Construction Calculator - Roofing

Rafter Length Calculator

Estimate rafter length from horizontal run, rise, and overhang.

Reviewed: August 26, 2026

Calculate Rafter Length

For the Rafter Length Calculator, enter horizontal run, roof rise, and overhang. It calculates sloped line length from horizontal run, rise, and horizontal overhang; cuts, bearing, member sizing, and connections remain outside the model.

Construction measuring tools arranged beside a residential building project

Formula used

Rafter length = (horizontal run + horizontal overhang) × √(run² + rise²) ÷ run

Rafter Length Calculator measurement plan showing Horizontal run feet, Roof rise feet, Overhang feet

Worked rafter length example

Rafter Length Calculator example: The basic slope factor is √(14.64² + 5.3²) ÷ 14.64 = 1.0635. Applying it to 15.64 ft of horizontal run gives 16.63 ft (199.6 in).

Read squares and sloped area

Use the diagonal as a layout check and select stock long enough for required cuts; verify against a full-size or framing-square layout.

Geometry and accessories outside the total

Ridge-board adjustment, birdsmouth location, tail treatment, plumb cuts, fascia, warped stock, and compound roof geometry alter the cut length.

Questions specific to this calculation

Does the reported rafter length include the overhang?

Enter horizontal run from bearing line to ridge reference, vertical rise over that run, and horizontal overhang. The calculator extends the same slope through the overhang.