---
type: chart
title: "Sheet Metal Bend Radius & K-Factor Chart"
tables: 4
url: https://manufacturingprocesses.org/charts/sheet-metal-bend-radius-k-factor
---

# Sheet Metal Bend Radius & K-Factor Chart

Minimum inside bend radius by alloy and temper as multiples of thickness, K-factor against R/t, and K by bending method — with the two conflicting published K sets shown side by side.

All radii on this page are **inside radius, on a 90° bend, at room
temperature, bending across the grain**, expressed as a multiple of material
thickness t.

Two radius columns are given because the sources genuinely disagree, and the
disagreement is meaningful. **Absolute min** is the crack threshold reported
by fabricators: achievable, with no margin. **Design default** is what to put
on a drawing — it adds margin for sheet thickness tolerance, lot-to-lot
ductility variation and the punch tips a shop actually owns.

The **K-factor** is the position of the neutral axis: distance from the
inside surface to the neutral axis, divided by thickness. It feeds the bend
allowance, BA = π · (R + K·t) · A / 180, with A in degrees. K can never
exceed 0.5 — that would put the neutral axis past mid-thickness, which is
physically impossible — and in practice it runs 0.30 to 0.50.

## Minimum inside bend radius by material (90°, across grain, multiples of thickness t)

| Material | Absolute min | Design default | Representative K-factor |
| --- | --- | --- | --- |
| Aluminum 1100-O / 3003-O | 0 × t | 0.5 × t | 0.38 |
| Aluminum 3003-H14 | 0.5 × t | 1 × t | 0.4 |
| Aluminum 5052-O | 0 × t | 0.5 × t | 0.4 |
| Aluminum 5052-H32 | 0.5 × t | 1 × t | 0.42 |
| Aluminum 6061-T4 | 1 × t | 1.5 × t | 0.41 |
| Aluminum 6061-T6 | 2 × t | 2.5 × t | 0.4 |
| Aluminum 6063-T5 | 1.5 × t | 2 × t | 0.41 |
| Aluminum 7075-T6 | 4 × t | 5 × t | 0.33 |
| Mild steel / CRS 1008-1018 | 0.5 × t | 1 × t | 0.41 |
| Hot-rolled steel | 1 × t | 1.5 × t | 0.41 |
| Galvanized steel G90 | 1 × t | 1.5 × t | 0.41 |
| HSLA 50 ksi | 1 × t | 1.5 × t | 0.42 |
| DP590 | 1.5 × t | 2 × t | 0.43 |
| DP780 | 3 × t | 4 × t | 0.45 |
| DP1180 | 5 × t | 6 × t | 0.45 |
| Stainless 304 / 304L | 0.5 × t | 1 × t | 0.38 |
| Stainless 316 / 316L | 0.5 × t | 1 × t | 0.38 |
| Stainless 430 | 1 × t | 1.5 × t | 0.4 |
| Copper C110 soft | 0 × t | 0.5 × t | 0.35 |
| Copper half-hard | 0.5 × t | 1 × t | 0.38 |
| Brass C260 | 0.5 × t | 1 × t | 0.38 |
| Titanium Grade 2 | 2.5 × t | 3 × t | 0.4 |

## K-factor against R/t — the dominant relationship

| R/t | K-factor |
| --- | --- |
| 0.1 | 0.21 |
| 0.2 | 0.22 |
| 0.3 | 0.23 |
| 0.4 | 0.24 |
| 0.5 | 0.25 |
| 0.6 | 0.26 |
| 0.7 | 0.27 |
| 0.8 | 0.3 |
| 1 | 0.31 |
| 1.2 | 0.33 |
| 1.5 | 0.36 |
| 2 | 0.37 |
| 2.5 | 0.4 |
| 3 | 0.42 |
| 5 | 0.46 |
| 75 | 0.5 |

## Typical K-factor by material — two published sets that disagree

| Material | Machinery's Handbook, 90° tables | Fabricator practical values |
| --- | --- | --- |
| Soft brass, soft copper | 0.35 | 0.35–0.38 |
| Hard brass/copper, mild steel, aluminum | 0.41 | — |
| Aluminum 5052 (soft) | — | 0.42 |
| Aluminum 6061-T6 | — | 0.4 |
| Cold-rolled steel | 0.45 | 0.38 |
| Stainless steel | — | 0.38 |
| Hard brass, bronze, spring steel | 0.45 | — |
| Hardened steel, 7075 | — | 0.33 |

## K-factor by bending method

| Method | K min | K max |
| --- | --- | --- |
| Air bending | 0.33 | 0.42 |
| Bottoming | 0.4 | 0.45 |
| Coining | 0.45 | 0.5 |

## Notes

**The dominant variable for K is R/t, not the material.** The neutral axis
shifts inward as the bend gets tighter, from K ≈ 0.21 at R/t = 0.1 to the
0.50 asymptote at very large R/t. Pick K from R/t first, then calibrate
against a test bend on the actual stock and tooling. A 0.05 error in K
produces roughly 0.3 mm of deviation per bend on 3 mm stock — four bends in
series and the part is 1.2 mm off.

