Thermoforming Calculator
Estimate draw ratio, wall thickness and formability before you cut a mould — from sheet gauge, cavity depth and width, with a feasibility verdict by material.
Inputs
Result
Estimates for a simple rectangular cavity — real wall distribution depends on tool design, plug, pre-stretch and process. Validate on tool. See the thermoforming guides.
How to use itCheck formability in one screen
- Pick the sheet material — it sets the forming-temperature window and the practical draw-ratio limits.
- Enter starting sheet thickness, cavity depth and cavity width (the smallest opening dimension the material has to draw across).
- Set whether the tool uses plug assist. The calculator returns the draw ratios, the estimated wall thicknesses and a feasibility verdict — so you know before cutting a mould whether the part will thin too much.
Why it mattersWhy draw ratio decides the part
Thermoforming conserves material: the sheet is stretched over a larger surface, so the deeper and narrower the cavity, the thinner the walls become. Push the draw ratio past what the material can handle and the corners thin to the point of pin-holing or splitting — often discovered only after the tooling is cut. Estimating the draw ratio and resulting wall thickness up front tells you whether a design is feasible, whether it needs plug assist or pre-stretch, and whether a thicker starting gauge is required.
The mathsThe formulas
Area draw ratio = 1 + 4 × (depth ÷ width)
Average wall = sheet thickness ÷ area draw ratio
Material is spread over the formed surface, so a bigger area draw ratio means thinner walls. The minimum wall (usually at corners) is estimated at roughly 55% of the average without plug assist, and about 80% with plug assist, which redistributes material more evenly. These are planning estimates for a simple rectangular pocket — round, multi-cavity and complex geometries behave differently and should be confirmed on the tool.
ReferenceForming window & draw-ratio limits by material
| Material | Forming temp (°C) | Max H:D (plain) | Max H:D (plug) |
|---|---|---|---|
| APET | 120–165 | ~1.0 | ~2.5 |
| PETG | 120–160 | ~1.5 | ~3.0 |
| PP | 150–165 | ~1.0 | ~2.0 |
| HIPS / PS | 130–180 | ~1.2 | ~2.5 |
| PVC | 100–150 | ~1.5 | ~3.0 |
| PLA | 90–110 | ~0.8 | ~1.5 |
Next stepImproving wall distribution
If the estimate warns of over-thinning, the levers are plug assist and pre-stretch (mechanical and pneumatic), a shallower or wider cavity, a thicker starting gauge, or generous corner radii. Tooling design does much of the work — see our guide on thermoforming tooling, and the thermoforming machine reference.
FAQFrequently asked questions
What is a good draw ratio for thermoforming?
As a rule of thumb, most materials form easily up to a depth-to-width (H:D) ratio of about 1:1 without plug assist, and up to roughly 2:1–3:1 with plug assist, depending on the polymer. Beyond that, walls thin excessively at the corners.
How do I calculate wall thickness in thermoforming?
Material is conserved, so the average wall equals the starting sheet thickness divided by the area draw ratio (1 + 4 × depth/width for a simple pocket). The thinnest point, usually a corner, is roughly 55% of that without plug assist and about 80% with plug assist.
Does plug assist really help?
Yes. A plug pre-stretches the sheet and carries material into the cavity before forming, so it distributes thickness far more evenly and lets you form deeper cavities from the same gauge without the corners thinning to failure.
What forming temperature should I use?
It depends on the polymer: roughly 120–165 °C for APET, 150–165 °C for PP, and 90–110 °C for PLA. The window is narrower for crystalline materials like PP, so temperature control matters most there.
Why is my part too thin at the corners?
Corners are the last area to form and stretch the most, so they thin first. If the draw ratio is high, add plug assist or pre-stretch, increase the corner radius, widen or shallow the cavity, or start from a thicker gauge.