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Roof Pitch Calculator

Convert between roof pitch (x:12), angle in degrees, slope percentage and rafter-length multiplier.

Choose a calculation

Enter the vertical rise and horizontal run — any matching unit works, since only the ratio matters.

Result

How to Use

  1. Choose whichever measurement you already have from the options above the form.
  2. Enter that one value — rise and run in any matching unit (inches, feet, centimeters, all work as long as they match each other), an angle in degrees, or a slope percentage.
  3. The result updates instantly, showing all four representations at once: pitch, angle, slope and the rafter-length multiplier.
  4. To get an actual rafter length, multiply your horizontal run length by the rafter-length multiplier shown in the result.

Formula

From rise / run
slope = rise ÷ run
Pitch (x:12)
pitch_x = slope × 12
Angle
angle = atan(slope) in degrees
Slope percentage
slope% = slope × 100
Rafter length multiplier
√(1 + slope²) — multiply your run length by this to get the sloped rafter length

Worked Example

A roof that rises 6 inches for every 12 inches of run — a classic "6:12" roof.

  1. slope = 6 ÷ 12 = 0.5
  2. pitch = 0.5 × 12 = 6, so this is a 6:12 pitch
  3. angle = atan(0.5) ≈ 26.5651°
  4. slope% = 0.5 × 100 = 50%
  5. rafter multiplier = √(1 + 0.5²) = √1.25 ≈ 1.1180

Result: Pitch: 6:12 Angle: 26.5651° Slope: 50% Rafter length multiplier: ×1.118

About This Tool

What this tool does

This calculator converts between the five common ways a roof's steepness gets described: rise/run, a pitch ratio (x:12), an angle in degrees, a slope percentage, and a rafter-length multiplier — enter any one and see all the others.

When to use it

Use it whenever a plan, product spec, or contractor quotes a roof's steepness in one form and you need it in another — or to find the multiplier that converts a horizontal run measurement into an actual sloped rafter length.

What the result means

Pitch, angle and slope percentage all describe the exact same steepness in three different, non-interchangeable scales — see the FAQ for why a "6:12 pitch" is not "6 degrees" and not "6% slope." The rafter-length multiplier is separate again: it's a ratio you multiply a flat run length by, not a pitch/angle/slope figure itself.

Assumptions & limitations

Rise and run must use the same unit as each other (both in inches, or both in feet, etc.) — the calculation only uses their ratio, so the actual unit doesn't matter as long as they match. This tool provides geometry only: it does not recommend a minimum pitch for any roofing material, check building-code or snow-load requirements, or offer any other structural guidance — consult a licensed professional and your local code for those.

Frequently Asked Questions

What is the difference between roof pitch, angle and slope?
Pitch (e.g. "6:12") is a ratio of rise to a fixed 12-unit run — a builder's shorthand, not a percentage or a degree measurement. Angle is the roof's actual angle from horizontal in degrees. Slope percentage is rise ÷ run × 100. A 6:12 pitch is a 26.57° angle and a 50% slope — three different numbers for the same roof, and none of them equal each other.
How do I convert roof pitch to degrees?
Divide the pitch's "x" value by 12 to get the slope, then take the arctangent of that slope and convert to degrees. A 6:12 pitch: atan(6÷12) ≈ 26.57°.
What does the rafter-length multiplier do?
It's the ratio between the sloped rafter length and the flat horizontal run it covers. Multiply your run length by this number to get the actual rafter length you'll need — for a 6:12 roof, a 10 ft run needs an 11.18 ft rafter (before overhang and any layout waste).

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