Roof Run Calculator

Calculate the horizontal run of your roof from pitch, rise, or rafter length for accurate framing layouts and material estimates

Calculate horizontal run from roof pitch and total rise

Quick presets

ft

Total Roof Area

0 sq ft

0.0 roofing squares • Pitch multiplier: 1.054

PRO

Professional Calculator

Calculate total area for complex roofs with multiple sections

ft
ft

Total Roof Area

1,389 sq ft

Sections

2

Total Squares

13.9

Cost Breakdown

Section 1: $1,054 (76%)
Section 2: $335 (24%)

Detailed Breakdown

Section 11,054 sq ft
Dimensions40 × 25 ft (footprint: 1,000 sq ft)
Pitch4/12 (×1.054)
Section 2335 sq ft
Dimensions20 × 15 ft (footprint: 300 sq ft)
Pitch6/12 (×1.118)
Total Roof Area1,389 sq ft
Total Squares13.9
Bundles (10% waste)46
How to Use This Calculator
Start on the From Pitch & Rise tab if you know the vertical rise (measured in your attic from ceiling joist to ridge) and the roof pitch (from plans, a pitch finder tool, or a smartphone inclinometer). Enter the total rise in feet and select the pitch. The calculator instantly returns the horizontal run, and also computes the rafter length and span for reference.

Use the From Rafter tab if you have measured the rafter length along its slope. This is useful when you can reach a rafter with a tape but cannot easily measure rise or run directly. Enter the rafter slope length and select the pitch to get the horizontal run.

The From Roof Dimensions tab lets you work from a ridge-to-eave slope measurement — the kind you get from walking a tape up the roof surface or reading from architectural plans. If your measurement includes the overhang beyond the wall, toggle that option on and enter the overhang portion so the calculator subtracts it to give you the structural run from wall to ridge.

The Formula
From Pitch & Rise: Run = Total Rise / (Pitch Rise / 12) Example: 6/12 pitch, 7.5 ft rise → Run = 7.5 / 0.5 = 15 ft From Rafter: Run = Rafter Length / Pitch Factor Pitch Factors: 4/12 = 1.054, 5/12 = 1.083, 6/12 = 1.118, 8/12 = 1.202, 10/12 = 1.302, 12/12 = 1.414 From Slope Distance: Run = Slope Distance x cos(Pitch Angle) Pitch Angle = arctan(Rise per foot / 12) Structural Run = Total Run - Overhang (horizontal) if measurement included overhang Span = 2 x Run (gable) or 1 x Run (shed)
Example Calculation
For a roof with 8/12 pitch and 8-foot total rise:
• Pitch ratio: 8/12 = 0.667
• Run: 8 / 0.667 = 12.0 ft
• Span (gable): 2 x 12.0 = 24.0 ft
• Rafter length: 12.0 x 1.202 = 14.42 ft
• Pitch angle: arctan(8/12) = 33.69 degrees

Verification: Rise² + Run² = 8² + 12² = 64 + 144 = 208 Rafter = sqrt(208) = 14.42 ft (matches)

Frequently Asked Questions

What is roof run and why does it matter?
Roof run is the horizontal distance from the outside face of the exterior wall to the center of the ridge board. It is the base of the right triangle formed by the rise, run, and rafter. Run matters because it determines: (1) how far rafters reach horizontally, which dictates interior bearing wall placement, (2) soffit width and gutter position, (3) the pitch when combined with rise, and (4) the structural span (span = 2 x run on a symmetrical gable). Every rafter birdsmouth cut, seat cut, and plumb cut is calculated from the run.
How do I calculate run from pitch and rise?
Run = Total Rise / (Pitch Rise / 12). For a 6/12 pitch with a 7.5-foot total rise: Run = 7.5 / (6/12) = 7.5 / 0.5 = 15 feet. Another way: since a 6/12 pitch means the roof rises 6 inches for every 12 inches of run, the run is always 12/6 = 2 times the rise. For a 4/12 pitch, run = rise x (12/4) = rise x 3. For a 12/12 pitch, run = rise x 1 (run equals rise).
How do I calculate run from rafter length?
Run = Rafter Length / Pitch Factor. The pitch factor (also called the rafter factor or secant of the pitch angle) converts slope distance to horizontal distance. Common pitch factors: 3/12 = 1.031, 4/12 = 1.054, 5/12 = 1.083, 6/12 = 1.118, 7/12 = 1.158, 8/12 = 1.202, 9/12 = 1.250, 10/12 = 1.302, 12/12 = 1.414. For a 15-foot rafter at 6/12 pitch: Run = 15 / 1.118 = 13.42 feet.
What is the difference between run and span?
Run is half the span on a symmetrical gable roof. Span is the full horizontal distance from exterior wall to exterior wall (the total width the roof covers). Run goes from one wall to the ridge center. If your run is 15 feet, your span is 30 feet. On an asymmetrical roof (unequal pitches on each side), each side has a different run but they add up to the total span. On a shed (single-slope) roof, the run and span are the same because there is only one slope.
Can I measure run directly without calculating it?
Yes. From inside the attic, use a level and tape measure. Place the level horizontally from the top of the exterior wall plate, then measure the horizontal distance from that point to the center of the ridge board. This direct measurement is the run. Alternatively, from outside, measure the horizontal distance from the exterior wall face to a plumb line dropped from the ridge — use a plumb bob or a level app on your phone. Direct measurement is the most accurate method when the attic is accessible.

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How to Measure a Roof Understanding Roof Pitch Roof Pitch Chart