Bend Allowance Calculator
Calculate flat pattern bend allowance and deduction from material thickness, bend angle, inside radius, and K-factor.
About this calculator
When sheet metal bends, the material on the inside of the bend compresses and the material on the outside stretches, but somewhere in between there's a neutral axis that neither stretches nor compresses -- its length stays the same before and after the bend. The K-factor expresses where that neutral axis sits as a fraction of the material thickness measured from the inside surface (0.33 is a common default for air bending mild steel, while bottoming or coining pushes it higher, toward 0.4-0.5, because the harder forming process shifts the neutral axis closer to the material's center). Bend allowance is simply the arc length of that neutral axis through the bend -- angle in radians times the neutral axis radius (inside radius plus K-factor times thickness) -- and it's the length you ADD to your flange leg dimensions to get the correct flat pattern length, since the flat sheet needs enough material to form that neutral-axis arc.
Bend deduction is the equivalent number expressed as a SUBTRACTION from the sum of the two outside flange dimensions instead, which is often more convenient because outside dimensions are what a drawing typically calls out. Getting K-factor wrong is a common source of flat pattern error: a smaller K-factor call (say 0.33 when the actual process behaves more like 0.44) under-predicts how much material the bend consumes, producing a finished part that's shorter than intended once formed.
Inputs
Results
Bend Allowance
3.92 mm
How to Use This Calculator
- Enter material thickness (mm) and inside bend radius (mm).
- Set the bend angle (degrees) and K-factor -- 0.33 for air bending mild steel, 0.40-0.50 for bottoming.
- Review bend allowance (mm) -- the arc length added to the flat pattern for the bend.
- Add bend allowance to your flange leg lengths (measured to the bend line), or subtract bend deduction from the total flange dimensions, to get the flat pattern length.
- Check the calculated outside radius, neutral axis radius, and minimum flange length before finalizing your flat pattern.
How the result changes with Bend Angle
| Bend Angle | Bend Allowance |
|---|---|
| 45 | 1.96 mm |
| 68 | 2.96 mm |
| 135 | 5.88 mm |
| 178 | 7.75 mm |
What each input means
- Material Thickness
- Sheet metal thickness (gauge converted to mm).
- Bend Angle
- Included bend angle in degrees (90° for a right-angle bend). Capped below 180° -- the setback/bend-deduction formula has a geometric singularity as the bend approaches a full flat hem, where the outside legs never converge.
- Inside Bend Radius
- Inside radius of the bend (typically 1-2× material thickness).
- K-Factor
- Neutral axis position (0.33 for air bending mild steel, 0.40-0.50 for bottoming).
What each result means
- Bend Allowance
- Length to add to flat pattern for the bend.
- Bend Deduction
- Length to subtract from total flange dimensions.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersMaterial Thickness = 1.5, Bend Angle = 90, Inside Bend Radius = 2, K-Factor = 0.33 = 4 input(s) provided
- Calculate Bend AllowanceBend Allowance3.919 = 3.919
- Calculate Bend DeductionBend Deduction3.081 = 3.081
- Calculate Outside SetbackOutside Setback3.5 = 3.5
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does K-factor change between air bending and bottoming?
The K-factor represents where the neutral axis -- the layer that neither stretches nor compresses -- sits within the material thickness during the bend, and that position depends on how the material actually deforms. Air bending (the punch doesn't fully seat the material against the die) tends to keep the neutral axis closer to the inside surface (K around 0.33 for mild steel), while bottoming or coining forces the material to conform more fully to the tooling, pushing the neutral axis toward the material's geometric center (K around 0.4-0.5).
Do I add bend allowance or subtract bend deduction -- can't I use either?
Both describe the same physical bend, just referenced from different dimensions -- bend allowance is added to flange leg lengths measured to the bend line's inside, while bend deduction is subtracted from the sum of the two OUTSIDE flange dimensions of a formed part. Use whichever matches how your drawing dimensions the part; mixing the two (adding bend deduction, or subtracting bend allowance) will produce a wrong flat pattern length.
Why does the minimum flange length depend only on material thickness?
The commonly used minimum-flange guideline (roughly 4x material thickness) exists because a flange shorter than that doesn't have enough material clear of the bend radius to seat properly against the die and form a controlled, repeatable bend -- the risk is a distorted or inconsistent bend near the flange edge. That risk scales with how thick the material is relative to the flange, not with the bend angle or how tight the bend radius is, which is why bend angle has no effect on this particular guideline.
Why does a larger K-factor increase bend allowance if the bend geometry didn't change?
Bend allowance is the neutral axis's arc length, and the neutral axis radius itself equals inside radius plus K-factor times thickness -- so a larger K-factor pushes the neutral axis farther from the inside surface (closer to the outside), which increases its radius and therefore the arc length it traces through the same bend angle, even though the physical inside radius and bend angle haven't changed at all.
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