# Sheet Metal Flat Blank Length Calculator

> Source: https://netray.co/tools/sheet-metal-flat-blank-length-calculator
> Publisher: Netray (netray.co), AI and integration engineering for ERP and manufacturing systems.
> Section: Free Tools

## Short answer

Turn outside leg dimensions and bend data into the flat pattern length, using outside setback, bend allowance and bend deduction.

## Category

manufacturing

## Industry

Sheet Metal Fabrication

## Audience

- Fabrication engineer
- CNC programmer
- Estimator

## Tool

**Kind:** calculator

**Currency:** $

**Inputs:**

#### Sum of outside leg lengths

**Key:** legs

**Type:** number

**Default:** 160

**Min:** 0

**Step:** 1

**Unit:** mm

**Help:** Add every leg measured to the virtual sharp (outside mold line) of each bend.

#### Material thickness

**Key:** t

**Type:** number

**Default:** 2

**Min:** 0.1

**Step:** 0.1

**Unit:** mm

#### Inside bend radius

**Key:** ri

**Type:** number

**Default:** 2

**Min:** 0

**Step:** 0.1

**Unit:** mm

#### Bend angle (deflection from flat)

**Key:** angle

**Type:** select

**Default:** 90

**Options:**

##### 30 deg

**Value:** 30

##### 45 deg

**Value:** 45

##### 60 deg

**Value:** 60

##### 90 deg

**Value:** 90

##### 120 deg

**Value:** 120

##### 135 deg

**Value:** 135

**Help:** A 90 degree bend is an L-shape. 0 means the part stays flat.

#### K-factor

**Key:** k

**Type:** number

**Default:** 0.33

**Min:** 0.25

**Max:** 0.5

**Step:** 0.01

**Help:** Neutral axis position as a fraction of thickness. 0.33 to 0.45 is typical for air bending.

#### Number of identical bends

**Key:** n

**Type:** number

**Default:** 1

**Min:** 1

**Step:** 1

**Help:** All bends are assumed to share the same angle, radius and thickness.

**Outputs:**

#### Bend allowance per bend

**Key:** ba

**Expr:** angle * pi / 180 * (ri + k * t)

**Format:** decimal

**Description:** Arc length of the neutral axis through one bend, in mm.

#### Tangent of half angle

**Key:** half

**Expr:** if(eq(angle,30),0.26795,if(eq(angle,45),0.41421,if(eq(angle,60),0.57735,if(eq(angle,90),1,if(eq(angle,120),1.73205,2.41421)))))

**Format:** decimal

#### Outside setback

**Key:** ossb

**Expr:** half * (ri + t)

**Format:** decimal

**Description:** Distance from the virtual sharp to the tangent point of the bend.

#### Bend deduction per bend

**Key:** bd

**Expr:** 2 * ossb - ba

**Format:** decimal

**Primary:** true

**Description:** Subtract this per bend from the sum of the outside legs.

#### Flat blank length

**Key:** flat

**Expr:** legs - n * bd

**Format:** decimal

**Primary:** true

**Description:** Developed length of the blank before bending, in mm.

**Chart:**

#### Flat blank length vs K-factor

**Type:** line

**X:** k

**From:** 0.25

**To:** 0.5

**Steps:** 26

**Series:**

- flat

**Disclaimer:** Results are theoretical. Confirm K-factor with a test bend on the actual material, tooling and press brake before cutting production blanks.

## Content

**Intro:** This calculator converts outside leg dimensions into a flat blank length for sheet metal parts. Enter the sum of the outside legs, thickness, inside radius, bend angle and K-factor. It returns bend allowance, outside setback, bend deduction and the developed length.

**Sections:**

#### How to use the calculator

**Body:** Measure each leg from the outside surface to the virtual sharp, the point where the two outside surfaces would meet if the bend were sharp. Add the legs together and enter the total. Set thickness, inside radius and the bend angle as the deflection from flat, so an L-bracket is 90. If all bends match, set the number of bends and the calculator subtracts one bend deduction per bend. Read the flat blank length from the primary result and use the sweep chart to see how sensitive the length is to your K-factor assumption.

#### The formulas behind it

**Body:** Bend allowance is the arc length of the neutral axis: BA = angle in radians times (inside radius + K times thickness). Outside setback is tan(angle / 2) times (inside radius + thickness). Bend deduction is twice the outside setback minus the bend allowance. The flat length is the sum of outside legs minus bend deduction for every bend. For a 90 degree bend the setback equals radius plus thickness, which makes the maths easy to check by hand.

**Bullets:**

- BA = angle (rad) x (Ri + K x t)
- OSSB = tan(angle / 2) x (Ri + t)
- BD = 2 x OSSB - BA
- Flat = sum of outside legs - n x BD

#### Worked example and common mistakes

**Body:** A 2 mm steel bracket with 100 mm and 60 mm outside legs, a 2 mm inside radius and K of 0.33 gives BA = 1.5708 x (2 + 0.66) = 4.18 mm. Setback is 4 mm, so BD = 8 - 4.18 = 3.82 mm and the flat length is 160 - 3.82 = 156.18 mm. Common errors are measuring legs to the tangent line instead of the virtual sharp, entering the included angle instead of the deflection, and assuming one K-factor suits every material, tool and radius.

**Bullets:**

- Use the deflection angle, not the included angle
- Measure legs to the virtual sharp
- Validate K-factor with a sample bend

**Cta:** If flat pattern data lives in CAD and bend tables live in a spreadsheet, we can connect them to your ERP routing and BOM so blank sizes and material usage flow into quotes automatically.

**Faq:**

#### What is the difference between bend allowance and bend deduction?

**Answer:** Bend allowance is the length of the neutral axis through the bend and is added to the flat legs. Bend deduction is subtracted from the sum of outside legs. They describe the same bend from two measurement conventions, linked by BD = 2 x setback - BA.

#### What K-factor should I use?

**Answer:** Many shops start near 0.33 for tight radii and 0.40 to 0.45 for larger radii relative to thickness. The right value depends on material, tooling and bend method, so measure a test part and back-calculate K.

#### Does this work for angles other than 90 degrees?

**Answer:** Yes. Select 30, 45, 60, 90, 120 or 135 degrees of deflection. The tool uses the exact tangent of half the angle for the setback and the full angle for the bend allowance.

#### Can I use inches?

**Answer:** The formulas are unit independent. Enter all lengths in inches and read the result in inches, as long as every dimension uses the same unit.

## Search terms

flat blank length calculator, sheet metal flat pattern length, bend deduction calculator, outside setback calculator, sheet metal developed length, flat length of bent sheet metal, sheet metal blank size calculation

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