Rafter Length Calculator
This rafter length calculator gives you the exact sloped length of a rafter from span, pitch, and overhang — the number you need before marking a birdsmouth cut where the rafter meets the wall plate.
Calculate Your Rafter Length
Enter your building specifications below. Results and the roof diagram update live as you type.
Live Roof Diagram
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Estimate Your Project Budget
Automatically calculated from your inputs above in the calculator.
Estimates only. Actual costs vary by region, supplier, and site conditions.
Rafter Length and the Birdsmouth Cut
Rafter length is the true sloped distance from the ridge to the tail of the rafter at the eave, found using the Pythagorean theorem from rise and run. It is always longer than the horizontal run because it follows the roof slope.
A birdsmouth is a notch cut into the underside of a rafter where it crosses the wall's top plate, allowing the rafter to sit flush and transfer its load directly into the wall. The notch has two cuts — a level "seat cut" that rests on the plate and a plumb "heel cut" — sized to the rafter's depth and the roof angle, not by this calculator directly.
Use the rafter length and roof angle from the calculator above as the starting numbers for laying out your birdsmouth cut, then confirm the seat-cut depth against your local code's minimum bearing requirement before cutting.
Frequently Asked Questions
Roof rafter cut calculator
The rafter length and roof angle shown above give you the two numbers needed to mark a plumb cut at the ridge and a birdsmouth notch at the wall plate.
Roof rafter birdsmouth calculator
A birdsmouth is a seat-and-heel notch cut where the rafter crosses the wall plate; this tool provides rafter length and angle, while notch depth should follow your local code's minimum bearing rules.
Truss length calculator
The Pitch & Angle and Rafter Length tools return exact member lengths.