Bend Allowance Calculator

About the Bend Allowance Calculator

The **Bend Allowance Calculator** solves one of the most common sheet-metal fabrication problems: how long should the flat blank be so that, after bending, the finished part has the correct flange dimensions? Bend allowance (BA) is the arc length of the neutral axis through the bend zone. Combined with the bend deduction (BD) and outside setback (OSSB), it determines the flat-pattern layout.

Accurate flat-pattern calculations save material, reduce scrap, and eliminate trial-and-error on the press brake. The K-factor — the ratio of the neutral-axis offset to thickness — varies by material, temper, bend method, and grain direction. This calculator includes presets for mild steel, stainless steel, aluminum alloys, copper, and brass.

Enter the material thickness, bend angle, inside radius, K-factor, and flange lengths. The calculator returns the bend allowance, bend deduction, flat blank length, outside setback, and neutral-axis radius. Use the angle and material tables to plan multiple bends or compare materials. The result keeps the neutral-axis geometry visible so the flat pattern can be checked against the bend setup before metal is cut.

When This Page Helps

Use this calculator when you need a reliable flat-pattern estimate before the part reaches the press brake.

It is useful for quoting, programming, fixture planning, and reducing trial-and-error when thickness, inside radius, or K-factor change from one job to the next. It also helps keep the bend assumptions consistent when different operators or shops use different setup conventions.

How to Use the Inputs

1. Select a material preset or enter a custom K-factor.
2. Enter the sheet thickness and inside bend radius in mm.
3. Set the desired bend angle in degrees.
4. Enter flange A and flange B lengths (mold-line dimensions).
5. Read the bend allowance, bend deduction, and flat blank length.
6. Use the reference tables for different angles and materials.

Example Calculation

Tips & Best Practices

• Measure inside radius with a radius gauge — do not assume it equals the punch radius.
• For multiple bends, calculate each bend allowance separately and sum them.
• Grain direction affects K-factor — bend perpendicular to the grain for tightest radii.
• K ≈ 0.33 for air bending, ≈ 0.44 for bottoming, ≈ 0.50 for coining.
• Always verify with a test bend on scrap material before production.

Practical Guidance

Flat-pattern work is easiest when the bend assumptions are tied to the real process. Material temper, tooling, air bending versus bottoming, and grain direction can all shift the neutral axis enough to matter, so a stored shop K-factor or bend table is often more valuable than a generic handbook number.

Common Pitfalls

Most fabrication errors come from mixing dimension conventions. If flange lengths are inside, outside, or mold-line dimensions, the blank-length math changes. Another common problem is assuming the inside radius equals the punch radius; the formed radius depends on material and tooling behavior, not just the punch label.

Frequently Asked Questions

• What is K-factor?

  K-factor is the ratio of the distance from the inner surface to the neutral axis to the total thickness. It ranges from 0 to 1, with 0.33–0.50 being typical for most metals.

• Why does K-factor vary?

  It depends on material ductility, bend radius relative to thickness, grain direction, tooling, and bending method. That is why many shops rely on tested bend tables instead of a single universal value.

• What is bend deduction?

  BD is the amount subtracted from the sum of mold-line flange dimensions to get the flat blank length. It accounts for the material that is effectively used up in the bend zone.

• What is outside setback?

  OSSB is the distance from the bend tangent line to the apex of the outside mold line. It is one of the geometric pieces used to convert flange dimensions into a flat pattern.

• When is springback significant?

  Springback is significant in high-strength or thick materials. Over-bend by 2–5° to compensate, and confirm the final bend with a test piece when the fit is critical.

• Minimum bend radius?

  A common rule is Ri ≥ 1 × t for ductile metals. Harder alloys may need 2–4 × t. Shop bend charts or supplier data should override the rule of thumb when the material is sensitive to cracking.