Look for #5 plastic or Tritan, not mystery plastic

Look for polypropylene (recycling #5) or Tritan, and skip anything that feels flimsy.

Top-rack dishwasher safe is a minimum, not a promise. That little symbol just means the plastic won’t melt in normal dishwasher heat. But warping happens over time, especially with thin walls and cheap molds. #5 polypropylene handles heat well and holds its shape. Tritan is a copolyester that does the same, plus it’s clear and doesn’t stain as easily.

Avoid acrylic, polycarbonate, and anything that feels like a disposable takeout container. Those will get cloudy, crack, or twist out of shape after a few trips through the top rack. Also, pay attention to the lid — a lot of lids are made from softer plastic or have silicone gaskets, and those can be the first thing to warp.

If you already own a lunch box you like but aren’t sure, run it through the top rack a few times and see what happens. Not all plastics behave the same, even among “safe” ones. When in doubt, buy one from a brand that specifically says “dishwasher safe” rather than just “top-rack safe,” or go with a stainless steel body and a silicone lid — that’s basically indestructible.