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BuildMath

Rafter Length Calculator · Free calculator

Rafter Length Calculator

Enter the building span (outside of wall to outside of wall), roof pitch and overhang. You get line length, total rafter length, the stock board to buy and the cut angles.

Quick answer

24' @ 6/12
14' 5-9/16"
24' @ 4/12
13' 7-5/8"
30' @ 6/12
17' 9-13/16"
20' @ 8/12
13' 1-3/4"

▍Rafter Length Calculator

ft
/12
in
Rafter length (incl. overhang)
14' 5-9/16"
Buy: 16' board
SPAN 24'RISE 5' 11-5/8"14' 5-9/16"
Line length
13' 4-3/16"
Overhang length
1' 1-7/16"
Run (− ½ ridge)
11' 11-1/4"
Total rise
5' 11-5/8"
Plumb cut
26.6°
Seat cut
63.4°

Rafter length chart (12" overhang)

Span4/126/128/1212/12
12'7' 3-3/4"7' 9-1/16"8' 4-1/16"9' 9-3/4"
16'9' 5-1/16"9' 11-15/16"10' 8-7/8"12' 7-11/16"
20'11' 6-3/8"12' 2-3/4"13' 1-3/4"15' 5-5/8"
24'13' 7-5/8"14' 5-9/16"15' 6-9/16"18' 3-9/16"
28'15' 8-15/16"16' 8-7/16"17' 11-7/16"21' 1-1/2"
30'16' 9-5/8"17' 9-13/16"19' 1-7/8"22' 6-7/16"
32'17' 10-1/4"18' 11-1/4"20' 4-1/4"23' 11-7/16"
36'19' 11-9/16"21' 2-1/16"22' 9-1/8"26' 9-3/8"

How the math works

  1. Run = span ÷ 2 − half the ridge thickness.
  2. Slope factor = √(12² + pitch²) ÷ 12.
  3. Line length = run × slope factor.
  4. Add overhang × slope factor for the tail.
  5. Round up to the next even stock length.

Trade pro tips

  • Always deduct half the ridge thickness from each run.
  • Order rafters 2 ft longer than calculated if you plan fascia trimming in place.

Frequently asked questions

How do you calculate rafter length?

Run (half the span, minus half the ridge) × slope factor for your pitch, plus overhang × slope factor.

What is the run of a rafter?

The horizontal distance from the outside of the wall to the center of the ridge — half the building span minus half the ridge board.

What is line length?

The rafter length along its top edge from the ridge plumb line to the heel of the birdsmouth, before overhang.

How long can a 2x8 rafter span?

It depends on species, grade, spacing and loads — check the IRC rafter span tables in Section R802. Rafter span tables use horizontal run, not rafter length.