The bottom of the lunch box is what fits the cup holder

To get a lunch box that sits securely in your car’s cup holder, you measure the cup holder’s diameter and then pick a box with a tapered or rounded base that matches it.

Most cup holders are about 3 inches across. A standard rectangular or square lunch box won’t stay put because the base is too wide. What works is a box that narrows toward the bottom—think cone-shaped or trapezoid. Also check depth: if the cup holder is shallow, a tall box will tip over the first time you hit the brakes.

If you already have a lunch box you like and it doesn’t fit, don’t buy a new one right away. A cheap cup holder expander or a rubber coaster adds enough grip to make it work. Some lunch boxes come with a detachable base that snaps into the cup holder—those are handy but not worth overpaying for unless you know your holder’s exact size.

Measure first, then buy. A wobbly lunch is a bad way to start the afternoon.