Plate Roll Blank Size Calculator

Why blank length uses the neutral axis

When plate is rolled, the outside surface stretches and the inside compresses — the same behaviour as a press brake bend, wrapped through 360°. The length that stays constant is the neutral axis, and for plate rolling it sits close enough to mid-thickness to use directly.

Blank length = π × (ID + t)

ID — finished inside diameter in mm
t — material thickness in mm

Adding one thickness to the inside diameter places the calculation on the mid-thickness line. Using ID alone gives a blank that is too short; using outside diameter gives one that is too long.

Partial arcs

For anything less than a full cylinder, the blank is that fraction of the full circumference:

Shape	Blank length
Full cylinder (360°)	π × (ID + t)
Half cylinder (180°)	0.5 × π × (ID + t)
Quarter cylinder (90°)	0.25 × π × (ID + t)

Seam allowance

A full cylinder needs a longitudinal weld to close it. Add 5–10 mm to the blank length for fit-up and weld preparation. Partial arcs have no closing seam, so no allowance is needed.

What this calculator does not include

Flat ends — the leading and trailing edges of a plate cannot be rolled without pre-bending. Plan for pre-bend or plan to trim.
Springback — plate relaxes slightly after rolling, so machines typically over-roll to land on target diameter.
Cone geometry — a tapered section develops as an annular segment, not a rectangle, and is a different calculation entirely.

Frequently Asked Questions

How do you calculate the flat blank for a rolled cylinder?

Blank length = π × (ID + t), where ID is the finished inside diameter and t is material thickness. Adding one thickness to the inside diameter puts the calculation on the neutral axis at mid-thickness, which is the length that stays constant through rolling.

Should I use inside or outside diameter?

Neither on its own. Use the neutral axis — inside diameter plus one material thickness. Calculating from inside diameter alone produces a blank that is too short, and from outside diameter one that is too long, with the error growing as material gets thicker.

How much weld seam allowance should I add?

Add 5–10 mm to the blank length for a full cylinder, to cover fit-up and weld preparation. Partial arcs such as half or quarter cylinders have no closing seam and need no allowance.

Does this work for cones?

No. A conical section develops as an annular (curved trapezoidal) blank rather than a rectangle, derived from the top diameter, bottom diameter and slant height. That is a separate layout calculation, normally done in CAD or nesting software.