One liter covers most adult lunches.

One liter (about 4 US cups) handles a normal adult lunch, and you should size up or down based on appetite, not age.

Standard capacities break down roughly like this: small kids need 0.5–0.75L (2–3 cups), older kids and light eaters sit around 0.75–1L (3–4 cups), average adults land at 1–1.5L (4–6 cups), and serious appetites should look at 2L (8 cups) or more. Bigger isn’t better — a huge box just invites you to fill it.

The honest test is to pack your actual lunch in whatever container you already own and look at the volume. No measuring cup? A normal adult lunch runs about a fist of protein, a palm of carbs, plus a fruit or veggie — that usually lands near 1L. If you’re mostly eating a sandwich and a yogurt, you’re a 750ml person. If you’re packing two sandwiches, a salad, and snacks, you’re 1.5L.

One practical rule: buy slightly too small, not slightly too big. A little squeeze keeps portions honest and the bag light. And check the claimed capacity on the product page — many “2L” bento boxes are counted with the lid on, which is cheating.

Your appetite varies day to day, so pick the size that fits a boring Tuesday, not the day you bring leftovers.