Most lunch boxes are dishwasher-safe. The lids are the problem.

Most lunch boxes can handle the dishwasher, but the lids and liners are usually what give up first.

Plastic is the big one to check. If it has a #5 (polypropylene) symbol, you’re fine — that’s the most common lunch box plastic and it handles the dishwasher fine. Tritan is also safe. Melamine is the one to avoid: it’s often used in “fancy” bento boxes and kids’ sets, and it cracks, crazes, and can leach after repeated hot washes. If it doesn’t have a recycling number and feels hard and ceramic-like, hand wash it.

Stainless steel is fine but not perfect. The metal itself won’t warp, but insulated steel containers can lose their vacuum seal if they’re heated regularly, which kills the insulation. Also, decent chance the outside gets water spots or mild discoloration from dishwasher detergent. If it’s a cheap thermos, top rack or hand wash. If it’s an expensive one, hand wash.

The lids are where all lunch boxes die. That rubber gasket around the lid traps water, degrades with heat, and eventually stops sealing — then you’ve got a leaky bag and a sad sandwich. Same with any silicone seal. Glass containers are fine on the top rack, but let them cool completely before washing so the thermal shock doesn’t shatter them. Wood or bamboo lids? Never. Ever.

Hand wash the gasket and you’ll double the life of any lunch box.