Measure the backpack's bottom, not the label.

To fit upright, your lunch box needs to be shorter than the backpack’s main compartment and have a base that fits between the sides—so grab a tape measure before you buy anything.

The “15-inch” label only tells you the laptop sleeve size. It says nothing about the interior. Most 15-inch laptop backpacks have a main compartment around 17-18 inches tall, but the bottom is often narrower than the opening because backpacks taper. The laptop sleeve also eats into depth, so you can’t just pick any tall lunch box and expect it to stand.

A safe bet is a rectangular lunch box around 9-10 inches tall, 7-8 inches wide, and 3-4 inches deep. That fits inside most standard backpacks without pressing on the laptop. Make sure the bottom is rigid—soft-sided bags slump and end up lying flat, which defeats the purpose.

If you can, test it with the actual backpack before you commit.