Rafter Length Calculator

Calculate common rafter length from run, roof pitch, and overhang using the Pythagorean theorem.

⏱ Updated: 18 Sep 2026

Calculator

Free rafter length calculator: find common rafter length from run, roof pitch, and overhang.

Total Rafter Length
′ ″
Total Length (decimal ft)
Rise
ft
Rafter Length (no overhang)
ft
Overhang Add-On
ft

Run is the horizontal distance from the wall plate to the ridge centerline (typically half the building span). Add extra length for the ridge board thickness and any birdsmouth cut, which this estimate doesn't account for.

Enter the horizontal run and roof pitch to find the total rafter length, including overhang.

How to Use the Rafter Length Calculator

Enter the run — the horizontal distance from the wall plate to the ridge centerline — along with the roof's pitch and any eave overhang. The total length appears both as feet-and-inches and as decimal feet as soon as a number changes.

A rafter's actual length is always longer than the horizontal distance it spans, because it travels at an angle rather than flat. How much longer depends on how steep the roof is — a 4/12 roof adds relatively little length over its run, while a 12/12 roof, a true 45-degree pitch, adds roughly 41% on top of the run alone.

The Formula

rise = run × (pitch ÷ 12)
rafter length = √(run² + rise²)
              = run × √(1 + (pitch ÷ 12)²)

This is the same rise-run-diagonal relationship a framing square is built around, applied algebraically. The overhang gets identical treatment — its horizontal length is multiplied by the same slope factor before being added on, since it travels along the same slope as the rest of the rafter.

What This Doesn't Account For

The figure here is the rafter's centerline length along the roof slope. Actual lumber needs a bit more: the ridge board's thickness eats into the run at the top end, and the birdsmouth notch where the rafter meets the wall plate needs its own margin. Framers typically add a few inches of working length before making the first cut, then trim to fit on site.