Use water, not math

The easiest way to measure a lunch box’s capacity is to fill it with water, then pour that water into a measuring cup.

That’s it. No geometry, no guessing. Lunch boxes have weird curves and dividers, so any math based on outside dimensions will be off. Water fills every nook, so it gives you the real number. If you measure in milliliters, just remember 1,000 ml = 1 liter. So if it comes out to 750 ml, that’s 0.75 liters.

Don’t want to get the box wet? Use dry rice or beans instead. Same trick, no cleanup beyond pouring them back.

If you don’t have a measuring cup, you can ballpark it with a rectangular box by multiplying length × width × height in centimeters and dividing by 1,000. But that only works for clean rectangular shapes. For anything else, water wins.

Your lunch box probably holds more than you think.