Roof Pitch Calculator

Please enter rise and run greater than 0.
–pitch (rise per 12 in)
–roof angle
–slope percent
–roof area multiplier

Rafter length does not include overhang.

How to use this calculator

Enter the rise (how many inches the roof goes up) and the run (usually 12 inches). Press Calculate to get the pitch, the angle in degrees, the slope percent and the roof area multiplier. Add the building width to get the rafter length.

The formula

Pitch = rise ÷ run × 12 (shown as x : 12)
Angle = arctan(rise ÷ run)
Slope % = rise ÷ run × 100
Area multiplier = √(1 + (rise ÷ run)²)

Example: A 6 in 12 roof has a 26.6° angle, a 50% slope and an area multiplier of 1.118. A 24 ft wide building needs rafters about 13.42 ft long.

What your roof pitch result tells you

Roof pitch describes how steep a roof is. This calculator turns your rise and run into four useful numbers: the pitch as x:12, the angle in degrees, the slope as a percentage and the area multiplier. Each one is used for a different job.

How each number is used in practice

  • Pitch (x:12): The standard way roofers and material makers describe a roof. Shingle and underlayment rules are usually written in this form.
  • Angle: Handy for setting a saw or speed square and for comparing against solar panel tilt.
  • Area multiplier: Converts a flat footprint into the real roof surface for ordering shingles.
  • Rafter length: When you enter building width, you get the sloped length from wall to ridge, before overhang.

Real-world examples

A 4/12 roof is about 18.4 degrees with an area multiplier of about 1.054. An 8/12 roof is about 33.7 degrees with a multiplier of about 1.202. So a house with a 1,500 sq ft roof footprint (including overhangs) has about 1,581 sq ft of roof surface at 4/12, but about 1,803 sq ft at 8/12. Roofing is often sold by the “square,” which is 100 sq ft. That is roughly 16 squares versus 18 squares before waste.

How to measure rise and run

The easiest safe method is from the attic. Hold a level horizontally against the underside of a rafter, mark 12 inches along it, then measure straight up to the rafter. That vertical distance is your rise. You can also measure on a gable end from the ground with a pitch gauge or a phone app, or use the building plans.

Tips and common mistakes

  • Use the footprint including the overhang when you multiply for roof area.
  • Remember that hips, valleys and dormers each need extra material. Add a waste allowance, usually around 10 to 15 percent.
  • Do not use the angle where a rule calls for pitch. A 45-degree roof is 12/12, not 45/12.
  • Low-slope roofs below about 4/12 often need special underlayment or a different roofing type.

Safety and planning

Steeper roofs are harder and more dangerous to walk on, and labor costs usually rise with pitch. Avoid climbing onto a roof to measure if you can get the numbers from the attic or plans. Roofing materials and framing must meet local building codes, so check with your building department or a licensed roofer before starting work.

Frequently asked questions

What is a 6/12 roof pitch?
A 6/12 pitch rises 6 inches for every 12 inches of horizontal run. That is an angle of about 26.6 degrees.
How do I convert roof pitch to degrees?
Divide the rise by the run and take the arctangent. For a 6/12 roof, arctan(0.5) is about 26.6 degrees.
What is the roof area multiplier?
It is the number you multiply the flat footprint area by to get the real roof surface area. A 6/12 roof has a multiplier of 1.118.
What is considered a low-slope roof?
Roofs below about 4/12 are usually called low slope and often need special roofing materials.
What is a common roof pitch for homes?
Most houses have a pitch between 4/12 and 9/12.
What is the minimum roof pitch for asphalt shingles?
Asphalt shingles generally need at least a 2/12 pitch. Roofs between 2/12 and 4/12 usually require extra underlayment protection, such as a double layer or an ice and water barrier. Below 2/12, shingles are typically not allowed and a membrane roof is used instead. Always check the shingle maker’s instructions and local code.
What is the difference between roof pitch and roof slope?
The terms are often used interchangeably. Strictly speaking, slope is rise over run, written like 6/12 or 6:12. Pitch in older carpentry use meant rise over the full span, so a 6/12 slope was a 1/4 pitch. Today most roofers and suppliers say “pitch” and mean the x/12 slope.
Is a steeper roof more expensive?
Usually yes. A steeper roof has more surface area for the same footprint, so it needs more material. Roofs steeper than about 8/12 also often need extra safety equipment and slower work, which can raise labor costs. On the plus side, steep roofs shed water and snow faster and can last longer.