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Generated on: July 20, 2026 at 09:33 PM

Rafter Length Calculator

Inputs

Run3.6 m
Roof pitch (in 12)6
Overhang0 m

This estimate gives the theoretical line length of a common rafter and does not account for ridge board thickness, the plumb and seat (birdsmouth) cuts, or lumber actual dimensions. Confirm measurements against your framing plans and lay out the cuts on the stock before making the final rafter.

Generated with OneCalc · https://onecalc.app/en/lifestyle/rafter-length

Lifestyle

Rafter Length Calculator

Find the length of a common rafter from the horizontal run and the roof pitch. Pitch is entered as "X in 12" — the vertical rise per 12 units of run — and the rafter is the sloped hypotenuse over that run and rise. An optional eave overhang can be added to the cut length.

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≥ 0.01 m
≥ 0

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Details

Rafter Breakdown

\begin{aligned} H &= R \times \frac{p}{12} \\ &= 3.6 \times \frac{6}{12} \\ &= ? \end{aligned}
\begin{aligned} L &= \sqrt{R^2 + H^2} \\ &= \sqrt{3.6^2 + ?^2} \\ &= ? \end{aligned}
\begin{aligned} T &= L + o \\ &= ? + 0 \\ &= ? \end{aligned}

A common rafter over a 3.6 m run at a 6 in 12 pitch rises ... and measures ... along the slope, or ... including the overhang.

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Rafter Length

A common rafter is the sloped member that carries a gable roof from the ridge down to the wall. Its length follows directly from two figures: how far the rafter reaches horizontally (the run) and how steeply the roof climbs (the pitch). Because the run, the vertical rise, and the rafter form a right triangle, the rafter length is the hypotenuse.

Run, rise, and pitch

The run is the horizontal distance from the outside of the wall to the point under the ridge — for a symmetric gable, half the building span. Roof slope is quoted as a pitch of "X in 12": the roof rises X units for every 12 units of run. The vertical rise over the whole run is therefore the run scaled by the pitch:

H=R×p12H = R \times \frac{p}{12}

Rafter length

The run and the rise are the two legs of a right triangle whose hypotenuse is the rafter. Its line length comes straight from the Pythagorean theorem:

L=R2+H2L = \sqrt{R^2 + H^2}

An eave overhang, measured along the slope, adds to the length of stock to cut:

T=L+oT = L + o

Worked example

Take a common rafter with a run of 12 feet3.6 metres on a roof with a 6 in 12 pitch, and no overhang.

The rise over the run is the run times the pitch over 12:

H=12×612=6 ft\begin{aligned} H &= 12 \times \frac{6}{12} \\ &= 6\ \text{ft} \end{aligned}

With a 12 ft run and a 6 ft rise, the rafter is:

L=122+62=144+36=180≈13.42 ft\begin{aligned} L &= \sqrt{12^2 + 6^2} \\ &= \sqrt{144 + 36} \\ &= \sqrt{180} \\ &\approx 13.42\ \text{ft} \end{aligned}
H=3.6×612=1.8 m\begin{aligned} H &= 3.6 \times \frac{6}{12} \\ &= 1.8\ \text{m} \end{aligned}

With a 3.6 m run and a 1.8 m rise, the rafter is:

L=3.62+1.82=12.96+3.24=16.2≈4.02 m\begin{aligned} L &= \sqrt{3.6^2 + 1.8^2} \\ &= \sqrt{12.96 + 3.24} \\ &= \sqrt{16.2} \\ &\approx 4.02\ \text{m} \end{aligned}

A steeper pitch raises the rise and so lengthens the rafter for the same run; a lower pitch shortens it.

From line length to a finished rafter

The figure above is the line length — the ideal length along the slope between the ridge line and the outer wall. A finished rafter still needs a plumb cut where it meets the ridge, a birdsmouth (seat and heel) cut where it sits on the wall plate, and often a subtraction for half the ridge board thickness. The overhang, or tail, extends past the wall to form the eave.

For a fuller material take-off, see the Wall Framing Stud Calculator for the walls below and the Roof Shingle Bundle Calculator for covering the finished roof.

Frequently Asked Questions (FAQ)

What counts as the run of a rafter?

The run is the horizontal distance the rafter spans, measured from the outside edge of the wall plate to the point directly under the ridge. On a symmetric gable roof each side has the same run, so the run is half the total building span.

Uneven or shed roofs measure the run per slope rather than halving the span. Use the horizontal projection, not a measurement taken along the slope.

How is roof pitch expressed as "X in 12"?

Roof slope is quoted as the vertical rise for every 12 units of horizontal run. A "6 in 12" pitch rises 6 inches for each 12 inches of run, which is why the rise equals the run times the pitch divided by 12.

The 12 is a fixed reference, so the same "6 in 12" describes the slope whether the run is measured in inches, feet, or metres. Steeper roofs carry a larger first number.

Is the overhang measured along the slope or horizontally?

This calculator treats the overhang as a length measured along the slope of the rafter, so it adds directly to the rafter length to give the total stock to cut.

If your plans give the overhang as a horizontal projection instead, convert it to a sloped length first (divide by the cosine of the roof angle) before entering it, or enter zero and add the eave separately.

Disclaimer

This estimate gives the theoretical line length of a common rafter and does not account for ridge board thickness, the plumb and seat (birdsmouth) cuts, or lumber actual dimensions. Confirm measurements against your framing plans and lay out the cuts on the stock before making the final rafter.

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