Sheet metal
Sheet metal bend calculator
In short
The flat length of a bent plate is worked out with the K-factor method: neutral-fibre arc of the bend BA = α (r + K e), setback R = (r + e) tan(α/2) for outside dimensions, correction BA − 2R per bend. The flat length is L = Σ legs + Σ corr, with K = k / 2 from DIN 6935.
Calculation
Example calculated with the tool’s default values.
Inputs
- Thickness
- 2 mm
- Inside bend radius
- 2 mm
- K-factor
- Automatic (DIN 6935 table, from r/t)
- Sheet width (bend length)
- 200 mm
- Leg dimensions taken
- Outside (virtual sharps)
- Leg 1
- 30 mm
- Bend 1 angle
- 90°
- Leg 2
- 50 mm
Results
- Flat length
- 76.2 mm
Worked example
Every value in this section belongs to the example, calculated with the tool’s default values.
Other results of the example
- K-factor used
- 0.325DIN 6935 table: k = 0.650 for r/t = 1.00, K = k / 2
- Sum of entered legs
- 80.0 mm
- Bend 1: correction (− = deduction)
- −3.84 mmNeutral-fibre arc 4.16 mm, setback 4.00 mm
- Bend line 1 (from the edge of leg 1)
- 28.1 mmBend zone from 26.0 to 30.2 mm
- Radius 2.0 mm is below the DIN 6935 minimum for S235 steel at 2.0 mm (2.5 mm, bend parallel to rolling): cracking risk, check the grade.
- Indicative value: the K-factor depends on the material, the bending method (air, bottoming) and the rolling direction. Bend a trial part before the batch.
Step-by-step calculation
K-factor
k = table DIN 6935 (r/e) ; K = k / 2
r/e = 2.0 / 2.0 = 1.00 → k = 0.650 → K = 0.325
Bend 1 (90,0°)
BA = α (r + K e) ; R = (r + e) tan(α/2) ; corr = BA − 2R
BA = 1.5708 × (2.0 + 0.325 × 2.0) = 4.16 ; R = 4.00 ; corr = -3.84 mm
Flat length
L = Σ cotes + Σ corr
80.0 − 3.84 = 76.2 mm
Bend lines
x = Σ longueurs droites + Σ arcs précédents + BA/2
pli 1 : 28.1 mm
How to use it
- Enter the thickness and the inside bend radius.
- Leave the K-factor automatic (DIN 6935 table from r/t) or enter your press value, from 0 to 0.5.
- Say whether the leg dimensions are taken outside or inside (virtual sharps).
- Enter the legs in order and the angle of each bend (0° = flat, 90° = right angle, negative = other way).
- Read the flat length.
Frequently asked questions
How do I calculate a sheet metal flat length?
Add up the leg dimensions and add, for each bend, the correction BA − 2R: BA = α (r + K e) is the neutral-fibre arc (α in radians), R the setback to the virtual sharp. With outside dimensions, R = (r + e) tan(α/2); with inside dimensions, R = r tan(α/2). Flat length: L = Σ legs + Σ corr.
What is the bend allowance and how is it calculated?
The bend allowance is the neutral-fibre arc of the bend: BA = α (r + K e), with α in radians. With 2 mm plate, a 2 mm inside radius, a 90° bend and K = 0.325: BA = 1.5708 × (2.0 + 0.325 × 2.0) = 4.16 mm. With outside dimensions, the bend deduction is 2R − BA, with R = (r + e) tan(α/2) = 4.00 mm, so 3.84 mm to take off the sum of the legs.
Which K-factor should I use?
By default, the calculation uses the DIN 6935 table: k = 0.65 + 0.5 log10(r/e), bounded between 0 and 1, and K = k / 2. For r/t = 1.00, k = 0.650 and K = 0.325; for r/t ≥ 5, k = 1 and K = 0.5 (fibre at mid-thickness). In air bending, K is often 0.3 to 0.45: calibrate on a trial part.
How do I work out the flat length of an angle?
Example: 2 mm plate, 2 mm inside radius, outside legs of 30 and 50 mm, 90° bend. K = 0.325; BA = 1.5708 × (2.0 + 0.325 × 2.0) = 4.16 mm; R = 4.00 mm; correction = −3.84 mm. Flat length: 80.0 − 3.84 = 76.2 mm, bend line 28.1 mm from the edge of leg 1.
What is the minimum bend radius of steel plate?
The calculation uses DIN 6935 table 1 for S235 steel, bend parallel to rolling, up to 120°: for example 2.5 mm for 2.5 mm plate, 6 mm for 4 mm, 10 mm for 6 mm, 20 mm for 10 mm. Below that, it flags a cracking risk. For the end legs, DIN 6935 suggests at least about 4 r.
Limits
Indicative results: the K-factor depends on the material, the bending method (air, bottoming) and the rolling direction. Bend a trial part before the batch. From 2 to 5 legs, bend angles strictly between 0° and 180°. Beyond r = 5 t, springback becomes significant: correct the angle on the press.
Sources
- DIN 6935 : pliage à froid des produits plats en acier, facteur de correction k selon r/e, longueur développée : Table k (r/e) et formules de longueur développée (link)
- DIN 6935 : rayons de pliage minimaux (tableau 1, acier S235), longueur de tronçon ≈ 4 r : DIN 6935, tableau 1 et figure 2 (link)
- Fibre neutre, arc de pli et retrait : méthode du facteur K : BA = α (r + K e) ; retrait = (r + e) tan(α/2) (link)