LumberCalcs: The Straight-Grain Figures for the Hand-Hewn Builder
Meet the Calcs Family →

Rafter Calculator

Find rafter length and roof angle from run and roof pitch.

Your numbers
Your project

Rafter length including overhang

Enter your numbers above.

The line length isn't the cut length

This calculator gives the theoretical rafter line length. Before cutting, subtract half the ridge board thickness at the top plumb cut and account for the birdsmouth depth at the seat cut, or your rafters will run long and throw off your ridge height.

How rafter length and roof angle are calculated

A common rafter forms the hypotenuse of a right triangle whose two legs are the horizontal run (half the building width for a gable roof, plus any overhang) and the total rise (how much the roof climbs over that run). Once run and rise are known, the rafter length follows directly from the Pythagorean theorem: Length = √(run² + rise²).

Roof pitch is expressed as rise per 12 inches of run (a "6/12" pitch rises 6 inches for every 12 inches of horizontal run). Total rise for a given run is therefore rise = run × (pitch ÷ 12), and the roof angle in degrees is angle = arctan(pitch ÷ 12).

This calculator adds the overhang as its own small right triangle (using the same pitch) and adds its length to the main span length to give a full ridge-to-tail rafter length. Actual cut length in the field will be shortened slightly at the ridge for the ridge board thickness and adjusted for the birdsmouth notch at the wall plate — this figure gives the working line length to start from.

Where these numbers come from

The formula and default values in this calculator are based on:

  • Right-triangle roof framing method (run, rise, and pitch expressed as rise-per-12) is the standard approach used in carpentry framing squares and framing reference tables such as those published by the American Wood Council and traditional rafter-square manuals.

These are planning estimates. Check your local building code, product specifications, and a licensed contractor before you buy materials or build.

Frequently asked questions

What is "run" in rafter framing?

Run is the horizontal distance the rafter spans, measured from the outside of the wall plate to the center of the ridge for a common rafter, not including the overhang. For a simple gable roof it's typically half the building width.

Does this account for the ridge board thickness?

Not automatically. This calculator gives the theoretical line length from the run and pitch; in the field, carpenters shorten the rafter by half the ridge board's thickness at the top cut so two opposing rafters meet correctly at the ridge.

How do I convert the roof angle to a pitch, or vice versa?

Pitch (rise per 12) and angle are related by angle = arctan(pitch/12). This calculator gives you the angle once you enter the pitch; to go the other way, pitch = 12 × tan(angle).

More Framing & Structural calculators

Board Foot Calculator Joist Span Calculator Plywood Calculator

All LumberCalcs tools