Many brands of whole-grain bread have slightly different calorie counts, due to minor differences in each manufacturer's recipe. One standard, 26-gram slice of whole-grain bread contains 69 calories, according to the U.S. Department of Agriculture's National Nutrient Database for Standard Reference. A large, 41-gram slice has 109 calories.
Of the 69 calories in one 26-gram slice of whole-grain bread, about 65 percent of them are from carbohydrates. About 20 percent of the calories come from the 3.5 grams of protein. Fat contributes 14 percent of the calories, with about 1 gram per slice. It is worth noting that a slice of whole-grain bread has almost 2 grams of fiber, whereas white bread has less than 1 gram of fiber per slice.