There’s no standard, but 10 x 4 x 3 inches works

A lunchbox that fits in a standard backpack side pocket is roughly 10 inches tall, 4 inches wide, and 3 inches deep. But “standard” is doing a lot of work there — side pockets vary, so treat that as a safe target, not a guarantee.

Most backpack side pockets are built around a water bottle, usually a 32 oz Nalgene, which is about 3.5 inches wide and 9 inches tall. That means the opening is the real bottleneck. If the lunchbox is soft-sided and around 4 inches wide, it can slide in. Hard-sided boxes won’t make it unless they’re almost exactly cylindrical.

Height matters too: if it’s much taller than 10 inches, it’ll stick out, catch on things, and eventually fall out when you swing the backpack around.

So aim for a soft lunchbox around 9.5 x 4 x 3 inches. If a Nalgene fits in your pocket, a lunchbox like that will probably fit too.