Sheet metal

galvanized steel 18-gauge bend allowance estimator

Prefilled bend allowance, deduction, and developed-length estimates for galvanized steel at 0.0478 in / 1.214 mm nominal thickness. Treat as a screening reference; verify against material certificates and a coupon bend before production.

galvanized steel18 ga0.0478 in1.214 mm

Interactive calculator

Prefilled Bend Allowance Calculator

MaterialGaugeThickness

Inside radius

Bend angle

deg

K-factor

Bend allowance0.1254 in

Neutral axis radius0.0798 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/.

K-factor variance

galvanized steel 18 ga process range

Process	Min K	Typical K	Max K	Use note
Air bending	0.33	0.42	0.50	Free bend in a V-die; most BendMath defaults start here.
Bottoming	0.42	0.44	0.46	Punch drives the sheet close to the die angle with higher contact.
Coining	0.46	0.48	0.50	High-tonnage forming that sets the bend more deeply into the tooling.

Source: SheetMetal.Me K-Factor chart; AUST IPE 2212 Manufacturing Processes-II Sessional generic K-factor table; shop starting estimate for this alloy family.

Values vary with grain direction, inside radius-to-thickness ratio, V-die opening, coating, temper, and tooling condition. Verify with a coupon test bend.

Material notes for this combination

Starting references for galvanized steel at 18 ga

Recommended starting V-die opening

About 0.287 in (factor 6T). Expect an air-bend inside radius near 0.046 in. Verify with the tooling supplier's chart.

Starting minimum inside bend radius

About 0.060 in (1.25T) for this material family. Bends across the grain typically need a larger radius.

Tonnage multiplier vs mild steel

1.05× the mild-steel baseline. Cross-check the press brake's rated tonnage and the punch/die load rating before forming.

Grain and process

Coating condition matters. Tight bends can crack the zinc layer at the outside fiber; check coating spec before committing radius.

Default K-factor (0.42), minimum-radius factor (1.25T), and tonnage multiplier (1.05×) for galvanized steel are starting estimates. See /reference/data-sources/ for provenance.

Other gauges in galvanized steel

galvanized steel · 22 ga galvanized steel · 20 ga galvanized steel · 16 ga galvanized steel · 14 ga galvanized steel · 12 ga galvanized steel · 11 ga galvanized steel · 10 ga galvanized steel · 7 ga

18-gauge in other materials

5052-H32 aluminum · 18 ga 6061-T6 aluminum · 18 ga 3003-H14 aluminum · 18 ga 304 stainless · 18 ga 316 stainless · 18 ga mild steel · 18 ga cold rolled steel · 18 ga

Static reference

Common thickness, radius, and angle combinations

Swipe table horizontally for more columns.

Gauge	Thickness	Inside radius	Angle	Bend allowance	Bend deduction
18 ga	0.0478 in / 1.214 mm	0.0598 in / 1.518 mm	45 deg	0.0627 in	0.0264 in
16 ga	0.0598 in / 1.519 mm	0.0897 in / 2.278 mm	90 deg	0.1804 in	0.1186 in
12 ga	0.1046 in / 2.657 mm	0.1308 in / 3.321 mm	120 deg	0.3659 in	0.4494 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

What thickness is 18 gauge galvanized steel on this page?

This page uses 0.0478 in / 1.214 mm as the nominal 18-gauge thickness for steel sheet. Mill thickness can vary; check the material certificate.

Can these galvanized steel bend values be used directly for production?

No. Use them as screening estimates. Confirm K-factor, bend radius, and tonnage with a coupon bend, the supplier's data, and the press brake's rated capacity before production.

What K-factor should I use for galvanized steel 18 gauge?

Use K=0.42 as a starting point for air bending galvanized steel 18 gauge. Bottoming and coining use different typical values (0.44 and 0.48 here), so verify the value with the actual tooling and a coupon bend.