Rafter Calculator

Enter your roof span and pitch to get the rafter length, plumb and seat cut angles, birdsmouth, a dimensioned diagram and a printable cut list.

Rafter length to cut (per rafter)

14 ft 5 9/16 in

Buildable stock length: body + overhang

Line length (to ridge): 13 ft 5 in
Plumb cut angle: 26.6°
Seat cut angle: 63.4°
Birdsmouth seat depth: 3 1/2 in
Heel height (HAP): 6 3/8 in
Rafters needed: 38

Dimension diagram

Cut list — common rafters
Tag	Name	Qty	Unit length	Notes (cuts)
R1	Common rafter	38	14 ft 5 9/16 in	Plumb cut 26.6° · seat cut 63.4° · birdsmouth seat 3.5" · HAP 6.36"
Total stock to purchase			549 ft 8 in	Rounded up to whole inch (buy long)

About this rafter calculator

This common rafter calculator turns your roof span and pitch into the numbers you need to cut straight (common) rafters: the rafter length, the plumb and seat cut angles, the birdsmouth, and a full material count. It is built for builders, roofers and DIYers who want a dimensioned diagram and a printable cut list instead of doing the trigonometry by hand. All lengths come from Pythagorean geometry, so every figure can be checked against the run, rise and pitch you enter.

If you only need to read or convert the slope itself — the angle in degrees, percentage grade or X-in-12 ratio — use our roof pitch calculator instead.

How to use the rafter calculator

1
Measure the span (building width)
Measure the full outside-to-outside width of the building that the rafters span. The calculator uses half of this span as the horizontal run for each common rafter, so an accurate span is the foundation of every result.
2
Determine the roof pitch
Enter the pitch as rise per 12 inches of run (for example 6 means a 6-in-12 roof). If you know the rise and run instead, pitch equals rise divided by run, multiplied by 12.
3
Set the ridge thickness and overhang
Enter the ridge board thickness so the calculator can shorten the rafter by half the ridge along the slope, and enter the horizontal overhang (the eave projection past the wall) so it can be added to the stock length.
4
Pick the lumber size and spacing
Choose the rafter lumber depth and the on-center spacing (commonly 12, 16 or 24 inches). Spacing and roof length together drive the rafter count. This tool computes geometry and cuts, not structural sizing.
5
Set the seat cut (birdsmouth) depth
Enter the seat cut depth, which is usually the width of the wall top plate the rafter rests on. The calculator returns the seat depth and the remaining heel height (HAP) above the plate.
6
Read the diagram and cut list
Review the dimensioned diagram and the printable cut list: rafter line length, total stock length including overhang, plumb and seat cut angles, birdsmouth, and the number of rafters. Verify the layout against your own measurements before cutting.

Frequently asked questions

How do I calculate rafter length?

For a common rafter, the horizontal run is half the building span. The rafter line length equals the run multiplied by the per-foot factor √(rise² + 12²) ÷ 12, where rise is the pitch rise per 12 inches of run. This is just the Pythagorean theorem applied to the roof triangle. This calculator performs that geometry for you and then adds the overhang to give the total stock length.

What is the difference between run, rise, and span?

The span is the full horizontal width the roof crosses (the building width). The run is the horizontal distance covered by one rafter, which for a common rafter on a symmetric gable roof is half the span. The rise is the vertical height the rafter climbs over that run. Run and rise are the two legs of the right triangle whose hypotenuse is the rafter length.

How far apart should roof rafters be?

Rafters are commonly spaced 12, 16 or 24 inches on-center, with 16 inches being a frequent choice. The correct spacing for a given project depends on the span, the roof loads (snow, wind, dead and live loads), and the lumber size and species. Always size and space rafters according to your local building code and applicable span tables rather than a rule of thumb.

What is a plumb cut and a seat cut (birdsmouth)?

The plumb cut is the vertical cut at the top of the rafter where it meets the ridge, and its angle equals the roof angle. The seat cut is the horizontal cut that lets the rafter sit flat on the wall top plate. Together the plumb cut and the seat cut form the birdsmouth notch, an L-shaped cutout near the lower end of the rafter that seats it on the wall.

What is heel height (HAP)?

Heel height, often labeled HAP (height above plate), is the vertical depth of rafter material remaining above the wall top plate after the birdsmouth seat cut is made. It is measured plumb from the seat cut up to the top edge of the rafter, and it affects how the roof meets the wall and the fascia.

How deep can a birdsmouth seat cut be?

A common guideline is that the seat cut should not remove more than about one-third of the rafter depth, so that enough uncut material (heel height) remains to carry the load. Treat this as general guidance only: the allowable birdsmouth depth is governed by your local building code and the structural design of the roof, so verify the notch depth against the code and span tables that apply to your project.

Does this calculator include the roof overhang?

Yes. The overhang you enter is converted to sloped length and added to the rafter, so the total stock length reported is the full length of material to cut, including the eave overhang past the wall. The line length figure, by contrast, is measured only from the seat cut up to the ridge.

What size lumber do I need for rafters?

Rafter size depends on the span, the on-center spacing, the roof loads, and the lumber species and grade, and it is determined from span tables in the building code. This calculator computes the geometry and the cuts, not the structural sizing, so use your local building code and the appropriate span tables (or a qualified professional) to choose the lumber size.

Can I use this for hip or valley rafters?

Not yet. This calculator handles common (straight) rafters only. Hip, valley and jack rafters use different run lengths and cut angles, and support for them is a planned addition. For now, use it for common rafters on gable and shed-style roofs.

What is standard rafter spacing?

The most common rafter spacing is 16 inches on-center, with 12 and 24 inches also widely used. There is no single universal standard: the right spacing depends on span, load and lumber size, and must follow your local building code and span tables.

Rafter terms glossary

Rafter: A sloped structural member that runs from the ridge or wall plate down to the eave and supports the roof deck.
Run: The horizontal distance covered by one rafter; for a common rafter it is half the building span.
Rise: The vertical height a rafter climbs over its run, expressed in the pitch as rise per 12 inches of run.
Pitch: The steepness of the roof, given as rise per 12 inches of horizontal run (for example 6-in-12).
Plumb cut: The vertical cut at the top of the rafter where it meets the ridge; its angle equals the roof angle.
Birdsmouth: The L-shaped notch (a plumb cut plus a seat cut) that lets the rafter sit on the wall top plate.
HAP (heel height): Height above plate: the depth of rafter material remaining above the wall plate after the seat cut.