Sheet Metal Bend Deduction Calculator — K-Factor, Flat Pattern & Bend Allowance

How It Works

1. Enter Material Thickness

   Input the material thickness in inches or millimeters. For sheet metal, this is the gauge-equivalent decimal thickness. For plate, use the nominal thickness. The calculator converts between gauge and decimal automatically if you select a standard gauge.

2. Set the Inside Bend Radius

   Enter the inside radius of the bend, which is determined by your die V-opening and punch tip radius. For air bending, the inside radius is typically the die opening divided by 6 to 8. For bottoming, it matches the punch tip radius exactly.

3. Enter the Bend Angle

   Specify the included bend angle in degrees. A standard 90-degree bend is the most common. For obtuse or acute bends, enter the actual included angle. The calculator handles any angle from 1 to 180 degrees.

4. Select or Enter K-Factor

   Choose a K-factor preset for your material and process, or enter a custom value. K-factor represents the location of the neutral axis as a ratio of material thickness. Values range from 0.3 to 0.5, with 0.44 being the most common default for mild steel air bending.

5. Review Bend Deduction and Flat Pattern

   The calculator outputs bend deduction (BD), bend allowance (BA), outside setback (OSSB), arc length, and the flat pattern length for each bend. For multi-bend parts, enter all bends to get the total flat blank size.

Built For

• Press brake operators calculating flat pattern blanks for production sheet metal parts
• Sheet metal fabricators developing flat patterns from engineering drawings with multiple bends
• CNC press brake programmers verifying bend deduction values before running first articles
• HVAC duct fabricators calculating flat patterns for custom duct transitions and fittings
• Job shop estimators calculating blank sizes for material ordering and nesting
• Engineering students learning the relationship between K-factor, bend allowance, and flat pattern development
• Prototype shops developing flat patterns for one-off parts without CAD bend tables

Features & Capabilities

Bend Deduction Method

Calculates BD using the standard formula: BD = 2 x OSSB - BA. This is the amount subtracted from the sum of flange lengths to get the flat pattern. The method most common in American fabrication shops.

Bend Allowance Method

Calculates BA using the arc-length formula along the neutral axis: BA = angle x (pi/180) x (radius + K x thickness). Use this when your shop works in bend allowance rather than bend deduction.

K-Factor Presets

Built-in K-factor values for mild steel (0.44), stainless steel (0.45), aluminum 5052 (0.40), aluminum 6061 (0.42), and copper (0.38). These are starting points for air bending with standard V-die tooling. Custom entry available for empirical values.

Multi-Bend Flat Pattern

Enter multiple bends with their flange dimensions to calculate the total flat blank length. The calculator sums the flange lengths and subtracts the bend deductions to give you the cut-to size for the blank.

Outside Setback Calculator

Calculates OSSB (outside setback) for each bend, which is the distance from the bend tangent point to the apex of the outside mold line. Essential for dimensioning bent parts and locating bend lines on flat blanks.

Gauge-to-Decimal Conversion

Built-in lookup table converting standard sheet metal gauges (10 through 30) to decimal inches for steel, stainless, and aluminum. Eliminates the need for a separate reference chart.

PDF Export

Export your flat pattern calculations as a branded PDF including all input parameters, calculated values, and a bend sequence summary. Attach to job travelers or shop floor work orders.

Frequently Asked Questions