Sheet metal

Bend Allowance & Developed Length Calculator

Estimate the bend allowance — the length of the neutral axis through a bend — from angle, inside radius, thickness, and K-factor. Use this for unfolding a part to a flat blank.

mild steel16 ga0.0598 in1.519 mm

Interactive calculator

Bend Allowance & Developed Length Calculator

MaterialGaugeThickness

Inside radius

Bend angle

deg

K-factor

Bend allowance0.1334 in

Neutral axis radius0.0849 in

Assumptions

Verify before production

Calculator values are estimates. Confirm bend deductions, springback, tooling, grain direction, coating condition, and safe tonnage with the material and tooling supplier before production.

Air bending and common gauge tables are assumed unless edited.
Actual results vary by tooling, grain direction, coating, temper, and springback.
Machine capacity and tooling load ratings must be checked independently.
Result is a screening estimate for shop planning. Verify against material certificate, tooling chart, machine rating, and a coupon test before production.
Single-bend math only. Multi-bend parts also need bend order, tooling clearance, and accumulated tolerance checks.
Bottoming and coining produce different K-factor behavior than air bending.

Default material and formula constants are documented at /reference/data-sources/.

Static reference

Common thickness, radius, and angle combinations

Swipe table horizontally for more columns.

Gauge	Thickness	Inside radius	Angle	Bend allowance	Bend deduction
16 ga	0.0598 in / 1.519 mm	0.0598 in / 1.519 mm	45 deg	0.0667 in	0.0324 in
12 ga	0.1046 in / 2.657 mm	0.1569 in / 3.985 mm	90 deg	0.3155 in	0.2075 in

Formula used

What this calculator computes

BA = (π / 180) × A × (r + K × t)

BA: Bend allowance (developed arc length along the neutral axis) (in or mm)
A: Bend angle (rotation angle of the bend, in degrees) (deg)
r: Inside bend radius (in or mm)
t: Material thickness (in or mm)
K: K-factor (neutral-axis position) (0 to 1)

Bend angle A is the angle of rotation through which the sheet is bent (the supplement of the included angle for less-than-180° bends). A flat sheet bent so its flanges meet at 90° corresponds to A = 90°. If your drawing labels the included angle, convert with A = 180° − included angle before entering it.
Developed flat length for a single bend = sum of straight flange lengths + BA. For multi-bend parts, add BA for every bend.

Worked example

One numeric walk-through

Inputs

Thickness t: 0.0598 in
Inside radius r: 0.0598 in
Bend angle A: 90°
K-factor: 0.42

Steps

A in radians: 90 × π/180 = 1.5708.
r + K·t = 0.0598 + 0.42 × 0.0598 = 0.0849 in.
BA = 1.5708 × 0.0849 ≈ 0.1334 in.

Result

Bend allowance ≈ 0.1334 in. Add this to the straight flange lengths to get the developed flat length for one bend.

Common mistakes

Things to check before trusting the number

Using degrees in the formula without converting to radians.
Adding bend allowance to outside-flange dimensions — that double-counts material; use bend deduction instead.
Forgetting to repeat the bend allowance addition for every bend in a multi-bend part.
Ignoring grain direction. Bending across the grain typically increases the inside radius needed and shifts K.

FAQ

Specific checks for this calculator

Is bend allowance used for flat pattern unfolding?

Yes. Bend allowance is the developed length of the neutral axis through the bend and is added to the straight flange lengths to get the flat blank length.

How do I calculate developed length from bend allowance?

Developed flat length = sum of straight flange lengths + bend allowance for every bend. For two flanges and one bend, add one bend allowance.

Bend Allowance & Developed Length Calculator

Bend Allowance & Developed Length Calculator

Common thickness, radius, and angle combinations

What this calculator computes

One numeric walk-through

Inputs

Steps

Result

Things to check before trusting the number

Move between connected shop calculations

Specific checks for this calculator

Is bend allowance used for flat pattern unfolding?

How do I calculate developed length from bend allowance?