Sheet Metal Bend Calculator

How It Works

Bending sheet metal is not the same as folding paper. When a metal sheet is bent around a die, the outer fiber stretches and the inner fiber compresses. Somewhere between the two lies a neutral axis where the fiber neither stretches nor compresses — and the length of the flat pattern is determined by the length of that neutral axis, not by measuring outer or inner corner geometry.

The neutral axis position is described by the K-factor — a fraction between 0 and 0.5 giving the distance from the inner surface to the neutral axis, as a fraction of material thickness. Values from Machinery's Handbook, 30th ed., Sheet Metal Working section, and the SendCutSend K-factor guide (sendcutsend.com/blog):

• Soft aluminum, thin gauge — K ≈ 0.33 for bends where R/T ≥ 2, dropping to 0.25 for tight bends where R/T < 1
• Mild steel, medium gauge — K ≈ 0.33-0.42 depending on radius
• Stainless steel — K ≈ 0.42-0.50, higher than mild steel because stainless work-hardens more during bending
• Copper, brass — K ≈ 0.33-0.40

The bend allowance — the arc length of the neutral axis through the bend — is:

BA = (π × A / 180) × (R + K × T)

where A is bend angle in degrees, R is inside bend radius, T is material thickness, and K is the K-factor. The total flat pattern length is: leg₁ + BA + leg₂, where each leg is measured from the flange end to the tangent point where the bend starts.

An alternative formulation uses bend deduction — how much material you subtract from the sum of outer flange lengths (the dimensions you would naively measure on the folded part). Both give the same result; some shops prefer one over the other based on how they lay out parts.

How to Use This Calculator

1. Choose your material — mild steel, aluminum, stainless, copper, or brass — from the dropdown. The tool preloads a typical K-factor.
2. Enter material thickness — in millimetres. Match to your actual gauge, not the nominal.
3. Enter inside bend radius — from the die you're using (typically 1×T to 4×T; the die manufacturer's spec sheet lists standard radii).
4. Enter bend angle — 90° is typical; 60° or 120° for angled brackets.
5. Enter the leg dimensions — measured from the flange end to the bend tangent point, for each leg of the bend.
6. Read the flat pattern length — the calculator reports both bend allowance and bend deduction so you can use either method.

Real-World Use Cases

Steel bracket

A hobbyist fabricates a 2 mm mild-steel L-bracket with a 4 mm inside bend radius and a 90° bend. The K-factor for that material and R/T ratio is 0.33. Bend allowance = (π × 90 / 180) × (4 + 0.33 × 2) = π/2 × 4.66 = 7.32 mm. If each leg needs to be 50 mm long from the bend tangent, the flat pattern is 50 + 7.32 + 50 = 107.32 mm. Cut the sheet exactly to that length before bending.

Aluminum enclosure

A maker folds a 1.5 mm aluminum enclosure box with three 90° bends. K-factor is 0.33 for the material/radius combo. Each bend adds a bend allowance of about 5.5 mm to the flat pattern. For a 100 × 100 × 60 mm box (six panels total), the pattern is a cross with three bend allowances between the panels. The tool computes and lays out all three bend allowances so the maker can cut the flat blank exactly to size.

Stainless steel formed pan

A metalworker forms a 1 mm stainless food service pan with a 3 mm inside radius on all four bends. Stainless K-factor is 0.42 for that R/T. Bend allowance = π/2 × (3 + 0.42 × 1) = 5.37 mm per corner. A pan with a 200 × 150 mm bottom and 50 mm walls needs the flat pattern to include 5.37 mm at each of the four corners, giving a slightly larger cut blank than a naive layout would suggest.

