Sheet Metal Tool

Flat Pattern Layout Calculator

Enter one bend and two outside leg dimensions to calculate bend allowance, bend deduction, cut length and the bend line position from either edge.

What this flat pattern calculator does

A folded part is not developed by simply adding its finished outside legs. Material on the outside of a bend stretches while material on the inside compresses. The calculator models the length of the layer between them that does not appreciably change length: the neutral axis. From that length it derives a bend deduction, subtracts the deduction from the two outside leg dimensions, and places the bend line on the resulting blank.

This focused tool handles a part with one bend and two legs: an angle bracket, cover return, flashing corner or test coupon. For a multi-bend part, calculate each bend with its own angle and radius, then account for every deduction using a consistent dimensioning method. The homepage calculator supports multiple bends and a live profile diagram.

How to enter the dimensions

Thickness and inside radius

Enter actual material thickness rather than relying blindly on nominal gauge. Paint, galvanising and mill tolerance can make actual stock different from a table value. Radius means the finished inside bend radius after springback. In an air bend that radius is influenced by die opening and material response and may not equal the punch nose.

K-factor

K-factor is the neutral-axis depth divided by sheet thickness. Typical starting values range from about 0.30 for tight bends to 0.50 for very large radius-to-thickness ratios. Mild steel is often estimated near 0.33 at R/T = 1, aluminium near 0.38, and stainless near 0.40. These are starting points, not certified material constants. Use a test bend made with the production stock and tooling whenever tolerance matters.

Bend angle

The calculator defines bend angle as the amount through which the material turns: 90° is a right-angle fold, 45° is a forty-five-degree fold, and a larger value consumes more arc length. Some drawings show the remaining opening rather than bend rotation. Converting that convention correctly is essential.

Outside leg lengths

Enter both finished legs as outside mould-line dimensions to their theoretical sharp intersection. This is the point where the two flat outside faces would meet if extended. Do not mix a tangent-point dimension on one leg with an outside dimension on the other. Dimensioning consistency matters as much as formula accuracy.

The formulas behind the result

Bend allowance is the arc length at neutral-axis radius. Angle is converted from degrees to radians before multiplying by radius:

Outside setback is the distance between a bend tangency line and theoretical outside sharp corner. Two setbacks surround a bend, and removing bend allowance from their total produces bend deduction:

The layout shown here splits deduction equally between adjacent outside legs. Bend line from edge 1 is Leg 1 − BD/2. The check dimension from edge 2 is Leg 2 − BD/2. These two positions add to the flat blank length. A shop may instead mark tangency points or program a backgauge datum, so label the convention on manufacturing drawings.

Worked layout example

Use the default values: 2 mm mild steel, 2 mm inside radius, K = 0.33, a 90° bend, and finished outside legs of 50 mm and 40 mm. Neutral-axis radius is 2 + 0.33 × 2 = 2.66 mm. Bend allowance is 1.5708 × 2.66 = 4.178 mm. At 90°, outside setback is 4.000 mm.

The two layout positions total 86.178 mm, matching blank length. Marking from one edge avoids stacking two independent measuring errors. The opposite-edge result is useful as a verification dimension after cutting.

From calculated blank to accurate part

Calculation is only the first-off prediction. Cut a coupon from the real sheet, keeping the intended grain direction. Bend with the actual punch, die and press-brake program. After springback, measure angle, inside radius and both legs. If the part is consistently too large or small, calculate an empirical deduction from measured outside legs minus original flat length, then derive the K-factor or store the proven deduction for that setup.

Avoid correcting a length problem by changing several variables at once. First establish that cut length, leg datum and bend angle are measured correctly. Then check finished radius and material thickness. Finally adjust K-factor or deduction. This sequence makes shop data reusable instead of hiding an angle or radius problem inside an arbitrary allowance.

Common flat pattern errors

• Using nominal instead of actual radius. Air-bent radius can differ substantially from the punch nose.
• Mixing inside and outside dimensions. This calculator expects outside mould-line legs.
• Using the wrong angle convention. A 30° opening and a 30° bend are not the same geometry.
• Assuming one K-factor fits all tools. Changing die opening changes R/T and often the effective factor.
• Rounding every intermediate value. Keep precision through BA and BD, then round the finished cut dimension.
• Ignoring grain and material temper. They affect cracking, springback and repeatability.

Metric and imperial use

The geometry is unit-independent. Select millimetres or inches and keep every length input in that system. K-factor and angle have no length unit. For inches, three decimal places are normally useful; for millimetres, two or three preserve calculation precision. Changing the selector changes labels only, so convert existing numbers before switching systems.

Frequently asked questions