A 2-liter lunch box fits most adults.

A 2-liter lunch box is the sweet spot for most adults, and kids do fine with 1 to 1.5 liters.

Adults usually need room for a main dish (sandwich, leftover pasta, big salad), a side or two, silverware, and maybe a small ice pack. That adds up to about 1.5 to 2 liters of actual volume. If you pack tall containers or like to bring a drink bottle inside the box, go up to 2.5 or 3 liters. Really anything over that is overkill unless you’re a construction worker or going on a day-long hike.

For kids, the math is simpler. A child in elementary school eats maybe half to two-thirds of what you do. A 1-liter box handles a sandwich, fruit, and a snack fine. Teens who eat like adults can jump to a 2-liter box, but for most kids under 12, 1 to 1.5 liters is plenty. Oversized boxes just mean more empty space, which lets food slide around and get squished.

One thing nobody tells you: a bigger box doesn’t equal a bigger lunch. It just makes you feel like you under-packed it.

When in doubt, measure what you normally pack in a plastic bag or container. That’ll tell you exactly what you need without guessing.