Tips & Safety Notes

• Test-bend on scrap first. K-factor tables are averages; a specific press brake, punch, and die combination will produce a slightly different K. Bend a scrap strip of the same material and thickness, measure the finished dimensions, and back-calculate your actual K.
• Radius the die, not the bend. The inside bend radius is set by the die you're using. If your material is thicker than the die's minimum, the bend radius follows the die; if thinner, the material may crack at the outside fiber before completing the bend.
• Watch for grain direction. Bending parallel to the rolling grain of the sheet cracks more easily than bending across the grain, especially on aluminum and copper.
• Spring-back is real. After releasing pressure, the bend springs back by 2-3° for most materials. Bend past your target angle by that amount to land on it.
• Deburr edges before bending. A rough or nicked edge will crack under the tension of a bend; a smooth edge won't.

Limitations & What This Tool Cannot Do

• K-factor is a material and radius approximation. K depends on material, radius, and bending method (air bend vs bottom bend vs coining). The tool uses published averages; a shop with a specific press brake and repeated parts should measure their actual K by test bending and enter the custom value.
• No spring-back compensation. Assumes the bent angle equals the target angle after release. Actual spring-back requires bending 2-3° past target for most materials — the tool does not model this; you adjust the bend angle input to compensate.
• No minimum-flange check. Does not warn if a required flange is too short to be safely held during bending on a press brake. Rule of thumb: the flange must be at least 4× the material thickness plus the die shoulder, so a 2 mm sheet on a standard V-die needs at least ~12-15 mm minimum flange.

FAQ

What is a K-factor and how do I choose the right one for my material?

The K-factor is the fraction of material thickness where the neutral bending axis lies, measured from the inner surface. Typical values: 0.33 for mild steel and most soft materials, 0.42 for stainless steel, 0.25 for tight bends where R/T < 1. If you have a specific press brake and die combo you use repeatedly, measure a test bend and use your own value.

What is the difference between bend allowance and bend deduction?

Bend allowance is the arc length of the neutral axis through the bend — the amount of material "hidden" inside the bend. Bend deduction is the amount you subtract from the sum of outer flange lengths to get the same flat pattern. Both give identical results; different shops prefer different methods based on how they draw the flat pattern.

How does the bend radius affect the flat pattern length?

Larger inside bend radius means longer bend allowance (more material curves through the bend) and a longer flat pattern. Smaller radius means shorter bend allowance but risks cracking at the outer fiber if R/T < 1. Every mm of radius change shifts the flat pattern by π/2 × radius change on a 90° bend.

Can I use this calculator for aluminum, steel, and stainless equally?

Yes, with material-specific K-factors. Aluminum and mild steel use K ≈ 0.33 for typical R/T ratios; stainless steel needs a higher K of 0.42-0.50 because it work-hardens more during bending. The calculator preloads K by material, but you can override with a shop-measured value.

What happens if the inside bend radius is smaller than the material thickness?

Cracking. When R/T < 1, the outer fiber stretches beyond the material's ductility limit and cracks appear at the outside of the bend. For most steels and aluminum, keep R ≥ T; for stainless keep R ≥ 1.5×T; for brittle alloys keep R ≥ 2×T. Some soft aluminum can go to R < T with careful die support.

How do I compute the flat pattern for a part with multiple bends?

Compute the bend allowance for each bend separately, then sum: flat = leg₁ + BA₁ + leg₂ + BA₂ + … + BA_n + leg_(n+1). The calculator handles multi-bend parts by prompting for each leg and bend in sequence, then reports the total flat blank size ready for shear or laser-cutting.

Related Tools

• Wax-to-Metal Casting Calculator — Same-lab metalworking tool for a different fabrication method
• Wood Shrinkage Calculator — Materials-lab sister tool for dimensional prediction
• Segmented Woodturning Calculator — Adjacent geometry math

References

• Machinery's Handbook, 30th ed., Sheet Metal Working section — bend allowance and bend deduction formulas
• SendCutSend K-factor and bend allowance guide (sendcutsend.com/blog) — material-specific K-factors and press brake practice
• SmartFab Press Brake Design Guide — die selection and minimum flange rules

Reviewed by the Craft Calc Lab team on August 4, 2026.