Spinning Blank & Disc Weight Calculator

Two numbers decide whether a spun part is feasible and what it costs in material: the flat blank you start from, and the weight of that disc. Work both out here before you cut anything.

Step 1

Developed Blank Diameter

Enter the finished part dimensions. The calculator works from the developed surface area of the shell — the flat disc that contains exactly the same area as the finished form.

Finished shape

Finished diameter D (mm) Wall height h (mm) Closed bottom

Trimming allowance (%)

3–5% is normal. Go to 8% on deep cones or thin material — the rim thickens and ears as it spins, and all of it gets trimmed after forming.

Blank diameter

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Formula used

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Step 2

Disc Weight & Material

The blank diameter above feeds straight into this. Pick a material, set the sheet thickness, and you get the weight of one disc — the figure your material cost is built on.

Blank diameter (mm)

Filled automatically from step 1 — overwrite it if you already know your blank size.

Sheet thickness (mm) Material

Density in g/cm³, shown next to each grade.

Weight per disc

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Blank area

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Method

How the Blank Diameter Is Worked Out

Cylinder, closed bottom

D_blank = √(D² + 4·D·h)

Side wall πDh plus base πD²/4, solved back to a single disc.

Cylinder, open ended

D_blank = √(4·D·h)

Side wall only — typical for sleeves, collars and rings.

Cone / frustum

D_blank = 2·√((R₁+R₂)·L + R₂²), L = √(h² + (R₁−R₂)²)

Lateral surface of the frustum plus the small-end base, if there is one.

Hemispherical dome

D_blank = D · √2

A hemisphere has twice the surface area of the disc it is spun from.

All four are developed-surface calculations: they assume the sheet keeps its thickness as it flows over the mandrel. That is a good assumption for most spinning work up to a 60° included angle, and it is what we start from when we size a blank in the shop. On very deep draws the nose thins, so the blank ends up slightly larger and the pass sequence is set to control it. If your part is at that end of the range, send the drawing and we will calculate the practical blank and the pass schedule rather than the textbook one.

Reference

Sheet Metal Density by Grade

Material	Density (g/cm³)	1 m² at 1 mm
Aluminium 1050 / 1100 / 3003	2.71	2.71 kg
Aluminium 5052	2.68	2.68 kg
Aluminium 6061	2.70	2.70 kg
Stainless steel 304	7.93	7.93 kg
Stainless steel 316L	8.00	8.00 kg
Stainless steel 430	7.70	7.70 kg
Carbon steel	7.85	7.85 kg
Galvanised steel	7.85	7.85 kg
Brass	8.50	8.50 kg
Copper	8.96	8.96 kg
Titanium	4.51	4.51 kg

Questions

Using These Numbers in a Real Quote

Why is the blank bigger than the finished part looks?

Because a flat disc has to contain the whole surface area of the finished shell. A 300 mm diameter cylinder 200 mm tall needs a disc roughly 536 mm across before allowance — the wall alone is 188,000 mm² of surface that has to come out of that disc. Add the trimming allowance and the blank is bigger still.

What trimming allowance should I use?

3–5% on straightforward work. Go to 8% when the cone is deep, the material is thin, or the rim has to be rolled — the edge thickens and can go wavy, and the trim is what squares it up. Under 3% usually costs more in scrapped parts than it saves in material.

Does the calculator account for wall thinning?

No. It is a developed-surface calculation and assumes constant thickness. On deep parts the nose thins by a few percent and the material at the rim thickens to match; that is controlled by the pass sequence and the blank allowance, not by the geometry alone.

How accurate is the weight figure?

Within about 2–3% of the real disc, most of the difference coming from mill tolerance on sheet thickness, which is typically ±0.05 mm on a 1.2 mm sheet. It is close enough to build a material cost on — and if you order to a nominal thickness, the mill tolerance is the supplier's allowance, not yours.

Can I spin a part from a blank size I already have in stock?

Often yes. If your stock disc is larger than the calculated blank, the surplus goes into the trim. If it is smaller, the part can sometimes still be made with a different pass sequence — but the height or the diameter has to give. Send the drawing and the disc size you have, and we will tell you which one has to move.

Working through the design rules before you cut a blank? The metal spinning design guide covers minimum radii, wall thickness limits and what has to be machined after forming. For the cost side, spinning vs stamping compares tooling and unit cost at different volumes.

Send the Drawing — We’ll Confirm the Blank

We come back with the practical blank size, material weight and unit price within 24 hours.

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