How to calculate & schedule sheet metal unfolded width in Revit.
An engineering guide for architects, facade consultants, and sheet metal fabricators on calculating bend allowances, stretch-out girth, and fabrication schedules in Autodesk Revit.
01Girth Fundamentals
Understanding Girth (Stretch-Out Blank Width)
When sheet metal fabricators bend flat aluminum or galvanized steel coils into parapet copings, drip trims, or facade sills, the blank sheet must be cut to an exact unfolded width. Under- or over-estimating girth causes coil waste or inadequate weatherproofing lap dimensions.
Mathematical Formula
Blank Width (Girth) = Sum of Flange Leg Lengths - Bend Deductions
Practical Takeaway
Always provide total stretch-out width on schedules so fabricators know coil slitting widths.
02Neutral Axis Mechanics
The K-Factor & Neutral Axis Shift
During air bending, sheet metal undergoes compression along its inside radius and tension along its outside radius. The neutral axis is the imaginary plane that neither stretches nor compresses. In architectural flashings (typically 0.7mm to 2.0mm thickness), the K-factor is approximately 0.33 to 0.40.
Mathematical Formula
Bend Allowance (BA) = (π / 180) × Angle × (Inside Radius + K × Thickness)
Practical Takeaway
For standard architectural trims with 90° bends, bend compensation is roughly 1.5× to 2× material gauge.
Standard Revit Wall Sweeps and In-Place generic models only report line length (linear run) or volume. They cannot compute 2D cross-sectional perimeter girth, leaving architects to maintain disconnected Excel spreadsheets that fall out of sync whenever facade walls shift.
Manual formulas in shared parameters break down when profiles contain variable drip return hems.
04Automated Workflow
Automated Blank Width Takeoffs with Karo.Segments
Karo.Segments automatically analyzes the 2D parametric vector cross-section of each parapet coping, sill, and drip flashing. It computes the unfolded girth width dynamically, factoring in hem folds, slope angles, and thickness.
Generates native Revit schedules and Excel cut lists ready for metal brake operators.
05Fabricator Handoff
Meeting Subcontractor Fabrication Requirements
Metal fabricators require three specific pieces of data to quote and cut architectural flashing packages: Mark tag, linear length of cut sections, and total blank width (coil gauge). Karo.Segments packages all three into a single clean schedule view.