Roof Pitch Calculator

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How to Use the Roof Pitch Calculator

1. 1

   Pick how you know the slope

   Choose Rise & run if you measured with a level, Angle if you have degrees from an app or plans, or Rise & span if you know the ridge height above the walls and the wall-to-wall width.

2. 2

   Enter the measurements

   Rise and run are in inches; the angle is in degrees; total rise and building span are in feet. In Rise & span mode the run is half the span.

3. 3

   Add the rafter run and overhang

   Enter the horizontal run from the wall to the ridge in feet (filled in automatically from the span in Rise & span mode) and an overhang in inches, measured level past the wall.

4. 4

   Read the results

   You get the pitch as X/12, the angle, the slope percentage, the pitch multiplier, the rafter length with and without overhang, and a note on the pitch category.

How the roof pitch calculator works

Pitch is the rise in inches for every 12 in of horizontal run. From any rise and run, the pitch is rise ÷ run × 12. From an angle, it is tan(angle) × 12. From a total rise and building span, the run is half the span, so the pitch is rise ÷ (span ÷ 2) × 12.

The angle is the arctangent of pitch ÷ 12, and the slope percentage is pitch ÷ 12 × 100. The pitch multiplier is the length of the sloped line for each foot of level run: √(1 + (pitch ÷ 12)²). Multiply a roof footprint by it to get the sloped area, or a horizontal run by it to get the rafter length.

Rafter length here is the theoretical line length from the wall to the ridge along the slope, plus the overhang (also measured level) times the same multiplier. It does not subtract half the ridge board thickness or add material for plumb and seat cuts.

Pitch (X/12) = rise ÷ run × 12
Angle = arctan(pitch ÷ 12)
Slope % = pitch ÷ 12 × 100
Multiplier = √(1 + (pitch ÷ 12)²)
Rafter = (horizontal run ft + overhang in ÷ 12) × multiplier

Worked example: a 6/12 roof with a 12 ft run

Rise & span works the same way. A house 24 ft wide with the ridge 4 ft above the wall plates has a 12 ft run, so the pitch is 4 ÷ 12 × 12 = 4/12 (18.43 degrees). Its rafters are 12.65 ft on the line, or 13 ft 8.4 in with a 12 in overhang. In Angle mode, 30 degrees works out to a 6.93/12 pitch.

• Pitch: 6 ÷ 12 × 12 = 6/12.
• Angle: arctan(6 ÷ 12) = arctan(0.5) = 26.57 degrees.
• Slope: 6 ÷ 12 × 100 = 50%.
• Multiplier: √(1 + 0.5²) = √1.25 = 1.118.
• Rafter for a 12 ft run: 12 × 1.118 = 13.42 ft, or 13 ft 5 in.
• With a 12 in overhang: (12 + 1) × 1.118 = 14.53 ft, or 14 ft 6.4 in. A 16 ft board covers it with room for the cuts.

Roof pitch to degrees chart

Common pitches from 1/12 to 12/12 with their angle, slope percentage, multiplier and the category the calculator shows.

Rafter lengths for common runs

Rafter line length for a 12, 14 or 16 ft horizontal run, without and with a 12 in overhang. Lengths are before ridge board deductions and cuts.

Low, conventional and steep roofs

• Under 2/12: usually treated as low-slope roofing. Asphalt shingle manufacturers generally do not allow shingles this flat; membrane or other low-slope systems are typical.
• 2/12 to under 4/12: shingles are generally possible only with extra underlayment, as the manufacturer instructions and local code require. Check before you buy.
• 4/12 to 9/12: conventional pitch that suits most roofing materials. Up to about 6/12 is usually walkable with care; 7/12 to 9/12 gets harder to stand on.
• Over 9/12: steep. Treat it as not walkable without roof jacks and fall protection, and expect more material per square foot of house, as the multiplier shows.
• Building stairs instead of a roof? Rise and run work the same way, and the stair calculator handles the code limits for steps.

Measuring pitch and common mistakes

• To measure on the roof or in the attic, hold a level horizontally against a rafter, mark 12 in along the level, and measure straight down (or up) to the rafter. That measurement is the rise per 12.
• Use horizontal run, not the sloped length. Measuring along the rafter and calling it run gives a pitch that is too low and a rafter that is too long.
• Overhang in this calculator is measured level from the wall. If you measured it along the slope, divide by the multiplier first.
• Rafter stock comes in 2 ft steps, so round up to the next length; the board foot calculator helps if you price by volume.

When to call a professional

Measuring pitch and checking rafter lengths for a shed or porch roof is straightforward. New roof framing, changes to rafter size or spacing, adding a heavier roofing material, and work on steep or high roofs involve structural and fall risks. Rafter size and spacing must follow span tables for your snow and wind loads, roofs often need a permit, and steep roofs call for proper fall protection. Get a roofer, builder or engineer involved for those jobs.

Features

• Three ways in: rise and run, angle, or rise and building span
• Pitch as X/12, angle in degrees and slope percent
• Roof pitch multiplier for converting footprint to roof area
• Rafter length for a horizontal run, with optional overhang
• Low, conventional or steep pitch note for roofing and walkability

Frequently Asked Questions