Rafter Length Calculator

Common rafter length from span and pitch, with the ridge-board deduction, overhang, cut angles and the lumber length to buy.

Common rafter length

Formulas

Run = span ÷ 2 − ridge thickness ÷ 2 Line length = run × √(pitch² + 144) ÷ 12 Tail length = overhang × same multiplier Rafter = line length + tail length Plumb cut = arctan(pitch ÷ 12) Seat cut = 90° − plumb cut Hip/valley = run × √(pitch² + 288) ÷ 12

Line length is measured along the top edge of the rafter, from the plumb cut at the ridge to the plumb line above the outside of the wall. The seat (birdsmouth) cut sits at that line; keep the seat no deeper than about a third of the rafter depth unless an engineer says otherwise.

Worked example

26 ft span, 7/12, 18 in overhang, 1-1/2 in ridge. Run = 13 − 0.0625 = 12.9375 ft. Line length = 12.9375 × 1.1577 = 14' 11-3/4". Tail = 1.5 × 1.1577 = 1' 8-13/16". Total 16' 8-9/16", so buy 18 ft stock. Plumb cut 30.3°, seat cut 59.7°.

Rafter length table (to ridge center, no overhang)

Common rafter line length by span and pitch
Span	4/12	6/12	8/12	10/12	12/12
16 ft	8' 5-3/16"	8' 11-5/16"	9' 7-3/8"	10' 4-15/16"	11' 3-3/4"
20 ft	10' 6-1/2"	11' 2-3/16"	12' 1/4"	13' 3/16"	14' 1-11/16"
24 ft	12' 7-13/16"	13' 5"	14' 5-1/16"	15' 7-7/16"	16' 11-5/8"
28 ft	14' 9-1/16"	15' 7-13/16"	16' 9-15/16"	18' 2-11/16"	19' 9-9/16"
32 ft	16' 10-3/8"	17' 10-11/16"	19' 2-3/4"	20' 9-15/16"	22' 7-1/2"
36 ft	18' 11-11/16"	20' 1-1/2"	21' 7-5/8"	23' 5-3/16"	25' 5-1/2"

Before you cut

Rafter size and spacing come from span tables for your lumber grade, snow load and dead load (IRC Tables R802.4.1 series). Length is only half the question.
Cut one pattern rafter, test it at both ends of the building, then use it as the template.
Building with trusses? The truss supplier does this math; you just need the span, pitch and overhang.

Frequently asked questions

How do I calculate rafter length?

Take half the building span (minus half the ridge thickness), multiply by the pitch multiplier, then add the overhang times the same multiplier.

What size rafter do I need?

Size depends on span, spacing, lumber species and grade, and snow and dead loads. Use the span tables in your building code or have an engineer size them; this calculator gives length and angles only.

What angle is the plumb cut on a rafter?

The plumb cut angle equals the roof angle: arctan(pitch ÷ 12). For 6/12 it is 26.6°, for 12/12 it is 45°.

Last reviewed 2026-10-10. Results are estimates for planning; confirm quantities with your supplier and requirements with your local building code and the manufacturer's instructions.