Sheet Metal Bend Allowance Calculator (Setback and Flat Length)

Bend a strip of sheet metal and the outside stretches while the inside squeezes, so the flat piece you start with is shorter than the two flanges added together. This calculator works out how much shorter. Enter the thickness, the inside bend radius and the bend angle, and it gives the bend allowance (the length of metal in the curve), the setback (where to put the bend line) and the bend deduction, then the flat pattern length for an L-shaped part. The default method is the empirical formula in the FAA’s Aviation Maintenance Technician Handbook; you can switch to a K-factor from your own test bends.

Sheet metal bend allowance calculator

How it works

Somewhere inside the thickness a line keeps its length when the metal bends: the neutral axis. The FAA handbook puts it about 0.445 × T from the inside of the bend. The length of that line through the bend is the bend allowance:

FAA formula: BA = (0.01743 × R + 0.0078 × T) × N, where N is the bend angle in degrees. This is the same as π/180 × (R + 0.4475 T) × N.
K-factor method: BA = N × π/180 × (R + K × T). K is usually between 0.3 and 0.5.
Setback: SB = tan(N ÷ 2) × (R + T). For a 90 degree bend, SB = R + T.
Bend deduction: BD = 2 × SB − BA. Flat length = flange A + flange B − BD, with both flanges measured to the outside mold line.

The bend angle is the angle the metal is bent through from flat, not the angle left between the flanges. A part drawn with 120 degrees between its flanges is a 60 degree bend.

Worked example

The FAA handbook’s own example: 0.040 in 2024-T3 aluminium, a 0.16 in inside radius (the minimum the handbook gives for that thickness and temper), bent 90 degrees, with flanges of 1 in and 2 in to the mold line:

Units: Inches
Material thickness: 0.04 in
Inside bend radius: 0.16 in
Bend angle: 90 degrees
Method: FAA formula (0.01743R + 0.0078T) x angle
Flange A, outside (mold line) length: 1 in
Flange B, outside (mold line) length: 2 in

The calculator gives:

Bend allowance: 0.2791 in
Setback (mold line to bend tangent line): 0.2 in
Bend deduction: 0.1209 in
Flat pattern length: 2.8791 in
Flat A (edge to bend line): 0.8 in
Flat B (edge to bend line): 1.8 in
K-factor used: 0.4475 (built into the FAA formula)

The bend allowance is 0.279 in, which matches the handbook’s 0.27 in from the formula and 0.273 in from its chart (read at 5/32 in radius). The setback is 0.040 + 0.16 = 0.2 in, so the flats are 0.8 in and 1.8 in and the blank is 0.8 + 0.279 + 1.8 = 2.879 in long. Mark the bend tangent lines 0.8 in from one end and 0.279 in apart.

Bend allowance for a 90 degree bend (inches, FAA formula)

Thickness	R 1/32	R 1/16	R 3/32	R 1/8	R 5/32	R 3/16	R 1/4
0.016 in	0.06	0.109	0.158	0.207	0.256	0.305	0.403
0.02 in	0.063	0.112	0.161	0.21	0.259	0.308	0.406
0.025 in	0.067	0.116	0.165	0.214	0.263	0.312	0.41
0.032 in	0.071	0.121	0.17	0.219	0.268	0.317	0.415
0.04 in	0.077	0.126	0.175	0.224	0.273	0.322	0.42
0.05 in	0.084	0.133	0.182	0.231	0.28	0.329	0.427
0.063 in	0.093	0.142	0.191	0.24	0.289	0.338	0.436
0.071 in	0.099	0.148	0.197	0.246	0.295	0.344	0.442
0.08 in	0.105	0.154	0.203	0.252	0.301	0.35	0.448
0.09 in	0.112	0.161	0.21	0.259	0.308	0.357	0.455
0.125 in	0.137	0.186	0.235	0.284	0.333	0.382	0.48

BA = (0.01743R + 0.0078T) × 90. For other angles, divide by 90 and multiply by the bend angle. A radius smaller than the thickness may crack hard alloys: check the minimum bend radius for the material and temper.

Notes and limits

Use the minimum bend radius for your alloy and temper. The FAA handbook takes it from a manufacturer’s chart (0.16 in for 0.040 in 2024-T3); a radius that is too tight cracks the outside of the bend.
K-factor depends on the material, the tooling and how the part is formed. A rule-of-thumb table on Wikipedia puts air-bent parts at about 0.33 to 0.40 for tight radii and 0.5 once the radius exceeds three times the thickness. A test bend in your own brake beats any table.
The FAA handbook says bend lines should run at 90 degrees to the grain of the sheet where possible.
For a part with several bends, subtract one bend deduction per bend from the sum of the mold line dimensions. The middle flat of a U-channel loses a setback on each side.
Gauge thicknesses for steel, galvanized, stainless and aluminium are in the sheet metal gauge chart.

Frequently asked questions

What is the formula for bend allowance?

BA = angle × π/180 × (R + K × T). The FAA handbook gives an empirical version for any angle from 1 to 180 degrees: BA = (0.01743 × R + 0.0078 × T) × degrees of bend.

What is the difference between bend allowance and bend deduction?

Bend allowance is the length of metal in the curved part of the bend. Bend deduction is how much to take off the sum of the two outside flange lengths to get the flat blank: twice the setback minus the bend allowance.

What K-factor should I use?

If you have no test data, about 0.44 to 0.45 is a reasonable start for many sheet parts (the FAA formula is equivalent to 0.4475). For an accurate figure, bend a test strip, measure it and work K back from the result.

How do I find the setback for a bend that is not 90 degrees?

Multiply R + T by tan(half the bend angle). For a 60 degree bend that factor is 0.577; for 120 degrees it is 1.732. The FAA handbook calls this the K chart, which is a different K from the K-factor.

Sources

Last reviewed: 25 October 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.