The calculation, without hidden assumptions
Calculate the neutral-axis arc consumed by a sheet-metal bend, then translate it into outside setback, bend deduction and flat blank length. The reverse mode can back-solve K-factor from a measured bend allowance instead of pretending one K-factor fits every material and press-brake setup.
How to use this calculator
Choose millimetres or inches; keep thickness, radius, flange lengths and measured allowance in the same unit.
Enter sheet thickness, inside bend radius, bend angle swept through, and a defensible K-factor.
For an outside-dimension flat blank, enter both outside flange lengths.
Use reverse mode with a measured bend allowance from a coupon when calibrating a real material/tooling setup.
Verify production blanks with the actual material, tooling and test bend before releasing a batch.
Where people use it
- •Press-brake flat-pattern planning for one bend.
- •Compare K-factor assumptions before cutting sheet.
- •Calculate bend deduction from outside dimensions.
- •Back-solve K-factor from a measured coupon bend allowance.
- •Check metric and imperial shop calculations with the same geometry.
Example: 2 mm sheet, 2 mm radius, 90° bend, K = 0.42
BA = (π/2) × (2 + 0.42 × 2) = 4.461 mm. OSSB = (2 + 2) × tan(45°) = 4 mm, so BD = 8 − 4.461 = 3.539 mm. Two 50 mm outside flanges give a 96.461 mm flat blank.
What the result does not assume
- •K-factor is an empirical neutral-axis model, not a universal material constant. It changes with material condition, radius/thickness ratio, tooling and forming method.
- •Bend angle here means the angle swept through by the bend; a square flange is 90°. Do not silently substitute a drawing included angle.
- •The outside-dimension flat-length result assumes two outside mold-line flange dimensions and one bend. Inside-tangent dimensions use a different bookkeeping convention.
- •Hems, curls, joggles, coined special forms and multi-bend interaction require process-specific methods.
- •Springback, grain direction, coating, tolerances and actual formed radius are not predicted by this geometry.
- •For production, calibrate against a coupon or approved bend table for the exact material, tooling and machine.
Frequently asked questions
What is bend allowance?+
Bend allowance is the arc length along the neutral axis through the bend: BA = angle in radians × (inside radius + K × thickness).
What is bend deduction?+
Bend deduction is 2 × outside setback − bend allowance. Subtract it from the sum of two outside flange dimensions to estimate the flat blank for one bend.
What K-factor should I use?+
Use your validated shop bend table or coupon result when available. Published typical values are only starting points because K-factor changes with material, radius/thickness ratio and forming process.
Is a 90° bend angle entered as 90°?+
Yes. This calculator defines bend angle as the swept forming angle, so a square bend is 90°.
Can I calculate K-factor from a test bend?+
Yes. Enter the measured bend allowance and the calculator back-solves K = (BA/θ − R)/t, then flags values outside the physically typical neutral-axis range.
Does this replace a press-brake bend table?+
No. It is transparent geometry for planning and checking. Production work should use validated machine/tooling/material data and test bends.
Semantic next steps
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