Bento boxes are Japanese style packed lunches containing protein, rice and vegetables. The size of the bento box determines the portion size and can impact the total calorie count, as can the food choices you make for your bento.
As a rough estimate, the number of milliliters a bento box holds is equivalent to the total calories contained within the bento, according to Makiko Ito, author of "The Just Bento Cookbook." To determine the capacity of your bento box, fill with water, then pour off into a measuring cup marked with milliliters.
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Bento boxes filled with generous portions of rice, fried foods or other fatty ingredients will be higher in calories. Choose bentos with a smaller amount of rice, lean protein and plenty of vegetables to lower the total calorie count.
Packing your own bento boxes is an easy way to control calories, but purchased bento box lunches can be a reasonable choice. A purchased bento box lunch from a Japanese fast food restaurant has about 525 calories, according to Livestrong's MyPlate. Larger portions or higher-fat foods may increase calorie counts.