Go for two to three liters

Most high school students do just fine with a lunch box in the 2–3 liter range.

That’s enough for a real sandwich, a piece of fruit, a snack, and a drink — which is basically the standard high school lunch. If your kid is a competitive athlete or eats like a bottomless pit, bump it up to 4 liters. Beyond that you’re just buying a cooler they’ll have to lug around.

Bigger isn’t better here. Oversized lunch boxes don’t fit in lockers, eat up backpack space, and quietly encourage overpacking. A kid who doesn’t eat everything feels bad, throws food away, or starts skipping the lunch box entirely.

The more important thing is insulation. A good insulated bag at 3 liters beats a giant non-insulated box every time. Also check whether they actually drink from a separate water bottle during the day — if so, you don’t need a bottle pocket eating into the food space.

If they’re packing every day, a hard-sided bento-style box is easier to clean. If they just want to grab and go, a soft insulated bag with one big compartment is fine. Don’t overthink it.