Roof Pitch Calculator

Pitch, angle, slope factor and common rafter length from rise and run, an X/12 pitch or an angle.

Start from

Rise and run Pitch Angle

Rise

Run

Any unit, inches or cm, as long as both match.

Rafter length

Units

Feet and inches Metric

Measure

Building span Rafter run

Span or run Outside of wall to outside of wall. The run is half.Level distance from the wall to the ridge centerline.

Overhang Measured level past the wall.

Ridge board thickness Half is deducted. Enter 0 to skip.

Your roof

Roof pitch –

Nearest whole pitch–
Angle–
Slope–
Slope factor–

Common rafter length –

Rafter run–
Run to ridge line–
Ridge deduction–
Overhang length–

Rafter length is measured along the rafter from the ridge to the end of the overhang, before plumb and seat cuts.

How to use the roof pitch calculator

Pick what you know: a rise and run you measured, a pitch such as 6/12, or an angle in degrees.
Read the pitch, angle, slope percent and slope factor. They update as you type.
For rafters, enter the building span (wall to wall) or the rafter run, the overhang and the ridge board thickness.

To measure pitch on site, hold a level flat against the underside of a rafter or on the roof, mark 12 in along the level and measure straight down (or up) to the roof. That distance in inches is the pitch.

Roof pitch formulas

Pitch (X/12)  = 12 × rise ÷ run
Angle         = arctan(rise ÷ run)
Slope %       = rise ÷ run × 100
Slope factor  = √(1 + (rise ÷ run)²)
Run           = building span ÷ 2
Rafter length = (run − ridge thickness ÷ 2 + overhang) × slope factor

The slope factor converts a level distance to a length along the roof. Multiply a run by it to get a rafter length, or multiply a plan area by it to get roof surface area. The ridge deduction is half the ridge board thickness, measured level, because each rafter stops at the face of the ridge board instead of its centerline. Skip it for a roof framed without a ridge board.

Worked example

A gable roof on a house 24 ft wide, wall to wall, with a 6/12 pitch, a 12 in overhang and a 2x ridge board (1 1/2 in thick).

Slope factor: √(1 + (6 ÷ 12)²) = 1.1180.
Angle: arctan(6 ÷ 12) = 26.57°. Slope: 50%.
Run: 24 ft ÷ 2 = 12 ft = 144 in.
Run to the ridge line: 144 × 1.1180 = 161.00 in.
Ridge deduction: 1.5 ÷ 2 = 0.75 in level, so 0.75 × 1.1180 = 0.84 in.
Overhang: 12 × 1.1180 = 13.42 in.
Rafter length: 161.00 − 0.84 + 13.42 = 173.57 in, or 14' 5 9/16". A 16 ft board leaves room for the plumb cut at the ridge.

Common roof pitch table

Pitch	Angle	Slope	Slope factor
1/12	4.76°	8.3%	1.0035
2/12	9.46°	16.7%	1.0138
3/12	14.04°	25.0%	1.0308
4/12	18.43°	33.3%	1.0541
5/12	22.62°	41.7%	1.0833
6/12	26.57°	50.0%	1.1180
7/12	30.26°	58.3%	1.1577
8/12	33.69°	66.7%	1.2019
9/12	36.87°	75.0%	1.2500
10/12	39.81°	83.3%	1.3017
11/12	42.51°	91.7%	1.3566
12/12	45.00°	100.0%	1.4142

Tips

Rafter length here runs along the measuring line, from the ridge to the tail. Add for the plumb cut at the ridge and any tail cut when you pick a board length.
Overhang is measured level from the outside of the wall. A 12 in overhang on a 6/12 roof adds 13.42 in of rafter.
For roof area, multiply the plan area of the roof, including overhangs, by the slope factor.
Shingles, membranes and metal panels each have a minimum slope. Check the product instructions and your local code before you choose a pitch.

Roof pitch FAQ

How do I find the pitch of my roof?

Measure 12 in level along a level held against the roof or a rafter, then measure straight to the roof surface. The rise in inches over that 12 in run is the pitch. A 6 in rise is a 6/12 pitch.

What angle is a 6/12 roof pitch?

A 6/12 roof is 26.57 degrees, a 50% slope, with a slope factor of 1.1180. A 12/12 roof is 45 degrees.

What is a roof slope factor?

It is the length along the roof for each unit of level run: √(1 + (rise ÷ run)²). Multiply a run by the slope factor to get a rafter length, or a plan area to get the roof surface area.

How do I calculate common rafter length?

Take the run (half the building span), subtract half the ridge board thickness, add the overhang and multiply by the slope factor. A 24 ft span at 6/12 with a 12 in overhang and a 1 1/2 in ridge gives 173.57 in.

Why subtract half the ridge board?

The run is measured to the centerline of the ridge, but each rafter stops at the face of the ridge board. Half the board thickness, times the slope factor, comes off every common rafter.

Can I use metric measurements?

Yes. Pitch is a ratio, so a rise and run in centimetres give the same pitch as inches. Switch the rafter units to metric to enter the span, overhang and ridge in centimetres.