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Sheet metal bend allowance & K-factor.

Enter material thickness, inside bend radius, bend angle, and K-factor to get bend allowance, bend deduction, and the developed (flat) blank length. Includes a sheet-metal gauge-to-thickness reference for steel, galvanized, stainless, and aluminum.

Bend Allowance
BA = (π/180)·A·(R + K·T)
Bend Deduction
BD = 2(R+T)·tan(A/2) − BA
Flat length
L = F₁ + F₂ + BA

Inputs

Single-bend parameters

Units:

Optional: two flange lengths for a flat blank

Flanges measured to the bend tangent lines (apex of the inside radius). Flat length adds the bend allowance once for this single bend.

Result

Developed bend geometry

Bend allowance
0.1338
3.40 mm
Bend deduction
0.1062
2.70 mm
Flat length
2.1338
54.20 mm
BA = (π/180) × 90 × (0.0600 + 0.42 × 0.0600) = 0.1338 in
BD = 2 × (0.0600 + 0.0600) × tan(45.0°) − 0.1338 = 0.1062 in
L = 1.0000 + 1.0000 + 0.1338 = 2.1338 in
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Outside setback (OSSB)
0.1200
3.05 mm

OSSB = (R + T)·tan(A/2). Distance from the bend tangent line to the apex of the outside mold lines.

If you measure to mold lines
2.1338
54.20 mm

Flat = (F₁ + F₂ + 2·OSSB) − BD when flanges are dimensioned to the outside mold lines instead of tangent lines.

Reference tool only. Real K-factors are best confirmed with a test bend on your press, tooling, and lot of material — published values vary with die width, grain direction, and forming method.

Reference

Sheet-metal gauge → thickness

Decimal-inch thickness by gauge number. Standard and galvanized steel use the Manufacturer's Standard Gauge (MSG); stainless uses the US Standard Gauge; aluminum and other non-ferrous metals use Brown & Sharpe (AWG). Because the standards differ, the same gauge number is a different thickness in each material — always confirm against the actual material.

Gauge Steel (MSG) Galvanized Stainless (US Std) Aluminum (B&S)
7 ga 0.1793″ 4.55 mm 0.1830″ 4.65 mm — — 0.1443″ 3.67 mm
8 ga 0.1644″ 4.18 mm 0.1681″ 4.27 mm 0.1719″ 4.37 mm 0.1285″ 3.26 mm
9 ga 0.1495″ 3.80 mm 0.1532″ 3.89 mm 0.1562″ 3.97 mm 0.1144″ 2.91 mm
10 ga 0.1345″ 3.42 mm 0.1382″ 3.51 mm 0.1406″ 3.57 mm 0.1019″ 2.59 mm
11 ga 0.1196″ 3.04 mm 0.1233″ 3.13 mm 0.1250″ 3.17 mm 0.0907″ 2.30 mm
12 ga 0.1046″ 2.66 mm 0.1084″ 2.75 mm 0.1094″ 2.78 mm 0.0808″ 2.05 mm
13 ga 0.0897″ 2.28 mm 0.0934″ 2.37 mm 0.0938″ 2.38 mm 0.0720″ 1.83 mm
14 ga 0.0747″ 1.90 mm 0.0785″ 1.99 mm 0.0781″ 1.98 mm 0.0641″ 1.63 mm
16 ga 0.0598″ 1.52 mm 0.0635″ 1.61 mm 0.0625″ 1.59 mm 0.0508″ 1.29 mm
18 ga 0.0478″ 1.21 mm 0.0516″ 1.31 mm 0.0500″ 1.27 mm 0.0403″ 1.02 mm
20 ga 0.0359″ 0.91 mm 0.0396″ 1.01 mm 0.0375″ 0.95 mm 0.0320″ 0.81 mm
22 ga 0.0299″ 0.76 mm 0.0336″ 0.85 mm 0.0313″ 0.80 mm 0.0253″ 0.64 mm
24 ga 0.0239″ 0.61 mm 0.0276″ 0.70 mm 0.0250″ 0.64 mm 0.0201″ 0.51 mm

Stainless 7 ga is omitted (not part of the standard US Standard Gauge stainless range). Galvanized values include the zinc coating, so a galvanized sheet runs slightly thicker than uncoated steel at the same gauge.

How to use this

From flange dimensions to a flat blank.

The bend allowance is the arc length the metal actually occupies through a bend, measured at the neutral axis. The neutral axis shifts inward as the metal is bent — the K-factor captures where it lands as a fraction of thickness (neutral-axis offset ÷ thickness).

  1. 1 · Pick thickness and inside radius

    Use the gauge chart above for stock thickness. The inside radius is set by your tooling; a common starting point is an inside radius roughly equal to the material thickness.

  2. 2 · Choose a K-factor

    0.42 is a reasonable air-bend default. Tighter radius-to-thickness ratios and bottoming/coining push K lower (~0.33–0.40); generous radii trend toward 0.50. Confirm with a test bend whenever tolerances are tight.

  3. 3 · Read bend allowance and deduction

    If you dimension your flanges to the bend tangent lines, add the bend allowance once per bend: flat = ΣF + BA. If you dimension to the outside mold lines, subtract the bend deduction: flat = ΣF − BD. Both routes give the same blank.

  4. 4 · Multiply across bends

    For a multi-bend part, sum the flat flange segments and add one bend allowance (or subtract one bend deduction) for each bend.

Need the machined mating part?

This calculator is a neutral engineering reference. True Precision Machining is a CNC machine shop — we produce machined components, brackets, and housings to print. If your assembly pairs a formed sheet-metal part with a machined detail, send us the machined part to quote, and we can help coordinate the forming through our supply network.

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Related references

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Material density table

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Material weight calculator

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Tolerances reference

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Keep exploring

Related tools & references

Material density table→

Sheet thickness and gauge-to-density reference.

Material datasheets→

Temper and formability notes by alloy.

Material weight calculator→

Weigh flat blanks from thickness and area.

Tolerances reference→

Dimensional tolerance grades for flat-pattern features.

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