K-Factor Calculator

How to use the K-factor calculator

What this calculator does

This calculator derives the sheet-metal K-factor from a known bend allowance. K-factor is the neutral-axis distance from the inside face divided by material thickness. It is useful when you have a real test bend, a bend table, or a trusted bend allowance and need a compact value for CAD or flat-pattern work. It does not select tooling, predict springback, certify a press-brake setup, or replace a shop trial for a new material and tooling combination.

When to use it

Use it to reverse-engineer a K-factor from a coupon bend, compare setups that use different radii, check whether a CAD bend rule matches measured fabrication, or document the neutral-axis assumption behind a flat pattern. For practical safety context, OSHA explains that powered press brakes form sheet metal with matched dies and require appropriate guarding.

How to calculate

1. The calculator opens with a complete demonstration: 1 mm thickness, 1 mm inside radius, a 90° bend, and 2.1 mm bend allowance. Its XLSX workbook is ready immediately.
2. Choose Length unit. Switching between millimeters and inches converts the three length fields without changing the physical bend.
3. Replace Material thickness (T), Inside radius (Rᵢ), Bend angle (θ), and Bend allowance (BA) with values from one consistent bend setup. Results update live.
4. Read K-factor (K) first, then use Neutral-axis location (t) to see the physical distance from the inside surface. The ratio and rebuilt allowance are consistency checks.
5. Select Download Excel to export the current typed inputs and calculated values. Reset clears the demonstration and results; Excel remains disabled until a complete valid setup is entered again.

Input guide

Length unit is required and accepts millimeters or inches. A typical choice is millimeters for metric fabrication drawings. Changing it converts thickness, radius, allowance, neutral-axis distance, and the workbook. Do not switch the label mentally while leaving mixed-unit source data; all three lengths must describe the same unit system. NIST's SI length guidance provides the relationship between millimeters and meters.

Material thickness (T) is a required positive decimal length, such as 1 mm or 0.040 in. A larger thickness changes both normalization terms in the formula. Enter the actual sheet thickness rather than a nominal gauge label unless that gauge has been converted to a measured thickness.

Inside radius (Rᵢ) is a required nonnegative decimal length, such as 1 mm. Raising the radius while holding the other inputs fixed lowers the derived K-factor. Use the finished inside radius, not the punch-tip radius when those differ.

Bend angle (θ) is required in decimal degrees, greater than 0 and no more than 180; 90 is a common right-angle bend. The value is the swept bend angle used in the bend-allowance equation. Do not substitute the complementary included angle used by some drawings or press-brake conventions.

Bend allowance (BA) is a required positive length, such as 2.1 mm. It is the arc length assigned to the bend along the neutral axis. Increasing BA raises K. Do not enter bend deduction or outside setback; those are different flat-pattern quantities.

Output guide and worked example

K-factor (K) is dimensionless and is the primary result. Neutral-axis location (t) is K × T in the active length unit. Radius-to-thickness ratio is Rᵢ ÷ T and helps describe bend severity. Rebuilt bend allowance applies the solved K back to the forward bend-allowance equation; it should match the input. The Calculation details table records each input, symbol, result, and role. These outputs are calculation identities based on the entered geometry, not a recommendation that a particular K-factor is suitable for every alloy, temper, grain direction, die opening, or process.