How to use this calculator
- Choose input method. Use a measured flat sample when you have outside leg lengths and blank length, or measured bend allowance when BA is already known.
- Choose a starter band. Pick copper, brass, steel, aluminum or bottoming/coining guidance so the solved K can be compared with a practical starting range.
- Enter bend geometry. Enter bend angle, inside radius and material thickness.
- Enter measured data. Enter the measured bend allowance or the outside legs and measured flat length.
- Review K-factor. Check the K-factor, neutral-axis offset, bend allowance, deduction, outside setback and delta from the selected starter band.
How it works
K-factor is the neutral-axis offset divided by material thickness. Starting from bend allowance: BA = theta x (R + K x t) Rearranged: K = (BA / theta - R) / t where theta is the bend angle in radians, R is inside radius and t is thickness.
The starter K band is a sanity check for the material and forming method you selected. It does not change the solved K-factor. It gives a low and high starting value, then the calculator reports delta from starter midpoint so you can see whether the measured coupon agrees with a typical copper, brass, steel, aluminum or bottoming/coining range before carrying the value into a batch.
If you measured a flat sample from outside dimensions, the calculator first finds bend deduction and bend allowance: BD = L_A + L_B - L_flat OSSB = (R + t) x tan(A / 2) BA = 2 x OSSB - BD Use the solved K with the bend allowance calculator, bend deduction calculator or box flat pattern calculator. The shorter K-factor calculator page and K-factor for copper page cover exact search paths; bend allowance K-factor, sheet metal K-factor chart, aluminum K-factor and brass K-factor handle adjacent material and formula questions before users are ready to back-solve a coupon.
| Use this result | What it answers |
|---|---|
| K-factor | The measured neutral-axis position from your bend allowance or flat sample. |
| Starter K low / high | The selected material/process starting range, such as ductile copper air bend. |
| Delta from starter midpoint | How far the measured coupon sits from the middle of the selected starter band. |
| Neutral-axis offset | The physical offset K x t from the inside bend surface. |
K-factor for copper
The K-factor for copper is not a single handbook constant. Copper sheet and copper alloys move the neutral axis according to temper, inside radius, thickness, grain direction, die opening and whether the bend is air-bent, bottomed or coined. Use the ductile copper air-bend starter band of about K = 0.30-0.40, or the hard-copper/brass band of about K = 0.40-0.50, only as a starting value. Then bend a copper coupon and use this calculator to back-solve the actual copper K-factor for that setup and compare it with the selected starter band. The copper K-factor page gives the coupon workflow in a tighter material-specific format; use the aluminum K-factor and brass K-factor pages for those material-specific bend searches.
Worked example
Verified against the live calculator
A 90 degree test bend in 2 mm sheet with a 3 mm inside radius has outside
legs of 50 mm and 50 mm, and a measured flat length
of 95.749 mm. The bend deduction is 4.251 mm,
outside setback is 5.000 mm, bend allowance is
5.749 mm, and the solved K-factor is about
0.330. With the ductile copper air-bend starter band selected
(K = 0.30-0.40), that coupon sits inside the band and is
0.020 below the band midpoint of 0.350.
Frequently asked questions
What is the sheet-metal K-factor?
K-factor is the neutral-axis position as a fraction of material thickness. If K = 0.33, the neutral axis is 0.33 times the thickness in from the inside bend surface.
How do you calculate K-factor from bend allowance?
Rearrange the bend allowance formula: K = (BA/theta - R) / t, where theta is the bend angle in radians, R is inside radius and t is material thickness.
How do you calculate K-factor from a flat sample?
Measure the outside leg lengths and the flat blank length. Bend deduction is outside leg A plus outside leg B minus flat length. Bend allowance is then 2 x outside setback minus bend deduction, and K is solved from that bend allowance.
Is K-factor a material table value?
Not by itself. K-factor depends on material, tooling, bend radius, die opening, grain direction and bend method. Use measured production bends when accuracy matters.
What K-factor should I use for copper?
Copper does not have one fixed K-factor. Ductile copper air bends often start around K = 0.30 to 0.40, while harder copper tempers and brass/copper alloys often move toward 0.40 to 0.50. Bend a copper coupon and back-solve K from the measured bend allowance for production flats.
What is the starter K band for?
The starter band is only a comparison range for the selected material and forming method. Use it for a first blank estimate, then replace it with the measured K-factor from a coupon made with the production material, punch, die, grain direction and bend method.
Method & assumptions
- The bend angle is the degrees the flange turns from flat, not the included angle between legs.
- Outside leg dimensions must be measured on the outside mold line for the flat-sample method.
- Starter K bands are practical first-pass ranges, not material standards; measured coupon data wins when the sample matches the production setup.
- K-factor is empirical. Build a coupon with the same material, grain direction, punch, die and bend method before using the value for production flats.
- This calculator does not choose tooling, minimum bend radius, springback allowance or forming limits.