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Rafter Calculator

Common rafter length and cut angles from span and pitch.

Your measurementsEvery assumption editable01

Outside to outside of the wall plates.

Rise in inches per 12 inches of run.

Horizontal projection past the wall, not measured along the slope.

RafterCalculated live02

Result

14.837ft rafter

178.05 in overall — 160.16 in line length

Runhalf the span, less half the ridge thickness
143.25 in
Rise
71.625 in
Slope factor
× 1.11803
Line lengthridge to the outside of the plate
160.158 in
Overhang along the slope16 in horizontal projection
17.889 in
Overall rafter length14.8372 ft
178.047 in
Plumb cutoff square — the ridge and tail cuts
26.565°
Seat cutthe level cut of the birdsmouth
63.435°

Half the ridge thickness is deducted from the run because the rafter stops at the face of the ridge board, not at the building centreline. Forgetting it makes every rafter in the roof too long by that amount.

The overhang figure is the horizontal projection, which is how eaves are specified. Along the slope it is longer by the slope factor, and cutting stock to the horizontal figure leaves the tails short.

This is a common rafter on a simple gable. Hip and valley rafters run at a different angle entirely and need their own layout, and anything structural — member size, span, load — is an engineering question rather than a geometric one.

Enter span, pitch, overhang and ridge thickness. Two details separate a rafter that fits from one that does not: half the ridge thickness comes off the run, and the overhang is longer along the slope than its horizontal projection.

Why use this tool?What it does differently03

Why use this tool?

Ridge deducted

The rafter stops at the ridge face, not the building centreline. Forgetting it makes every rafter long.

Overhang along the slope

Eaves are specified as horizontal projection but cut along the slope, and the two differ.

Both cut angles

Plumb for the ridge and tail, seat for the birdsmouth — complements of each other.

Scope stated

Common rafters on a simple gable. Hips and valleys run at a different angle entirely.

How this worksThe method04

How this rafter calculator works

Run is half the span less half the ridge thickness, because the rafter lands on the face of the ridge board rather than at the centreline of the building. Rise is that run times the pitch over twelve.

Line length is the run times the slope factor — the hypotenuse of the rise-and-run triangle. The overhang is added along that same slope, which is why the total is more than run plus overhang.

The plumb cut is the pitch angle measured off square, and the seat cut is its complement. Those two cuts form the birdsmouth that seats the rafter on the plate.

How to use itStep by step05

How to use it

  1. Step 1: Measure the span

    Outside to outside of the wall plates.

  2. Step 2: Enter the pitch

    Rise in inches per twelve of run. Use the roof pitch calculator if you only have an angle.

  3. Step 3: Set the overhang

    Horizontal projection past the wall, which is how eaves are specified.

  4. Step 4: Deduct the ridge

    Enter its actual thickness — a nominal 2x is 1.5 in.

Example usageWorked figures06

Example usage

A 24 ft span at 6/12
Run 143.25 in after deducting half a 1.5 in ridge, rise 71.625 in, line length 160.16 in. With a 16 in overhang the rafter is 178.05 in overall — 14.837 ft.
The cut angles
At 6/12 the plumb cut is 26.565° and the seat cut 63.435°. Those hold for any span at that pitch.
Forgetting the ridge
Skipping the 0.75 in deduction makes every rafter three quarters of an inch long, which pushes the ridge up and the walls out.
Frequently asked questionsCommon questions07

Frequently asked questions

How do I calculate rafter length?

Half the span less half the ridge thickness gives the run. Multiply by the slope factor for the line length, then add the overhang times the same factor.

Why subtract half the ridge board?

Because the rafter stops at the face of the ridge, not at the centreline of the building. Without the deduction every rafter is long by half the ridge thickness.

What is a birdsmouth?

The notch that seats the rafter on the wall plate, formed by a plumb cut and a level seat cut. Their angles are complements — at 6/12, 26.565 and 63.435 degrees.

Is the overhang measured along the slope?

It is specified as horizontal projection but cut along the slope, which is longer by the slope factor. Cutting to the horizontal figure leaves the tails short.

Does this work for hip rafters?

No. Hip and valley rafters run diagonally and have their own, shallower effective pitch. This is common rafters on a simple gable.

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