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.

How the Roof Pitch Calculator works

The roof pitch calculator works in three modes and converts a roof slope into an angle, slope percentage, rafter factor, and, in one mode, an actual rafter length.

Pitch is expressed as rise over run, almost always as X-in-12: the roof rises X inches for every 12 inches of horizontal run. In Rise & Run mode you enter both numbers; in Pitch mode you enter just the rise against a fixed 12 run. The angle in degrees is the arctangent of rise ÷ run. The slope percentage is (rise ÷ run) × 100. The rafter factor, also called the roof-area multiplier, is √(rise² + run²) ÷ run; multiply your flat footprint area by this factor to get the true sloped surface area for ordering shingles, underlayment, or panels.

In Building Width mode the calculator sizes an actual rafter. It takes half the building width as the horizontal run (half-span), computes the ridge height as half-span × (pitch ÷ 12), then finds the rafter length as √(half-span² + ridge-height²) and adds 1.5 feet for the overhang.

Worked example: an 8/12 pitch rises 8 inches per 12 inches of run. The angle is arctan(8 ÷ 12) = 33.7 degrees, the slope is 66.7%, and the rafter factor is √(8² + 12²) ÷ 12 = √208 ÷ 12 = 1.202, meaning the sloped roof needs about 20% more material than the flat footprint. For a 30-foot-wide building at a 6/12 pitch, the half-span is 15 feet, the ridge sits 7.5 feet above the wall plate, and each rafter is √(15² + 7.5²) + 1.5 = 18.3 feet including the overhang.

Frequently Asked Questions

What is the rafter factor and how is it calculated?

The rafter factor is √(rise² + run²) ÷ run, the ratio of the sloped rafter length to the horizontal run. It doubles as the roof-area multiplier. A 6/12 pitch gives √(36 + 144) ÷ 12 = 1.118, so the sloped roof area is 11.8% larger than the flat footprint below it.

How do I convert a roof pitch to degrees?

Take the arctangent of rise divided by run and convert to degrees. A 4/12 pitch is arctan(4 ÷ 12) = 18.4 degrees, and a 12/12 pitch is arctan(12 ÷ 12) = 45 degrees. The calculator does this automatically for whichever mode you use.

How does Building Width mode find rafter length?

It uses half the building width as the run (half-span), computes ridge height as half-span × pitch ÷ 12, then applies the Pythagorean theorem: rafter length = √(half-span² + ridge-height²). It adds a fixed 1.5 feet for the eave overhang. This is the theoretical length before deducting for ridge thickness or birdsmouth cuts.

Is slope percentage the same as pitch?

No. Pitch is the rise-in-12 ratio, while slope percentage is (rise ÷ run) × 100. A 6/12 pitch equals a 50% slope, and a 12/12 pitch equals a 100% slope. Both describe the same roof, just in different units.