Roof Pitch Calculator

Convert roof rise and horizontal run into x-in-12 pitch, degrees, percentage slope, pitch factor, and a theoretical common-rafter line length for a symmetrical gable roof.

Vertical risein Use the same unit as the horizontal run. Enter a rise of zero or more

Horizontal runin This is horizontal distance, not the sloped rafter length. Enter a run greater than zero

Full building spanft For the rafter estimate, the calculator assumes a centered ridge on a symmetrical gable roof. Enter a span greater than zero

Horizontal overhangin Measured horizontally beyond the wall line. Enter an overhang of zero or more

Calculation Results

Normalized roof pitch

—:12

Rafter output is geometric line length only and does not design or cut a roof member.

How to calculate roof pitch

Measure rise and run

Measure the vertical rise and horizontal run over the same section of roof. Both values must use the same unit.

Normalize the pitch

The calculator converts any rise-to-run pair to the conventional amount of rise per 12 units of horizontal run.

Enter span and overhang

For theoretical common-rafter length, enter full building span and horizontal overhang. The calculation assumes a centered ridge and equal roof sides.

Use the output as geometry

Angle, slope percentage, and pitch factor are geometric conversions. Structural sizing, cuts, loads, and fall protection require separate project-specific work.

Roof pitch and rafter geometry formulas

Rise divided by run is the roof slope ratio. Multiplying this ratio by 12 expresses the pitch in x-in-12 form. The angle comes from the arctangent of the ratio, and the pitch factor is the hypotenuse for one unit of horizontal run.

Pitch per 12 = (Rise ÷ Run) × 12

Roof angle = arctan(Rise ÷ Run)

Slope percentage = (Rise ÷ Run) × 100

Pitch factor = √(1 + (Rise ÷ Run)²)

Rafter line = (Span ÷ 2 + Horizontal overhang) × Pitch factor

The rafter result is the straight-line hypotenuse based on half the building span plus horizontal overhang. It excludes ridge thickness, seat cuts, plumb cuts, tail details, fascia, structural deflection, and field adjustments.

Common roof pitches and angles

Rounded geometric conversions.

Pitch	Angle	Slope	Pitch factor
2:12	9.46°	16.67%	1.0138
4:12	18.43°	33.33%	1.0541
6:12	26.57°	50.00%	1.1180
8:12	33.69°	66.67%	1.2019
12:12	45.00°	100.00%	1.4142

Methodology, assumptions and sources

All outputs follow right-triangle geometry. The pitch factor is derived from the Pythagorean theorem and the angle from the arctangent of rise divided by run. Results are rounded only for display.

OSHA uses a slope greater than 4 in 12 as part of its steep-roof definition for construction fall-protection rules. That terminology is included as a safety reference, not as structural or roofing-system design guidance.

Sources:

OSHA — Fall protection on steep roofs — steep-roof terminology and construction safety context.
NIST Handbook 44, Appendix C (2026) — customary unit relationships.

Last reviewed: August 2026. This calculator is not a substitute for structural design, code review, manufacturer instructions, or jobsite safety planning.

Frequently asked questions

What does a 6:12 roof pitch mean?

It means the roof rises 6 units vertically for every 12 units of horizontal run.

Is horizontal run the same as rafter length?

No. Run is the level horizontal distance. Rafter line length is the sloped hypotenuse.

What is the angle of a 4:12 roof pitch?

A 4:12 pitch has a geometric angle of about 18.43 degrees above horizontal.

Can this calculator size rafters?

No. Rafter sizing requires design loads, span, spacing, species and grade, supports, connections, code criteria, and often engineering.