What Size Blank Makes Your Cup?

A drawn part starts as a flat circle. Enter the cup you need and see the blank it takes, what it weighs, and whether it is likely to draw in one stroke.

Cup diameter, in 10.00 Cup height, in 4.00

Bottom corner radius, in Flange diameter, in optional Trim allowance, in Gauge, in Or pick a gauge

Blank diameter: –
Draw ratio: –
Likely draws: –
Blank weight: –
Steel area: –
Cup depth to diameter: –

Draw plan, stroke by stroke
Stage	Diameter	Height	Reduction

Quote These Numbers Nest This Blank

Enter the cup's mean diameter, halfway through the wall, and its height to the top of the wall. Steel area is the same before and after drawing, so the blank is the flat circle with the cup's surface area. The draw plan assumes steel draws about 1.8 times its diameter in the first stroke and about 1.25 times in each redraw; heights between strokes ignore the corner radius. Your real blank and draw plan are set on the print and the die.

How It's Calculated

Sharp-cornered cup: D = √(d² + 4dh)
With a bottom corner radius r: D = √(d² + 4dh − 1.72dr − 0.57r²)
With a flange of diameter F: add F² − d² under the root
Trim allowance t: added to the height, or to each side of the flange
First draw: d₁ = D ÷ 1.8
Each redraw: dₙ = dₙ₋₁ ÷ 1.25, until the cup diameter is reached
Height at each stage: h = (D² − d²) ÷ 4d, or (D² − F²) ÷ 4d with a flange

Questions About Cup Blanks

How do you calculate the blank size for a drawn cup?

In drawing the steel's thickness stays close to the same, so the surface area of the finished cup equals the area of the flat blank. Add up the bottom, the corner, the wall and any flange, then find the circle with that area. For a straight-walled cup with sharp corners, the blank diameter is the square root of d² + 4dh.

What is the draw ratio?

It is the blank diameter divided by the cup diameter. The higher it is, the harder the draw. Drawing-quality steel usually manages a ratio of about 1.8 to 2 in the first stroke; past that the wall tears, so the part is drawn partway and then redrawn.

Why add a trim allowance?

The top edge of a drawn cup comes out uneven, because steel stretches differently in different directions. Parts are drawn a little tall and trimmed level, so the blank needs extra steel for the trim. A quarter inch or so on the height is common; the die designer sets it.

Does the corner radius change the blank size?

Yes, slightly. A rounded bottom corner uses less steel than a sharp one, so the blank gets smaller. It matters most on short cups with large radii.

Where We Do This Stamping