Dnipro, Robocha St.Mon–Fri 9:00–18:00Shipping across Ukraine
CalculatorsDelivery and paymentAbout the companyContacts
Catalog
+380 50 362-09-72Viber · Telegram · +380 96 269-61-72
Request a quoteCart
Catalog
VisesDrill pressesMetal lathesPressesBand sawsBelt grindersWoodworkingGrindersElectric hoist spare partsPress spare partsDiamond toolsAll categories →
Request a quoteCalculatorsArticlesDelivery and paymentReviewsAbout the companyContacts
Sheet metal

Flat pattern and bend radius

Enter the outside flange dimensions, thickness, inside radius and bend angle - the calculator returns the blank length before bending and the bend allowance.

126.4 mm blank
K-factor
0.40
Neutral-axis arc
4.40 mm
Bend deduction
3.60 mm
A 50B 8090°PartR 2, s 2Blank 126.4 mm46.0bend 4.476.0
Not sure about the numbers? An engineer will check the calculation and pick equipment from stock or to order.

Matching products

Benders and rolls →

How it is calculated

When sheet is bent, the outer fibres stretch, the inner fibres compress, and somewhere in between lies a layer whose length does not change - the neutral axis. The flat pattern is calculated along it. Its position is set by the K-factor: the fraction of thickness measured from the inside surface.

For one bend: L = (A - R - s) + (B - R - s) + BA, where A and B are outside flange dimensions, R is the inside radius and s the thickness. Bend allowance BA = π × (R + K × s) × angle / 180.

Example: A = 50 mm, B = 80 mm, s = 2 mm, R = 2 mm, 90°. R/s = 1, so K = 0.4. Straight sections: 46 + 76 = 122 mm, BA = 3.14 × (2 + 0.8) × 90 / 180 ≈ 4.4 mm. Flat length - 126.4 mm.

K-factor by R/s ratio

R/sKBA at s = 2 mm, 90°
below 1 (R = 1 mm)0.332.6 mm
1 to 3 (R = 2 mm)0.44.4 mm
3 and above (R = 6 mm)0.511.0 mm

Practical tips

  • A and B are outside dimensions, as measured with a caliper on the finished part. If the drawing gives inside dimensions, add the thickness.
  • In air bending the inside radius is set by the die, not the punch: about V/6. Work out the die and tonnage with the bending force calculator.
  • K depends on the material and tooling. For a batch, bend a test strip, measure it and refine K.
  • For a part with several bends, calculate each bend separately and add all straight sections and all BA values.
  • The angle in the formula is how far the sheet was bent (90° for a right angle), not the included angle between flanges when that is obtuse.

When you need a more accurate method

For precision parts, stainless and aluminium with strong springback, and for bending on rolls or press brakes with non-standard tooling, use the tool maker's tables or test bends.