How this instrument works
When sheet metal bends, the material on the outside of the bend stretches and the material on the inside compresses, but somewhere through the thickness there's a neutral axis that neither stretches nor compresses — its length stays the same as the flat sheet before bending. K-factor describes where that neutral axis sits, as a fraction of the material thickness measured from the inside surface: K = t/T, where t is the distance to the neutral axis and T is the total thickness.
Bend allowance is the length of flat material a single fold actually consumes, and it's what a fabricator works with when flattening a bent part back into a cut pattern: BA = angle (in radians) × (inside radius + K × thickness). A tighter radius, a sharper angle, or a K-factor closer to the inside surface all shrink that allowance, since less material is being drawn from the neutral axis to complete it.
K-factor is not one universal number — it depends on the material, its temper, the bending method (air bending versus bottom bending or coining), and the ratio of inside radius to thickness. 0.44 is a widely cited general-purpose default for mild steel, but aluminum commonly runs lower, roughly 0.35–0.42, and stainless steel higher, roughly 0.46–0.50; this calculator leaves K-factor fully editable so you can use a shop-tested value for your actual material and process rather than a generic default.
- Enter Material thickness (T) — the sheet's actual gauge thickness in inches.
- Enter Inside bend radius (R) — the radius of the bend as measured on the inside surface of the material.
- Enter Bend angle — how many degrees the material bends through, commonly 90° for a right-angle bend.
- Enter K-factor — 0.44 is a common mild-steel default; adjust for your material and bending method (aluminum, stainless, or a shop-tested value).
- Read Bend allowance — the flat-pattern length this bend consumes, used to lay out an accurate flat pattern before bending.
Worked example — a 90° bend in 14-gauge steel
Set Material thickness to 0.075in (14-gauge mild steel), Inside bend radius to 0.125in, Bend angle to 90°, and K-factor to 0.44, a widely-cited general-purpose default for mild steel. Bend allowance = 90 × (π/180) × (0.125 + 0.44 × 0.075) = 1.5708 × 0.158 = 0.2482in. The neutral axis sits 0.44 × 0.075 = 0.033in in from the inside surface.
That 0.2482in is the length of flat material this single 90° bend consumes — a fabricator accounts for that figure when converting a finished part's dimensions back into a flat cut pattern. Switch to 16-gauge (0.0625in) aluminum with a K-factor of 0.41, inside aluminum's commonly cited 0.35–0.42 range, and the same 90° bend at a matching radius produces a smaller 0.1384in bend allowance, since both the thickness and K-factor shrank.
Questions
What K-factor should I use for my material?
0.44 is a commonly cited general-purpose default for mild steel, but it's a rough industry compromise, not a measured property of your specific sheet. Aluminum typically runs lower, roughly 0.35–0.42, and stainless steel typically runs higher, roughly 0.46–0.50; the exact value also shifts with temper, bending method (air bending, bottom bending, or coining), and how the inside radius compares to material thickness. For precision work, the most reliable K-factor comes from a physical test bend on your actual material and tooling, not a table default.
What's the difference between bend allowance and bend deduction?
Bend allowance is the length of material a fold itself consumes, used when building a flat pattern up from individual straight segments. Bend deduction is a related but different number used the other way — trimming the total of two adjacent flange lengths down to account for that fold — calculated as 2×(R+T) minus the bend allowance for a 90° angle. This calculator computes that allowance specifically; converting to a bend deduction requires that extra subtraction step.
Does bend angle mean the angle of the bend or the resulting flange?
It's the included angle the material actually bends through — how many degrees the flat sheet rotates around the bend line, not the angle between the two resulting flanges. A simple right-angle bracket bent to a 90° flange corresponds to a 90° bend angle in this formula; a shallower bend ending in a 135° flange angle corresponds to a 45° bend angle, since the material rotated through only 45° from flat, not 135°.
Why does a tighter inside radius produce a smaller bend allowance?
Because bend allowance is proportional to the radius from the neutral axis out to the inside surface (R + K×T) — a tighter radius means the neutral axis itself sweeps through a shorter arc for the same bend angle, so less material length is consumed by the bend. This is why very tight, near-zero-radius bends common in thin sheet draw noticeably less flat-pattern length than the same angle bent over a generous radius.
Can I use this formula for bends other than 90 degrees?
Yes — the formula works for any bend angle from just above 0° up to 180°, since angle enters the calculation directly as a variable, not a fixed assumption. A shallower bend, like a 45° bend, produces a proportionally smaller bend allowance than a 90° bend at the same radius and K-factor, since less of the material's rotation — and therefore less neutral-axis arc length — is involved in the bend.