Sheet Metal Bend Allowance
Bend allowance and flat blank length from K-factor and angle.
Example
You enter
- Thickness (in) 0.06
- Bend angle (deg) 90
- Inside radius (in) 0.125
- K-factor 0.44
- Leg A (in) 2
- Leg B (in) 3
You get
- Bend allowance 0.2378 in
- Flat blank length 4.8678 in
Checked against published worked examples from: SendCutSend.
Details, formula, and sources
Formula or tableBA = (π / 180) × angle × (R + K × t) where K is the K-factor (neutral-axis offset) and t is sheet thickness; outside setback OSSB = (R + t) × tan(angle / 2); bend deduction BD = 2 × OSSB - BA; flat blank = leg1 + leg2 - BD with both legs measured to the outside mold line.
Source and editionMachinery's Handbook by name; SME Sheet Metal Forming by name.
Free accessOlder Machinery's Handbook editions free at archive.org.
What governsEstimate. AHJ and licensed professional govern.
API field namesthickness_in, bend_angle_deg, inside_radius_in, k_factor, leg_a_in, leg_b_in, bend_allowance_in, flat_blank_in
- Default K-factor 0.44 unless the user supplies the shop's own (the neutral axis sits between 0 and 0.5 t)sheet-metal practice; SendCutSend bend guide prints K .43