You need about 2 liters for a normal lunch

For a normal lunch — a main dish plus a side or two — grab something in the 2-to-3-liter range. That’s roughly identical in quarts, so don’t overthink the units.

Liters and quarts are almost the same thing: 1 liter is 1.06 quarts. So a bag that says “2.5 quarts” is a 2.5-liter bag for all practical purposes. The conversion math is a trap; ignore it.

What actually matters is what you pack. A sandwich, yogurt, apple, and chips fits in 1.5–2 liters. Throw in a thermos of soup or a big salad container and you’re at 3. A meal-prep person stacking four glass containers needs 3.5–4.

My rule: look at your biggest container, add about a third of that volume for the rest, and buy that size. And resist the urge to go big — an oversized box means stuff slides around and you feel weirdly obligated to pack more. The right lunch box is one you can close without sitting on it.