Minimum Bend Radius Calculator

Why minimum bend radius exists

Bending stretches the outside surface of the material. Past a certain point that surface can no longer elongate and it cracks. The minimum bend radius is the tightest inside radius a given material and thickness will tolerate before that happens.

It is normally expressed as a multiple of material thickness — an "R/t ratio". A 1.0× material at 2 mm thick needs at least a 2 mm inside radius.

Typical multipliers by material

Material	Multiplier (× t)
Aluminum 5052-H32	0.3
Cold-rolled mild steel (CRMS)	0.5
Aluminum 6061-T6	0.5
Hot-rolled mild steel (HRMS)	0.6
Stainless 304 — 2B	1.0
Galvanized steel (G90)	1.0
Stainless 304 — No.4 / brushed	1.5
High-strength steel (Grade 350)	1.5

Grain direction doubles the requirement

Rolled sheet has a grain running along the rolling direction. Bending across the grain is the preferred orientation. Bending with the grain — where the bend line runs parallel to the grain — roughly doubles the minimum radius needed, because the crack path follows the grain.

Design fix before machine fix

If a part cracks, rotating the blank 90° on the nest is usually cheaper and faster than changing tooling or material. Where a flat pattern allows either orientation, nest it to bend across the grain.

Other factors that tighten the limit

Temper — harder tempers crack sooner. 6061-T6 needs a larger radius than annealed 6061.
Surface finish — brushed stainless (No.4) tolerates less than 2B, which is why it carries a higher multiplier.
Edge condition — a rough sheared or laser-cut edge with microcracks will start a crack well before a clean edge does.

Frequently Asked Questions

What is minimum bend radius?

It is the smallest inside radius a sheet metal part can be bent to without cracking the outer surface. It is normally expressed as a multiple of material thickness — for example, 1.0 × t means the inside radius must be at least equal to the material thickness.

Does grain direction affect minimum bend radius?

Yes, substantially. Bending with the grain — bend line parallel to the rolling direction — roughly doubles the minimum radius required, because cracks propagate along the grain. Bending across the grain is always the preferred orientation where the flat pattern allows it.

Which materials need the largest bend radius?

Brushed stainless (304 No.4) and high-strength steel (Grade 350) are the most demanding at around 1.5 × thickness. Aluminum 5052-H32 is the most forgiving at around 0.3 × thickness. Harder tempers always require larger radii than annealed material.

My part cracked even though I used the minimum radius — why?

The most common causes are bending with the grain instead of across it, a rough cut edge with microcracks that act as crack initiators, or material with a harder temper than assumed. Published minimums are guidelines — always test on sample material from the same batch before a production run.