Roof Pitch Calculator

Calculate angle, slope percentage, rafter length, and roof area multiplier from any pitch input.

Calculation Mode

Rise (inches)

Vertical rise in inches

Run (inches)

Horizontal run in inches (typically 12)

check_circle

Conventional

All material types

Pitch Diagram

Roof Angle

18.43°

Slope: 33.33% · Factor: 1.05

Pitch ratio	4/12
Angle (degrees)	18.43°
Slope percentage	33.33%
Rafter factor	1.05
Roof area multiplier	1.05

How to use the area multiplier

Multiply your flat (horizontal) footprint area by 1.05 to get the actual sloped roof area for material takeoffs.

Methodology

Rise-over-run ratio expressed as X/12. Rafter length = √(rise² + run²). Angle = arctan(rise ÷ run) converted to degrees. Roof area multiplier accounts for slope — multiply your horizontal footprint area by the slope factor to get actual roof area for material takeoffs. For building width mode, half-span is used as the horizontal run, and 1.5 feet of overhang is added to the calculated rafter length.

Frequently Asked Questions

What is a 4/12 roof pitch?

A 4/12 pitch means the roof rises 4 inches for every 12 inches of horizontal run — an angle of approximately 18.4 degrees. It is one of the most common pitches in residential construction, offering a good balance between drainage performance and ease of installation. Most standard roofing materials are approved for 4/12 and above.

How do I measure roof pitch from the ground?

Use a level and tape measure on an accessible rafter or in the attic. Hold the level horizontally on the rafter and mark 12 inches from the end. Measure straight up from that 12-inch mark to the underside of the rafter — that vertical measurement is your rise. A 4" rise at 12" run = 4/12 pitch. You can also use a smartphone pitch finder app or a digital angle gauge on the roof surface.

What pitch do I need for metal roofing?

Standing seam metal roofing can be installed on pitches as low as 1/2:12 with proper underlayment and panel design. Exposed fastener metal panels require a minimum of 3:12. Corrugated metal is generally rated for 3:12 or greater. Always verify with the manufacturer's installation guidelines for the specific panel profile you are using.

How does pitch affect material quantity?

A steeper pitch means more actual roof surface area for a given footprint. A 6/12 pitch has a slope factor of approximately 1.118, meaning it requires 11.8% more shingles than a flat footprint. A 12/12 pitch has a factor of 1.414 — requiring 41.4% more material than the flat area suggests. Always multiply your horizontal footprint by the slope factor before ordering shingles, underlayment, or metal panels.