Bend allowance vs bend deduction

Bend allowance and bend deduction both answer the same question — how long does the flat blank need to be so that, after bending, the part comes out to size? They just measure from different references, so it helps to keep them straight before you cut stock.

Why a blank isn't just the sum of the sides

When a sheet bends, the outer surface stretches and the inner surface compresses. Somewhere between them sits the neutral axis, a layer whose length doesn't change. Its position is captured by the K-factor — the fraction of the thickness T from the inside surface to the neutral axis (see what is the K-factor). Because the part's nominal dimensions are taken on the outside, you have to correct for the material that "wraps around" the bend. Bend allowance and bend deduction are two ways to apply that correction.

Bend allowance (BA): add it to the flats

The bend allowance is the arc length of the neutral axis through the bend — the actual amount of material consumed by the curve:

BA = (π / 180) · angle · (R + K · T)

where angle is the bend angle in degrees, R the inside bend radius, K the K-factor and T the material thickness.

To get the blank length, you measure each flat segment to the bend line (the tangent points where the flat meets the curve) and then add one bend allowance for every bend:

flat length = Σ flat segments + Σ BA

This is the natural fit for CAD and any "sum of flats" approach, where you already model each flat out to where the bend begins.

Bend deduction (BD): subtract it from the flanges

The bend deduction works the other way. Instead of measuring to the bend line, you measure each flange to the apex — the mould-line intersection where the two outside faces would meet if extended. Those dimensions overlap in the corner, so you subtract one deduction per bend:

flat length = (flange₁ + flange₂) − BD

This is the classic press-brake shop number: read the two outside flange dimensions off the print, add them, subtract the deduction. Our bend deduction calculator works directly in these terms.

How the two relate

The link between them runs through the outside setback (OSSB), the distance from the bend line to the apex along each flange:

OSSB = (R + T) · tan(angle / 2)

Going from "to the bend line" to "to the apex" adds one OSSB on each side, so the deduction is the two setbacks minus the allowance:

BD = 2 · OSSB − BA

For the common 90° bend, tan(45°) = 1, so the setback simplifies to OSSB = R + T and BD = 2·(R + T) − BA. Feed both methods the same K-factor and they return the same blank — they're just two bookkeeping systems for the same physics.

Worked example

Take a 90° bend in T = 2 mm sheet with an inside radius R = 2 mm and K = 0.44.

BA = (π / 180) · 90 · (2 + 0.44 · 2) = 4.52 mm

OSSB = (2 + 2) · tan(45°) = 4 mm

BD = 2 · 4 − 4.52 = 3.48 mm

Now suppose the part is an L-bracket with two flanges that each measure 50 mm to the apex. Using bend deduction:

blank = (50 + 50) − 3.48 = 96.52 mm

You would reach the same 96.52 mm by the allowance route: each flat measured to the bend line is 50 − OSSB = 46 mm, so 46 + 46 + 4.52 = 96.52 mm. (These are exactly the numbers the calculator returns for these inputs.)

Which method to reach for

Use bend allowance when you're driving from a flat-pattern model and summing segments measured to the bend lines — it's the CAD-friendly form. Use bend deduction at the press brake, where the operator works from outside flange dimensions and a single deduction per bend. Both are correct; the only thing that matters is using a K-factor that matches your material, radius and bending method, because that one number sets where the neutral axis sits — and therefore both results.

Frequently asked questions

What is the difference between bend allowance and bend deduction?

Both find the flat (blank) length of a bent part, but from different references. Bend allowance is added to the sum of the flat segments (measured to the bend lines). Bend deduction is subtracted from the sum of the outside flange dimensions (measured to the apex). With a consistent K-factor they give the same blank length.

How are bend allowance and bend deduction related?

BD = 2·OSSB − BA, where OSSB (outside setback) = (R + T)·tan(angle/2). For a 90° bend the setback simplifies to OSSB = R + T, so BD = 2·(R + T) − BA.

Which one should I use?

Bend allowance suits CAD and sum-of-flats workflows where you model each flat to the bend line. Bend deduction is the classic press-brake shop number: flange + flange − deduction. Either is correct as long as the K-factor is consistent.

Last reviewed: 2026-05-29.