Rafter Length Calculator

What a rafter calculator is solving

A common rafter follows the diagonal line from the top plate to the ridge. If you know the roof span and the pitch, the roof geometry becomes a right-triangle problem: half the span gives the horizontal run to the ridge, the pitch gives the rise over that run, and the rafter length follows from the hypotenuse.

That makes a rafter length calculator useful for sheds, garages, porches, and simple gable roofs. It helps you compare whether the roof pitch and overhang you want still fit the stock lengths you plan to buy before you move on to birdsmouth detail, ridge thickness, and connector layout.

Core rafter formulas

The calculator first turns full roof span into half-span run, then uses the pitch ratio to calculate rise. The rafter to the seat cut comes from the basic roof triangle, and the overhang is added as an extension along the same roof slope rather than as a horizontal-only guess.

Using the result when framing a roof

The most useful outputs are usually the seat-cut length, the total rafter length including overhang, and the roof angle. Together they tell you how much stock you need, how steep the roof is, and how much of the length sits outside the plate line at the eaves.

For example, a modest pitch may produce only a small difference between seat-cut length and total rafter length, while a steep roof with a generous overhang can add much more material than the plan view suggests. That is why it helps to check the overhang extension as its own number before you buy the timber.

What this estimate does not cover

This is a common-rafter geometry calculator, not a structural design tool. It does not size rafters, check snow or wind load, allow for ridge-board thickness, or model birdsmouth depth and seat length rules from a specific code or engineered detail. It also assumes a simple symmetrical roof rather than hips, valleys, trusses, or irregular framing conditions.

Use the result as a planning estimate, then confirm the final framing detail from the applicable code tables, engineered design, and your actual roof assembly.