Rafter Length Calculator

Rafter = (run + overhang) × √(1 + (pitch ÷ 12)²). A 24 ft span at 6/12: 12 ft run × 1.118 = 13 ft 5 in, or 14 ft 6 7/16 in with a 1 ft overhang. The run is half the span, and the overhang is measured level past the wall, then multiplied by the same factor.

Rafter length with overhang

14 ft 6 7/16 in

14.53 ft along the slope

Estimate only. Check dimensions, loads and local code requirements with a qualified professional before you build.

Run (level): 12 ft, half the span
Slope factor: × 1.118 (6/12, 26.57°)
Wall to ridge, no overhang: 13 ft 5 in
Overhang along the slope: 1 ft 1 7/16 in
Rafter per foot of run: 13.42 in

Measured along the rafter’s top edge line: buy a longer board so the plumb and tail cuts fit.

Worked examples

24 ft span at 6/12 with a 1 ft overhang

Run = 24 ÷ 2 = 12 ft
Factor = √(1 + 0.5²) = 1.118
12 × 1.118 = 13.416 ft = 13 ft 5 in
(12 + 1) × 1.118 = 14.53 ft = 14 ft 6 7/16 in

Result: 13 ft 5 in from wall to ridge, 14 ft 6 7/16 in with the overhang.

Checked by the automated test “rafter — 24 ft span, 6/12 → 13.416 ft (13 ft 5 in); + 1 ft overhang → 14.53 ft”.

Curtis Lumber's example

Run 12 ft 6 in = 12.5 ft; 4/12 factor = 1.054
12.5 × 1.054 = 13.175 ft = 13 ft 2 in

Result: 13.176 ft with the unrounded factor: 13 ft 2 1/8 in.

Checked by the automated test “rafter — Curtis Lumber 4/12, 12.5 ft run → 13.175 ft (13 ft 2 in)”.

How it works

Run = span ÷ 2

Slope factor = √(1 + (pitch ÷ 12)²)

Rafter, wall to ridge = run × slope factor

Rafter with overhang = (run + overhang − ridge board ÷ 2) × slope factor

Assumptions

Curtis Lumber: “Multiply Horizontal Span by” the slope factor; its table gives 1.118 for 6/12 and 1.414 for 12/12.
The overhang is added to the run before multiplying, as InspectAPedia does (“increase the run length” by the overhang).
The length runs to the ridge centre unless you type the ridge board: half its thickness then comes off the run (InspectAPedia: “half the thickness of the ridge board”).
Common rafters of a symmetrical gable. No lumber size or allowable span: that is structural, see the IRC span tables or a professional.

Reference table

Slope factor and rafter length per foot of run
Pitch	Slope factor	Rafter per foot of run	Angle
3/12	1.031	12.37 in	14.04°
4/12	1.054	12.65 in	18.43°
5/12	1.083	13 in	22.62°
6/12	1.118	13.42 in	26.57°
7/12	1.158	13.89 in	30.26°
8/12	1.202	14.42 in	33.69°
9/12	1.25	15 in	36.87°
10/12	1.302	15.62 in	39.81°
11/12	1.357	16.28 in	42.51°
12/12	1.414	16.97 in	45°

Factors as in Curtis Lumber's table; 13.42 in per foot at 6/12 is the framing square figure quoted by InspectAPedia; angles as in the City of Dallas chart.

Before you cut

Span or run. Measure the span from the outside of one wall to the outside of the other; the run is half of it.
Ridge board. Type its thickness to take half of it off each rafter; leave 0 to measure to the ridge centre.

Common mistakes

Using the whole span The run is half the span: a 24 ft span at 6/12 takes 13 ft 5 in rafters, not 26 ft 10 in. Curtis Lumber's “rafter span” of 12 ft 6 in is already a run: choose Run in the calculator.
Adding the overhang without the factor Curtis notes that “the overall rafter length depends on the length of the overhang”: a 1 ft level overhang adds 1 ft 1 7/16 in at 6/12, not 1 ft.
Buying the exact length The figure runs along the rafter's line: buy a longer board so the plumb and tail cuts fit, and count the lumber with the board foot calculator.
Choosing the lumber size here This calculator gives a length, not a size: what a rafter can span depends on load, species and spacing (IRC tables or a professional).

Frequently asked questions

How do you calculate rafter length?

Measure the span between the outer faces of the wall top plates and halve it to get the run, then multiply by the slope factor √(1 + (pitch ÷ 12)²): 12 ft × 1.118 = 13 ft 5 in for a 24 ft span at 6/12.

What is the rafter length formula?

Rafter = run × √(1 + (pitch ÷ 12)²), or (run + overhang) × the same factor with an overhang. Curtis Lumber's table gives the factor: 1.054 at 4/12, 1.118 at 6/12.

How do you work out a rafter with an overhang?

Add the level overhang to the run before multiplying: (12 + 1) × 1.118 = 14 ft 6 7/16 in for a 24 ft span at 6/12 with a 1 ft overhang.

Sources

Formula, examples and sources checked on October 17, 2026 by the Yardstick Math team. How we check our formulas.

Roof Rafter Length: conversion factors for roof rafters — Curtis Lumber, accessed Oct 17, 2026
Framing Square Rafter Table Use — InspectAPedia, accessed Oct 17, 2026
Visual examples of roof pitch — City of Dallas, accessed Oct 17, 2026