How to calculate bending dimensions in metal plates

Updated February 21, 2017

Metal fabrication requires bending a product to exacting standards. Calculating the bending dimensions of the metal plate is crucial to the final product. Many factors determine where a bend in a metal plate should be located. This includes the thickness of the metal plate, whether the dimensions of the product are outside (O.D.) or inside (I.D.) and the radius of the dies to be used to bend the metal plate. Missing one factor will result in the bend dimensions being incorrect and ruining the metal plate.

Read the blueprint to determine the areas of the metal plate that require a bend and whether the bends are O.D. or I.D. dimensions. If the listed dimensions are I.D., the bend dimensions are per the blue print, meaning you do not have to deduct the metal thickness or the radius of the bending dies. For O.D. dimensions continue to step two.

Measure the thickness of the metal plate with the tape measure.

Secure the bending dies into the press brake per the manufacturer's operating manual.

Slide the metal test plate into the bending dies and bend the test plate.

Measure the radius of the bend on the test plate with the tape measure. If the bend radius is different from the metal thickness, you will adjust the bend location to compensate for the difference. For example, a set of bending dies designed to bend 3 mm (1/8 inch) metal plate may make a 4mm (5/32 inch) radius bend.

Deduct the thickness of the metal plate from the dimension specified by the blueprint. For example, if you were bending a 3 mm (1/8 inch) metal plate into a 100 by 150 mm (4 by 6 inch) angle, the bend location would be 100 mm (3 7/8 inches) in from one end of the metal plate and the same mark would sit 15 mm (5 7/8 inches) in from the other end of the metal plate.

Adjust the bend dimension to compensate for the difference between the metal plate thickness and the radius placed on the metal test plate by the bending dies in Step 4. For example, in Step 6 the test radius is 0.8 mm (1/32 inch) larger than the metal thickness, so you would need to move the bend location over 0. 4 mm (1/64 inch), one half of the difference between the metal thickness and the bend radius. This would leave the final bend location from the example at 95 mm (3 55/64 inch), leaving 145 mm (5 55/64 inch) if you trimmed the length of the metal plate to compensate for the difference.


At times a fabrication blueprint will allow a tolerance of +/- 1.5 mm (1/16 inch) and you can skip adjusting the bend location for the bending radius.

Things You'll Need

  • Blue print
  • Metal plate
  • Bending dies
  • Tape measure
  • Press brake
  • Metal test plate
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