Kaimera Industries
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KM-T-03 / Sheet Metal · Press Brake

Bend Allowance / Flat Pattern

Bend allowance · bend deduction · K-factor. Standard press-brake geometry, plus a reverse solver that back-calculates the K your brake actually produces.

Live

A free sheet metal bend allowance calculator for press-brake work. Enter material thickness, inside bend radius, bend angle, and K-factor to get bend allowance, bend deduction, outside setback, and total flat pattern blank length. If your parts come out long or short, use the reverse solver: measure one test bend and it back-calculates the K-factor your brake and tooling actually produce.

Flat Pattern Reference

Bend allowance · Bend deduction · K-factor

Bend setup

in
in
deg

K-factor

K
Published K tables from different shops genuinely disagree (stainless is quoted anywhere from 0.38 to 0.50) because K depends more on your process — air bend vs. bottoming, die width, tooling — than on the material alone. Use a preset to get close, then dial it in with the test-bend panel.
Bend deduction — subtract per bend from summed outside dims
0.1036in
90° bend · T 0.0600 · R 0.0600 · K 0.4468
Bend allowance
0.1364in
neutral-axis arc length
Outside setback (OSSB)
0.1200in
tangent to sharp apex
Neutral-axis radius
0.0868in
R + K·T

Flat length

Outside dimensions measured to the apex (mold line), as on a typical print. For an L-bracket: two flanges, one bend. For a channel or hat with identical bends, sum the outside dims into the two fields and set the bend count.
in
in
×
Cut the blank at
1.8964in
= A + B − 1 × 0.1036

Dial in K from a test bend

Cut a coupon of known flat length, bend it once with the setup above, measure both formed flanges (outside, to apex), and this back-solves the K your brake actually produced. Use that K for the real parts.
in
in
in
Your measured K-factor
—
enter all three measurements
Measured bend allowance
—in
flat − A − B + 2·OSSB
Conventions: bend angle is the swept (exterior) angle — 90° for a right-angle bend. Flange dimensions are outside dimensions to the theoretical sharp apex (mold line). Bend allowance = θrad × (R + K·T); outside setback = (R + T)·tan(θ/2); bend deduction = 2·OSSB − BA. These are the standard press-brake relations; the geometry is exact — all real-world uncertainty lives in K and in what inside radius your setup actually forms.

On K: the 0.4468 generic average and the 0.33–0.50 range trace to the Machinery's Handbook–based chart widely reproduced in fabrication references. Per-material tables published by different shops disagree with each other because K is a process property as much as a material one — treat any table value as a starting point and calibrate with test bends on your own brake, your own tooling, and the actual formed radius. Y-factor (as used by Creo and some CAD systems) is the same physics re-parameterized: Y = K × π/2.

§ Common Questions

Frequently asked

What is bend allowance in sheet metal?
Bend allowance is the arc length of the neutral axis through a bend. You add it to the flat lengths of the legs to get the total blank length, because material stretches on the outside of a bend and compresses on the inside.
What K-factor should I use?
0.44 is a common starting point for mild steel with a bend radius close to material thickness; softer aluminum often lands near 0.40–0.42 and harder stainless higher. The only reliable number is the one your own brake produces, so bend a test coupon and use the reverse solver here to measure it.
What is the difference between bend allowance and bend deduction?
Bend allowance is added to the leg lengths measured to the tangent points. Bend deduction is subtracted from the sum of the outside dimensions. Both give the same blank length — they just start from different measurements on the drawing.

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