Get the tall skinny lunch bag, not the big wide one

For a hungry teenage boy, aim for a 7-to-9-liter lunch bag, but prioritize a tall, narrow shape so it slides into a backpack without taking over the whole bag.

That size range is enough for a big main dish, a couple of snacks, and a drink bottle — but the real trick is shape. A wide rectangular cooler might hold the food, but it eats the bottom of the backpack and jams against books. A taller, slimmer lunch tote, about the size of a loaf of bread standing up, fits better beside binders and still carries a surprising amount.

Check the backpack’s main compartment dimensions before you buy. If the lunch bag is taller than the backpack is wide, it can lie flat on top of books instead of fighting them for space. Also go with a soft-sided insulated bag, not a hard cooler — soft bags compress a little when the pack is packed tight.

One more thing: don’t chase max capacity. If the bag is too bulky, he’ll stop using it and just shove food loose into his backpack. A 7-liter model with a few good containers is enough — as long as the containers are more vertical than flat.

Get the one that disappears into the backpack, and he’ll actually take it.