Thermoforming Draw Ratio Calculator

Thermoforming stretches a flat, uniformly thick sheet down into a mold cavity, and the sheet doesn't gain any material as it does so — it just spreads thinner. The draw ratio, the ratio of the formed part's total surface area to the original opening's area, is the direct measure of how much stretching happened, and this calculator computes it geometrically for two cavity shapes. For a rectangular box it sums the bottom plus the four trapezoidal side walls (accounting for the mold's draft angle, which shrinks the bottom footprint relative to the opening); for a cylindrical cup it treats the wall as a conical frustum and uses the standard frustum lateral-area formula with the slant height derived from draw depth and the radius difference caused by draft.

Once it has that areal draw ratio, it estimates wall thickness two ways: dividing the original sheet gauge by the full ratio gives a conservative minimum (roughly what the thinnest corners and bottom edges will measure), while dividing by the square root of the ratio gives a more realistic part-average, since material doesn't thin perfectly uniformly. Feasibility is then just a threshold read on the areal ratio — under 2:1 is routine, above 5:1 is pushing (or past) what most sheet materials can survive without tearing or excessive thinning at the corners. A common mixup: draft angle matters more than people expect at deep draws, since it directly changes both the bottom area and the wall's slant length — zero draft is the worst case for thinning, and even a few degrees measurably eases the draw.