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Rafter Length Calculator: Common Rafters, Cuts and Count

Work out common rafter length from the roof's span, pitch and overhang, with ridge reduction, plumb cut angle and rafter count. Enter the pitch as x/12 or degrees and see the run, rise, bird's mouth, the board length to buy and the hip or valley line length.

Your numbers

ft
in

Outside of wall to outside of wall.

Rise in inches per 12 in of run (6 for 6/12), or the angle in degrees.

in

Actual: a 2x ridge is 1-1/2 in. 0 if the rafters meet.

in

Measured level from the outside of the wall to the tail cut.

ft

Along the ridge, for the rafter count.

For the height above plate and the board length.

in

The wall plate it sits on: 3-1/2 in on a 2x4 wall, 5-1/2 in on a 2x6.

Rafter length

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To the ridge face
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Tail (overhang)
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Run (half the span)
–
Rise
–
Pitch
–
Length per foot of run
–
Line length to the ridge centre
–
Plumb cut
–
Seat cut
–
Hip or valley line length
–

Lengths are measured along the rafter, from the outside of the wall line to the ridge; cut the first rafter as a pattern and check it before cutting the rest.

Rafter size and spacing come from the span tables (IRC R802.4.1) and the local code, not from this calculator.

The calculation runs in your browser and ParrotNotes does not store your numbers. They sit in the page address so you can bookmark or share the result.

How to calculate rafter length

A common rafter is the hypotenuse of a right triangle: the run (half the span) along the bottom and the rise up the side. Its length is the run × √(1 + (pitch ÷ 12)²), the slope factor. For a 6/12 roof the factor is 1.118, or 13.42 in of rafter for every foot of run.

Worked example: a 24 ft building (outside of wall to outside of wall) with a 6/12 roof has a run of 12 ft and a rise of 6 ft. The line length to the ridge centre is 144 × 1.118 = 161.0 in. Take off the ridge reduction for a 1-1/2 in ridge board (0.84 in along the rafter) to get 160.16 in, then add the tail for a 12 in overhang (13.42 in): the rafter is 173.57 in, or 14 ft 5-9/16 in.

Ridge reduction

The calculation runs to the centre of the ridge, but each rafter stops at the ridge board's face. Shorten it by half the ridge thickness, measured level (square to the plumb cut): 3/4 in for a 2x ridge, which is 0.84 in along a 6/12 rafter. The code asks for a ridge board at least 1 in nominal thick and no shallower than the cut end of the rafter. On a 6/12 roof the plumb cut on a 2x8 is 7.25 × 1.118 = 8.1 in deep, more than a 2x8 ridge (7-1/4 in), so it takes a 2x10 ridge.

Plumb cut, seat cut and the bird's mouth

Both ends of a common rafter are cut plumb: vertical once the rafter is in place. On a speed square, pivot to the roof's angle (26.57° for 6/12) or to the rise on the common scale and mark along the edge. The plumb line meets the rafter edge at 90° minus the roof angle.

The bird's mouth is the notch that sits on the wall plate: a level seat cut as long as the plate is wide (3-1/2 in on a 2x4 wall) and a plumb heel cut. On a 6/12 roof the heel is 3.5 × 6 ÷ 12 = 1-3/4 in high, which leaves a 2x8 rafter 6.36 in of wood above the plate. Keep the notch shallow; the local code limits how deep you may cut into a rafter.

How many rafters, and what length to buy

For each side, divide the building length by the spacing, round up and add one; double it for both sides. A 40 ft building at 16 in on center has 30 spaces, so 31 rafters a side and 62 in all, plus any doubled rafters at openings and the gable ends.

Plumb ends make the cut rafter a parallelogram, so the board has to be longer than the rafter by its depth × pitch ÷ 12: 173.57 + 7.25 × 0.5 = 177.2 in for a 2x8 on a 6/12 roof. The calculator then picks the next length from a typical yard list of 8 to 20 ft in 2 ft steps, here 16 ft. Ask the yard what they stock.

Hip and valley rafters

On a hip roof where every side has the same pitch, the hip runs at 45° in plan, so its run is the common run × √2 while its rise is the same. Its line length is the run × √(2 + (pitch ÷ 12)²): 18 in per foot of common run on a 6/12 roof, 18 ft for the 12 ft run in the example. The figure is to the ridge centre, before the hip's own shortening and side cuts, which depend on how it meets the ridge.

Rafter size comes from the span tables

This calculator gives lengths and angles, not sizes. Under the International Residential Code, rafters are sized from the span tables in R802.4.1, with the span measured along the horizontal projection, by species, grade, spacing, snow load and dead load. Your local code, the lumber grade stamp and, where needed, an engineer decide the size and spacing.

Sources, checked October 2026. All results are estimates; the local code decides:

More for roofing, framing and building-materials sales: Roofing sales training, Building materials sales, Construction daily report.

The pitch is on the plans. The rest is on the walk-through.

Span and pitch come from the site visit, and so does everything that moves the price: the roofing the customer picked, the overhang they want, vents and skylights, who hauls the old roof away. Record the walk-through in ParrotNotes and get the summary and next steps, so the quote matches what the customer asked for.

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Rafter length calculator FAQ