Rafter Length Calculator: Span, Pitch and Overhang

Enter the span from outside wall to outside wall, your pitch, and any overhang. The overhang is the level distance past the wall, not the length along the slope.

Building span (feet)Pitch (inches per 12)Overhang, level (inches)Ridge board thickness (inches)

Total rafter length14 ft 5 5/8 inAlong the slope, ridge to tail, before any plumb cut allowance
Ridge to wall13 ft 4 1/8 inOverhang adds 1 ft 1 3/8 in
Rise at the ridge5 ft 11 5/8 inOver a level run of 11 ft 11 1/4 in
Shortest stock length16 ftNext even length; check that it leaves room for cuts

Nothing you enter leaves your browser. Gives length only. Rafter size and spacing come from your local building code or an engineer.

Show the math with your numbers

Run: (288 in − 1.5 in) ÷ 2 = 143.25 in
Multiplier for 6/12: 1.118
Ridge to wall: 143.25 × 1.118 = 160.16 in
Overhang: 12 × 1.118 = 13.42 in
Total: 173.57 in = 14 ft 5 5/8 in

What if: pitch per 12

Pitch per 12	Total rafter length
3 in	13 ft 4 in
4 in	13 ft 7 5/8 in
5 in	14 ft 0 1/4 in
6 in (yours)	14 ft 5 5/8 in
8 in	15 ft 6 5/8 in
10 in	16 ft 10 1/8 in
12 in	18 ft 3 1/2 in

How the calculator works

Run: (span − ridge board thickness) ÷ 2
Slope multiplier: √(1 + (pitch ÷ 12)²)
Ridge to wall: run × multiplier
Overhang length: level overhang × multiplier
Total: ridge to wall + overhang length

Worked example

A 24-foot span with a 6/12 pitch, a 12-inch overhang and a 1½-inch ridge board:

Step	Math	Result
Run	(288 in − 1.5 in) ÷ 2	143.25 in
Multiplier for 6/12	√(1 + 0.5²)	1.118
Ridge to wall	143.25 × 1.118	13 ft 4 ⅛ in
Overhang	12 × 1.118	1 ft 1 ⅜ in
Total		14 ft 5 ⅝ in

The next even stock length is 16 feet, which leaves room for cuts. Framing lumber is typically sold in even-foot lengths, so check what your supplier stocks.

What this calculator doesn't do

Size the rafter: the depth, grade and spacing depend on span, load and your local code. Check your building department's rules or ask an engineer.
Lay out the cuts: it doesn't work out plumb cuts or birdsmouth notches. Ridge-board and tail details can change the cut length slightly, so measure your first rafter before you cut the rest.
Handle hips and valleys: hip and valley rafters run at an angle across the plan, so they need a different calculation.

To find your pitch first, use the roof pitch calculator. To work out the area these rafters cover, use the roof area calculator.

How pitch changes the lumber you buy

On a 24-foot span with no ridge board or overhang, pitch alone can move you up a stock length:

Pitch	Rafter length	Shortest even stock
4/12	12.65 ft	14 ft
6/12	13.42 ft	14 ft
8/12	14.42 ft	16 ft
10/12	15.62 ft	16 ft
12/12	16.97 ft	18 ft

Adding an overhang lengthens each one further, so check the stock length before you price the job.

Frequently asked questions

Is the span measured inside or outside the walls?

This calculator expects the distance between the outside edges of the walls, where the rafters land. Measure the way your plans do, because some details differ.

Why does the ridge board matter?

Each rafter stops at the face of the ridge board rather than the center of the building. So each run is shorter by half the board's thickness.

What if I have no ridge board?

Enter 0. That gives the theoretical length from the center of the building to the wall line.

What size rafter do I need?

This tool can't tell you. Sizing depends on span, spacing, lumber grade and roof loads, and those vary by location. Use your local code's tables or an engineer.

Sources

We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.

2021 International Residential Code, Chapter 8, Section R802 (wood roof framing, including rafter rules and span tables)