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What is D of bend?

Tube fabricators sometimes indicate the centerline radius using D of bend.

We say it by speaking three words: "D of bend".

Some tube fabricators also say "D factor" to mean the same thing.

It is a ratio calculated like this: D of bend = Centerline Radius / OD

How Is It Used?

FOR FABRICATORS

The D of bend value helps fabricators know if the radius is tight or large compared to the OD.
It serves as a general guide for how difficult it will be to create a quality bend.
It helps fabricators know what kind of tooling will be necessary to achieve the bend. It is useful, therefore, in building quotations for customers.
It is common for fabricators to share the lower limit of their D of bend capabilities with customers.

FOR DESIGNERS

It is helpful to designers as a limit for what is practical in choosing radii.

Example Values

In most applications, a lower D of bend valued radii is less complex and therefore less expensive.

D of bend = 2.00 or greater is generally considered not difficult.
D of bend = 1.50 is moderately difficult.
D of bend = 1.00 is difficult.
D of bend = 0.75 is very difficult and much more expensive.

D of bend values at 1 and below are possible - but are rare and require special benders that can press material into the bender's dies. (Most call this "carriage boost".)
Note: Other variables, like wall factor, impact what is difficult. Wall factor = OD / Wall Thickness