Rafter Length Calculator

How to work it out

1. Measure the span

   Outside face to outside face of the wall plates. The run is half that, because a common rafter climbs from the wall to the ridge in the middle.

2. Take half the ridge off

   The rafter stops at the face of the ridge board, not its center. Half the ridge thickness comes off the run — 0.75 in for a 2x ridge — and forgetting it is the classic first-roof mistake.

3. Cut one and try it

   Cut a single rafter, offer it up, and check the birdsmouth sits flat and the plumb cut meets the ridge tight. Only then use it as the pattern for the rest.

4. Mark from the pattern

   Trace every other rafter from that first one. Measuring each one separately is how a roof ends up with a wave in it.

Rafter length per foot of run

Multiply your run in feet by this figure to get the sloping length.

Pitch	Length per ft of run	Plumb cut	Seat cut
3 in 12	1.031 ft	14.0°	76.0°
4 in 12	1.054 ft	18.4°	71.6°
5 in 12	1.083 ft	22.6°	67.4°
6 in 12	1.118 ft	26.6°	63.4°
7 in 12	1.158 ft	30.3°	59.7°
8 in 12	1.202 ft	33.7°	56.3°
10 in 12	1.302 ft	39.8°	50.2°
12 in 12	1.414 ft	45.0°	45.0°

How to work out rafter length

A common rafter is the hypotenuse of a right triangle: the run is the base, the rise is the height. Length per foot of run is √(rise² + 144) ÷ 12, which is 1.118 ft at 6 in 12.

So a 24 ft span at 6 in 12 has a 12 ft run, less 0.06 ft for half a 2x ridge, giving about 13 ft 4 in from ridge to birdsmouth, plus the tail.

The two cuts

The plumb cut at the ridge is vertical when the rafter is in place, and equals the roof angle — 26.6° at 6 in 12. Set that on a circular saw or use the same number on a speed square.

The seat cut of the birdsmouth is horizontal, so it is 90° minus the plumb angle. The birdsmouth should never remove more than a third of the rafter depth, or the tail loses its strength.

How many rafters

Rafters are spaced 16 or 24 in on center, with 12 in used under heavy snow or tile. The count is the building length divided by the spacing, plus one for the far end, and then doubled because every pair has two sides.

Add extra for gable overhangs, and remember that openings for chimneys or skylights need headers and doubled rafters either side.

How this is worked out

No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.

1. run = span ÷ 2 A common rafter climbs from the wall to the ridge at the middle.
2. length per ft of run = √(rise² + 144) ÷ 12 Pythagoras, with 12 in of run.
3. − half the ridge thickness The rafter stops at the face of the ridge board.
4. plumb cut = arctan(rise ÷ 12) The roof angle, cut at the ridge.
5. seat cut = 90° − plumb cut The horizontal face of the birdsmouth.
6. rafters = (length ÷ spacing + 1) × 2 Both slopes, plus the closing rafter at the end.

Common questions

How do I calculate rafter length?

Multiply the run in feet by √(rise² + 144) ÷ 12. For a 12 ft run at 6 in 12, that is 12 × 1.118 = 13.4 ft, before the ridge deduction and the tail.

What is the plumb cut for a 6/12 roof?

26.6° from vertical reference — the same as the roof angle. The seat cut of the birdsmouth is 63.4°.

Do I subtract the ridge board?

Yes. Take half the ridge thickness off the run: 0.75 in for a 1.5 in ridge board. Both rafters meet its faces, not its centerline.

How deep should the birdsmouth be?

No more than a third of the rafter depth, and the seat must bear fully on the wall plate. A deeper notch weakens the tail where it cantilevers out.

How far apart should rafters be?

16 or 24 in on center for most homes. The right answer depends on the rafter size, the span, the snow load and the roof covering — your local code span tables decide it.

How long a board do I need to buy?

Round the rafter length up to the next even foot, since lumber comes in 2 ft steps. A 14 ft 2 in rafter needs a 16 ft board.

Does this work for hip or valley rafters?

No. Hips and valleys run diagonally, so they use a 17 in run instead of 12. This calculator is for common rafters.

Where these figures come from

Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.

• International Residential Code, roof-ceiling construction — bearing, notching limits and rafter spacing
• American Wood Council span tables — allowable spans by size, species and load
• Framing squares and rafter tables — the per-foot-of-run length figures used here

Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 27, 2026.