Sheet Metal Flat Pattern Calculator

Example Data Table

Numbers are rounded; your shop factors may differ.

Formula Used

Angle conversion: A(rad) = A(deg) × π / 180

Setback (SB): SB = (R + T) × tan(A/2)

Bend allowance (BA):

• BA = A(rad) × (R + K × T) when computing from K-factor
• Or enter BA directly when using shop-tested allowances

Bend deduction (BD): BD = 2×SB − BA

Flat length:

• Outside dimensioning: Flat = Σ(Flange outside lengths) − Σ(BD)
• Inside dimensioning: Flat = Σ(Flange inside lengths) + Σ(BA)

If you enter BD directly, the calculator back-solves BA for reporting.

How to Use This Calculator

1. Select units and choose outside or inside dimensioning.
2. Enter thickness, then pick your calculation method.
3. Set the number of bends for the part.
4. Enter flange lengths L1…L(N+1) in your chosen style.
5. Enter bend angles and inside radii for bends 1…N.
6. If using allowance or deduction input, fill those fields.
7. Click Calculate to see the flat pattern above the form.
8. Download CSV or PDF for a clean shop summary.

1) Why flat pattern accuracy matters

Accurate developed length reduces scrap, rework, and brake time. A small bend error compounds across multiple bends, shifting hole locations and causing fit-up problems on frames, ducts, and equipment guards.

2) Neutral axis and K-factor basics

The neutral axis is where material neither stretches nor compresses during bending. K-factor estimates its position as a fraction of thickness from the inside surface. Typical values often fall between 0.30 and 0.50, depending on material, tooling, and bend method.

3) Thickness and inside radius selection

Thickness (T) and inside radius (R) drive setback, allowance, and deduction. As a practical rule, many shops target R close to 1×T for mild steel air bends, while softer materials may use larger radii to avoid cracking and surface marking.

4) Bend angle handling for consistent results

This calculator uses degrees converted to radians for the curved length contribution. For tight production, record the actual achieved angle from first-article inspection. Even a 1° deviation can shift the developed length by measurable fractions across several bends.

5) Outside vs inside dimensioning in drawings

Drawings may specify flange lengths as outside (overall) or inside (clear opening). Outside dimensioning typically uses total bend deduction (BD) to remove overlap from intersections. Inside dimensioning typically uses total bend allowance (BA) to add the developed bend arc.

6) When to input allowance or deduction directly

If your shop maintains a bend table, you can enter BA per bend or BD per bend to match proven tooling. This is useful when K-factor varies by punch/die set, material batch, grain direction, or when coating thickness affects forming behavior.

7) Multi-bend parts and data you should capture

For parts with 2–6 bends, capture bend order, angle, R, and measured leg lengths after forming. Track the method used, then compare predicted vs measured flat lengths. Over time, this builds a reliable database for faster quoting and better first-pass yield.

8) Quality checks and shop-floor workflow

Confirm units, verify thickness with calipers, and match dimensioning style to the print before cutting. After the first part, measure overall and internal features, then adjust K-factor or bend table values. Save the final report for repeat work.

1) What is a good starting K-factor?
Many shops start around 0.40–0.45 for air bending mild steel, then tune using first-article measurements. Tooling geometry, material, and bend method can shift the best value noticeably.

2) Should I use outside or inside dimensioning?
Use the same style as your drawing. Outside matches overall flange lengths; inside matches clear openings. Mixing styles can double-count or miss bend effects, causing wrong developed length.

3) Why does bend deduction sometimes look negative?
With certain angles, radii, or entered allowances, the calculated overlap can be small or reversed. Negative BD is a signal to verify inputs, tooling assumptions, and whether you should use a bend table instead.

4) Can I use this for stainless or aluminum?
Yes, but expect different K-factors and radius choices. Softer alloys may need larger radii; harder alloys may spring back more. Calibrate with a test bend and update your preferred settings.

5) What if I already have bend allowances from the shop?
Choose “Enter bend allowance per bend” and input BA for each bend. The calculator will still compute setback and report the resulting bend deduction for documentation.

6) How many bends can I calculate?
This calculator supports 1 to 6 bends per part, which covers many brackets, channels, and transitions. For more bends, run the part in segments or expand the form fields in code.

7) Does this replace a press brake bend table?
It complements it. Use the K-factor method for estimates and early planning, then rely on validated bend tables for production. The best process is to compare both and keep the settings consistent.