Why PET Seedling Trays Are Measured by Weight & PS by Thickness

If you’ve shopped for seedling trays, you’ve noticed something confusing:

PET trays are labeled by weight (grams), but PS trays are labeled by thickness (mm).

This isn’t random. It’s based on material properties, manufacturing, industry standards, and real‑world performance.

In this guide, we explain exactly why the two most popular plug tray materials use different measuring systems — so you can buy confidently, avoid low‑quality traps, and choose the right trays for your operation.

Quick Summary

Weight reflects total material, is stable for thermoforming, and matches real durability.

PET trays → measured by weight (grams)

PS trays → measured by thickness (mm)

Thickness is consistent, predicts rigidity, and prevents “fake weight” from fillers.

Why PET Seedling Trays Use Weight (Grams)

PET (polyethylene terephthalate) is tough, flexible, UV‑resistant, and ideal for reuse.

PET is thermoformed & thickness varies

Most PET trays are made via thermoforming (vacuum forming).

The plastic sheet stretches during production, so:

• Cell bottoms become thinner

• Edges and rims stay thicker

  A single thickness measurement cannot represent the whole tray.

Weight = true material & real strength

For PET, weight equals quality.

Higher gram weight = more plastic = stronger, longer‑lasting, more tear‑resistant trays.

Weight is:

• Easy to measure

• Fair for international trade

• Hard to fake

Industry standard supports weight

Agricultural standards recognize weight as the primary quality metric for PET seedling trays.

In short:

PET → weight is the most reliable measurement.

2. Why PS Seedling Trays Use Thickness (mm)

PS (polystyrene) is rigid, smooth, and widely used for standard and automated seeding lines.

PS is consistent in thickness

PS trays are usually injection‑molded or thick‑sheet formed.

Thickness is uniform across the entire tray.

One measurement shows real strength.

PS can be “weight‑cheated”

Some cheap PS trays add heavy mineral fillers to raise weight without improving strength.

This gives you:

• Heavy but brittle trays

• Poor durability

• Bad performance in automated lines

Thickness cannot be faked.

Thickness = rigidity & machine compatibility

PS rigidity depends almost entirely on thickness.

Greenhouse automation systems (seeding, transplanting) require strict thickness consistency.

Standard PS thickness guide:

• 0.6 mm: lightweight, single‑use

• 0.8 mm: general purpose

• 1.0 mm: durable, multi‑use

• 1.2 mm: heavy‑duty, commercial reuse

In short:

PS → thickness is the only trustworthy measurement.

How to Buy the Right Tray

For PET trays

• Check weight per tray

• Ignore single‑point thickness

• Heavier = stronger

For PS trays

• Check thickness in mm

• Don’t trust weight alone

• Thicker = longer life

Final Thoughts

The difference isn’t a marketing trick — it’s material science and industry logic.

• PET uses weight because thickness is uneven.

• PS uses thickness because weight can be misleading.

At seedlingtrayexperts.com, we provide clear, honest specs so you always get the quality you pay for.