Bend Allowance Calculator

Free tool

Bend allowance calculator

Work out bend allowance, bend deduction and flat pattern length for a single air bend. Enter your real tooling numbers — the inside radius from the punch you actually have, not a theoretical value.

Material

Thickness — T (mm)

Inside radius — R (mm)

Bend angle (degrees of bend)

Flange A — outside (mm)

Flange B — outside (mm)

K-factor used—

R / T ratio—

Neutral axis offset—

Bend allowance (BA)—

Outside setback (OSSB)—

Bend deduction (BD)—

Flat length—

Live section view

The drawing follows your inputs. Focus any field to highlight what it controls.

How the numbers are worked out

BA = (π / 180) × A × (R + K·T)

OSSB = tan(A / 2) × (R + T)

BD = (2 × OSSB) − BA

Flat length = Flange A + Flange B − BD

A = degrees of bend (90 for a right angle) · R = inside radius · T = material thickness · K = K-factor

Flange lengths are outside dimensions — measured to the outside of the bend, which is how a part is normally dimensioned on a drawing. If your flanges are dimensioned to the tangent point instead, add the bend allowance rather than subtracting the deduction.

K-factor by R/T ratio

K-factor is not a fixed material property. It shifts with how tight the bend is relative to thickness, because the neutral axis moves inward as the radius tightens. The material presets above are starting points — these are the ratios that actually drive it.

R / T ratio	Typical K	What it means
Under 1	0.33 – 0.38	Tight bend. Neutral axis shifts well inside centre. Watch for cracking on the outside face.
1 – 3	0.38 – 0.43	The normal working range for most sheet metal work.
Over 3	0.43 – 0.50	Generous radius. Neutral axis approaches the centre of the material.

The honest caveat: published K-factors get you close, not exact. Tooling wear, die opening, grain direction and the specific coil all move the result. If the part matters, bend a test coupon, measure it, and back-calculate your own K. Then use that number — it will beat any table, including this one.

Minimum inside radius — starting points

Bend tighter than this and you risk cracking on the outside of the bend, particularly across the grain. Values are multiples of material thickness.

Material	Min inside radius	Note
Mild steel (CR)	1.0 × T	Forgiving. Can often go tighter with good tooling.
Galvanised steel	1.0 × T	Coating may flake on tight bends.
Stainless 304 / 316	1.3 × T	Springback is significant — expect to overbend.
Aluminium 5052	1.5 × T	Good bending alloy.
Aluminium 6061-T6	2.5 × T	Cracks readily. Bend across the grain where possible.
Copper / brass, annealed	1.0 × T	Very formable while soft.

Grain direction matters more than most drawings admit. Bending parallel to the rolling direction needs a noticeably larger radius than bending across it. On a critical part, specify the grain direction on the drawing — the shop cannot guess it.

Need the drawings as well as the numbers?

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Bend Allowance Calculator