**K-factor by material is genuinely contested.** *Machinery's Handbook* gives
cold-rolled steel K = 0.45; production fabricators use 0.38 for the same
material. Both are published in good faith: the difference is bending method
(bottoming and coining versus air bending) and R/t. Both columns are shown
rather than averaged.

**Minimum bend radius is the other place the sources conflict.** For mild
steel one fabricator source says 0.5 t while two design guides say 1.0 t;
for 304 stainless the spread is 0.5 t to 1.5 t; for copper it runs from
0 t (soft) to 1.0 t. Rather than average — which would produce a number no
source supports and no fabricator would honour — both positions are shown.
The one place a design guide was rejected is 6061-T6, where it claims 1.5 t
against 2.0 t from two other sources; 8–10% elongation makes 2.0 t the
defensible figure.

**Terminology trap.** Some guides label the bend-radius multiplier itself
"k factor" (min radius = k × t). That is not the K-factor. The K-factor is
the neutral-axis ratio and is always ≤ 0.5. The two are unrelated quantities
that happen to share a letter.

**Grain direction:** bending parallel to the rolling direction stretches the
weak axis. Add 50–100% to the minimum radius for with-grain bends, or rotate
the flat pattern. **Feature clearance:** keep holes and slots at least
2.5 × t + bend radius from the bend line, measured to the near edge, or they
distort. **Springback:** expect 1–3°, so a 90° bend is typically overbent to
about 87° in cold-rolled steel or 88° in 5052.

**Thickness matters too.** Minimum radius grows with thickness even in the
same alloy. A working rule for steel: up to about 6 mm use 0.8–1.0 t, 6–12 mm
use about 1.2 t, above 12 mm expect 1.5 t or more. And the bend takes the
punch tip radius plus springback correction — if the theoretical minimum is
1.0 mm but the smallest tip on the floor is 1.5 mm, you get 1.5 mm.

## FAQ

### What is the minimum bend radius for 6061-T6 aluminum?

2.0 × t as an absolute minimum and 2.5 × t as a design default. 6061-T6 has only 8–10% elongation and cracks easily. If you need a tighter radius, bend in the T4 or O temper and age afterwards, or switch to 5052-H32, which bends at 0.5 × t.

### What is a good default K-factor?

Pick it from R/t rather than from the material: about 0.31 at R/t = 1.0, 0.36 at 1.5 and 0.42 at 3.0. If you have to pick blind, 0.40 to 0.42 is a reasonable air-bending default — then calibrate against a test bend.

### Can the K-factor be greater than 0.5?

No. K is the distance from the inside surface to the neutral axis divided by thickness, so K > 0.5 would put the neutral axis past mid-thickness. In practice it runs 0.30 to 0.50, approaching 0.50 only at very large bend radii.

### What is the bend allowance formula?

BA = π · (R + K·t) · A / 180, where R is the inside radius, t the material thickness, K the K-factor and A the bend angle in degrees.

### How far should a hole be from a bend line?

At least 2.5 × t plus the bend radius, measured from the bend line to the near edge of the hole. Closer than that and the hole distorts into an oval as the material stretches through the bend.

### Does grain direction change the minimum bend radius?

Yes, substantially. Bending parallel to the rolling direction stretches the weak axis; add 50–100% to the minimum radius for a with-grain bend, or rotate the flat pattern so the bend runs across the grain.

### How much springback should I allow?

One to three degrees for common sheet metals, so a 90° bend is typically overbent to roughly 87° in cold-rolled steel or 88° in 5052 aluminum. Higher-strength alloys spring back more.

## Sources

- [PrecisionSmith — Sheet Metal Bend Radius Chart](https://precisionsmith.com/guides/sheet-metal-bend-radius-chart/)
- [MachineMFG — K Factor Calculator for Sheet Metal Bending](https://www.machinemfg.com/k-factor-calculator/)
- [DurmaPress — Minimum Bend Radius for Sheet Metal: Chart, Formula & Design Guide](https://www.durmapress.com/minimum-bend-radius-for-sheet-metal-chart-formula-design-guide/)

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- [Roll Forming](https://manufacturingprocesses.org/processes/forming/roll-forming.md)

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---

*Source: [Manufacturing Processes](https://manufacturingprocesses.org/charts/sheet-metal-bend-radius-k-factor)*

*Last updated: August 11, 2026*