Most lunch boxes are not microwave-safe — check the symbol, but honestly just use glass.

You can microwave a lunch box, but only if it says “microwave-safe” and even then, there are catches.

The safest materials are glass and ceramic. They won’t warp and don’t leach. For plastic, look for the microwave-safe symbol (wavy lines) and the resin code #5 (polypropylene). That’s the most common microwave-safe plastic. Avoid #1 (water bottles — melts), #3 (PVC — leaches), #6 (styrofoam — melts and leaches).

The tricky part: many lunch boxes are a mix — a PP base but a lid made of #4 or a different plastic that’s not rated for heat. So even if the base is fine, the lid can warp. And two, “microwave-safe” means the plastic won’t melt or deform easily, but it doesn’t mean it won’t leach chemicals into greasy, hot food. If you’re reheating leftovers, the safest move is to empty the food into a glass or ceramic dish.

As for brands — nobody can promise a specific brand will never warp because batches vary. Your best bet is buying something with an explicit “microwave-safe” label from a reputable brand like Rubbermaid, Pyrex (glass), or Glasslock. Pyrex or any borosilicate glass container is the actual answer.

If it doesn’t say microwave-safe, assume it isn’t. Your lunch deserves better than melted plastic.