Roof Pitch Calculator · Free calculator

4/12 Roof Pitch Calculator

A 4/12 roof rises 4" for every 12" of run — an angle of 18.43° and a rafter multiplier of 1.054. It's a conventional (walkable up to ~6/12) roof. Rise and rafter lengths for common runs are below.

Quick answer

Angle: 18.43°
Slope factor: 1.054
Grade: 33.3%
Rafter per ft of run: 1' 0-5/8"

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Roof pitch

4/12

18.4° · 33.3% grade

Angle

18.43°

Slope factor

1.054

Grade

33.3%

Rafter / ft run

12.65in

4/12 rise and rafter length by run

Run = half the building span. Rafter length is ridge to building line, no overhang.

Run	Total rise	Rafter length
4'	1' 4"	4' 2-5/8"
6'	2' 0"	6' 3-7/8"
8'	2' 8"	8' 5-3/16"
10'	3' 4"	10' 6-1/2"
12'	4' 0"	12' 7-13/16"
14'	4' 8"	14' 9-1/16"
16'	5' 4"	16' 10-3/8"

Roof pitch comparison chart

Pitch	Angle	Slope factor	Grade	Class
1/12	4.76°	1.003	8.3%	flat / low-slope
2/12	9.46°	1.014	16.7%	low-slope
3/12	14.04°	1.031	25.0%	low-slope
4/12	18.43°	1.054	33.3%	conventional
5/12	22.62°	1.083	41.7%	conventional
6/12	26.57°	1.118	50.0%	conventional
7/12	30.26°	1.158	58.3%	conventional
8/12	33.69°	1.202	66.7%	conventional
9/12	36.87°	1.250	75.0%	steep
10/12	39.81°	1.302	83.3%	steep
11/12	42.51°	1.357	91.7%	steep
12/12	45.00°	1.414	100.0%	steep

How the math works

Angle = arctan(rise ÷ run) = arctan(4 ÷ 12) = 18.43°
Slope factor = √(12² + 4²) ÷ 12 = √160 ÷ 12 = 1.0541
Grade = 4 ÷ 12 × 100 = 33.3%
Rafter length = run × 1.0541

Trade pro tips

Asphalt shingles are fine at this pitch with standard underlayment.
Up to about 6/12 most roofers can walk the roof without jacks.
Measure pitch in the attic: level 12" off a rafter and measure straight up to its bottom edge.

Frequently asked questions

What angle is a 4/12 roof pitch?

18.43 degrees (arctan 4/12). The plumb cut on a rafter is 18.4° and the seat cut is 71.6°.

What is the rafter multiplier for a 4/12 pitch?

1.0541. Multiply the run (in feet or inches) by it to get the rafter length: a 12 ft run gives 12' 7-13/16".

Is a 4/12 roof steep?

A 4/12 roof (18.4°) is conventional (walkable up to ~6/12).

What is 4/12 pitch as a percentage?

33.33% grade — the roof rises 4 inches per foot, or 0.333 ft per ft.