Rafter Length & Roof Pitch Calculator

Common rafter length, pitch in degrees and X/12, plumb cut, and birdsmouth seat cut

Enter building span or rafter run, then pitch as 6/12 or total rise in feet, plus horizontal overhang. The tool returns common-rafter line length (plate to ridge), tail length, total stock ridge-to-fascia, pitch angle, X/12, plumb-cut angle, and birdsmouth seat-cut angle. Centered gable only — hip and valley use different geometry.

How to mark the cuts: rafter length and pitch guide. Short rafter or crooked ridge: rafter troubleshooting. When not to use this math: common vs hip/valley. Hip/valley length: hip/valley calculator.

Stairs: Stair Stringer Calculator and Deck Stair Calculator.

Buying the stock: Board Foot Calculator (board feet / optional cost for rafter lumber).

Floor and deck joists: Joist Span Calculator (tabulated spans — verify local code and the grade stamp).

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How to Use This Rafter Length Calculator

Pick building span (the tool halves it) or enter rafter run directly. Then either type pitch as 6/12, or switch to rise+run and enter total rise in feet. Add the horizontal overhang. Read common-rafter line length, tail, total stock, pitch in degrees and X/12, plumb-cut angle, and birdsmouth seat-cut angle.

run = span ÷ 2 · rise = run × (X ÷ 12) · length = √(run² + rise²)

Marking plumb cuts and the birdsmouth: rafter length and pitch guide. Wrong angle or short rafter: rafter troubleshooting. When this math does not apply: common vs hip/valley rafter.

Stairs still live next door: Stair Stringer Calculator and Deck Stair Calculator.

What This Number Is (and Is Not)

The hero length is ridge-to-fascia stock for a common rafter: theoretical line length plus the tail from a level overhang. It is the hypotenuse of the rise/run triangle, not a hip. Set the framing square to 12 on the body and the pitch number on the tongue; that is the same X/12 the calculator prints.

Plumb cut vs birdsmouth seat

The plumb cut is the pitch angle. Set the circular saw (or the square) to that degree for the ridge cut and for the heel of the birdsmouth. The seat cut that sits on the plate is 90° minus pitch — the level cut. Depth of the birdsmouth is a remaining-rafter-depth decision, not a length decision. Walkthrough: how to mark plumb cuts and the birdsmouth.

Ridge thickness and overhang

If a 1.5" ridge board is in the middle, each common rafter's run is 0.75" shorter than span/2. Subtract that from run, or the pair will be long and the ridge will sit high. Overhang is always the level eave; converting it along the slope is what the tail line is for. Do not add fascia thickness twice.

Need hips or valleys? Different plan run, different plumb. Use the hip/valley calculator, not this one.

Hip / Valley Calculator →

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Rafter Layout Gear

A framing square set to 12 and the pitch number is the same math as this calculator. The saw only follows the plumb and seat lines.

Swanson Speed Square

Pivot to the pitch degree for a fast plumb cut at the ridge and fascia. Confirm the number against the calculator's pitch angle before the first kerf.

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