Measure the opening, not just the depth.

Typical lunch boxes run about 9–11 inches long, 7–9 inches wide, and 4–6 inches tall — but the real test is your backpack pocket’s opening and the shelf height.

Those numbers are for the classic soft-sided or basic bento box. If you’re looking at a hard cooler-style box or a tall insulated tote, add an inch or two in every direction. Most are around 11 x 8 x 5, which is roughly a 4- to 6-quart capacity. Enough for a sandwich, snack, and a drink can.

To measure one properly: grab a tape measure and get the length, width, and height at the widest points. For a backpack, don’t just compare that to the pocket’s interior dimensions — measure the zipper opening. A box can be the right size on paper but impossible to get through a narrow opening. Same goes for a fridge shelf: measure the vertical clearance between shelves, plus the shelf depth. Door shelves are smaller than you think.

If it doesn’t fit when empty, it won’t fit when full. And if it’s for a kid’s backpack, round down your estimate — they will find a way to jam it in sideways.