plan under 1926.502(k). A calculator result does not replace site-specific safety procedures, training, access equipment, or applicable workplace requirements.
Rafter Calculator FAQ
What is the difference between run and rafter length?
What does 6/12 mean?
How is the overhang handled?
Does the result include a ridge-board deduction?
Can I use this to choose a rafter size?
Continue Your Roof or Framing Calculation
References Used for This Calculator
- American Wood Council — Span Options Calculator for Wood Joists and Rafters: structural rafter span context, species/grade and loading variables.
- American Wood Council — Online Span Calculator Help: horizontal span, loading, bending, shear, deflection and bearing context.
- OSHA — 29 CFR 1926.501: fall-protection requirements applicable to construction work, including residential construction provisions.
- CalcsDone Methodology: how calculator formulas, assumptions, limitations and sources are documented.