Blank Size Calculator
Enter the cup's mean diameter, its height and a trim allowance. The calculator gives the diameter of the flat circular blank to cut, the draw ratio and how many draws the part needs.
- Blank diameter
- Trim allowance
- Number of draws
Use the mean diameter, measured to the middle of the wall. A trim allowance of 5% to 10% of the height is common.
A drawn cup uses the same amount of metal as the flat blank it starts from. Setting the blank's area equal to the cup's base plus its wall gives D = √(d² + 4dh). A 50 mm cup 50 mm tall needs a blank about 112 mm across before trim.
Blank size chart
50 mm cup, no trim. Search for a height.
| Cup height (mm) | Blank (mm) | Draw ratio | Draws |
|---|---|---|---|
| 10 | 67.1 | 0.75 | 1 |
| 25 | 86.6 | 0.58 | 1 |
| 50 | 111.8 | 0.45 | 2 |
| 75 | 132.3 | 0.38 | 3 |
| 100 | 150 | 0.33 | 3 |
Over about one diameter tall, most materials need a second draw.
Cup height and blank
50 mm diameter, 5% trim. Slide the height.
The flat blank on the left becomes the cup on the right. The dashed circle is the cup's base.
How to work out a blank size
Equal areas. Step through it.
Mean diameter
Middle of the wall.
For cups with a corner radius, the blank is a little smaller. Subtract about 1.72 × d × r for a radius r.
One draw, redraw or ironing
Forming methods. Compare them.
One press stroke.
Several smaller steps.
Thins the wall.
Annealing between draws lets some metals go further before they tear.
Before tooling
Press checks. Tick each item.
Tick each item as you go.
Earing, wavy rims on drawn cups, comes from the sheet's grain. Extra trim allows for it.
What you get
Blank design. Each tile explains one result.
With trim.
Exact size.
d ÷ D.
Steps needed.
Sheet used.
Steel, aluminium and brass all draw well. Stainless steel work hardens fast and may need annealing between draws.
Frequently Asked Questions
No question matches that search.
How do I calculate blank size for deep drawing?
For a cylindrical cup, D = √(d² + 4dh), where d is the mean diameter and h the height.
What is the limiting draw ratio?
The smallest ratio of cup to blank diameter that can be drawn in one step, about 0.5 to 0.55 for steel.
How much trim allowance should I add?
About 5% to 10% of the cup height, more for materials that ear badly.
Why use the mean diameter?
Because the metal's area is measured at the middle of its thickness.
When does a cup need a second draw?
When the draw ratio falls below about 0.55, usually when the cup is taller than about two thirds of its diameter.
Does wall thickness change in drawing?
A little. The formula assumes it stays the same, which is close for thin sheet.
Can I use this for a box or rectangular part?
No. Rectangular parts need a different, corner by corner method.
Start with the right blank
Find the blank diameter from the cup size, add trim and check the draw ratio. Then plan as many draws as the part needs.
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