Stainless is fine. Aluminum isn't.

Stainless is fine. Aluminum isn’t.

If the dividers are stainless steel, your lunch box is probably fine in the dishwasher; if they’re aluminum, it’s not.

Stainless steel can handle dishwasher detergent without rusting or warping. It might lose a little shine over time, but that’s cosmetic. Aluminum, though, reacts with the detergent and can get dark, pitted, or leave a weird residue that tastes like metal.

Also check what else is attached. Glued-in dividers, a wooden lid, or a rubber gasket won’t survive the dishwasher well. Same for plastic parts that aren’t marked dishwasher-safe.

If it’s all stainless and nothing is glued on, load it up. When in doubt, hand-wash it — that lunch box doesn’t need a rinse cycle.