Don't shop by age — shop by appetite

The right lunch box capacity comes down to how much you actually pack, not whether you’re shopping for an adult or a child.

A 10-year-old swimmer can out-eat a desk-bound adult, so age is a terrible guide. Rough numbers: most kids’ lunch boxes run 4–6 liters, and adult boxes go 8–12 liters. But those numbers lie a bit, because containers and any drink bottle eat up space fast.

More reliable method: count the containers you’ll pack. If it’s a small bento with 2–3 containers, a 5–7 liter box handles either a big kid or a light adult. If you’re carrying a full meal plus a thermos, a drink, and a snack, go 10 liters or bigger. The box should fit the containers you already own, not some abstract “adult portion” size.

If in doubt, bring your actual containers into the store and see if they fit before you buy.