Bend Allowance Calculator

Calculate bend allowance, bend deduction, and K-factor to develop accurate flat patterns for any bend.

Using this calculator responsibly

The Bend Allowance calculator is an engineering aid for checking a shop-floor estimate before a setup, quote, or process review. Enter values in the units shown, confirm material properties against the applicable specification, and keep a record of the assumptions used. Calculator output does not override machine capacity, tooling limits, part requirements, or a qualified engineer's approval.

Before using a result in production, repeat the calculation with the extreme values allowed by the print or process window. Check that the selected material, thickness, geometry, and safety factor reflect the actual job. If a result approaches a press, die, fastener, or machine limit, stop and verify it with the equipment manufacturer or responsible engineer.

Inputs and results

Read every field label before entering data and avoid mixing inch and metric values. A useful result should be traceable to the job number, revision, material certification, and machine or tooling record. Treat warnings, invalid ranges, and unusual outputs as reasons to inspect the inputs rather than as values to work around.

When you fold sheet metal, the bend consumes material along the neutral axis. To cut the right flat blank, you account for that consumed length — the bend allowance — at every bend.

The K-factor places the neutral axis inside the thickness. Calibrate K from a test bend in your own material and tooling for the best accuracy.

Formula: BA = (π / 180) × Angle × (R + K × T)

Worked example

  1. A 90° bend, inside radius R = 0.125 in, thickness T = 0.060 in, K = 0.44.
  2. BA = (π/180) × 90 × (0.125 + 0.44 × 0.060) = 1.5708 × 0.1514 = 0.238 in.
  3. Add the two flat flange lengths plus this 0.238 in to get the blank length.

Frequently asked questions

What is bend allowance?

Bend allowance is the arc length of the neutral axis through a bend. It is added to the flange lengths to find the flat blank length.

What is a typical K-factor?

K-factor commonly ranges from 0.33 to 0.50 depending on material and bend radius; 0.44 is a common starting point for mild steel air bending.

Related references