How Many Calories Are in a Tuna & Mayo Sandwich?

A tuna sandwich makes a nutrient-rich lunch or dinner. Tuna contributes protein, vitamin B-12 and healthy omega-3 fatty acids. It fulfills one of the two servings of fish per week recommended by the American Heart Association to lower your risk of cardiovascular disease. Whole-grain bread delivers fiber, B vitamins and minerals. You can gain all of these nutrients for as few as 302 calories -- or significantly more -- depending on the types of ingredients you choose.

A 3-ounce serving of tuna contains anywhere from 99 to 168 calories, depending on the variety you buy. Light tuna canned in water has 99 calories, compared to 109 calories in white tuna canned in water. When tuna is canned in oil, you can expect to add 49 to 69 calories to the total. If you make your sandwich using fresh, cooked tuna, you'll get a different number of calories depending on the type of tuna. A 3-ounce serving of yellowfin tuna has 118 calories, while bluefin has 156 calories.

When it comes to mayonnaise, portion control is important if you want to keep calories as low as possible. If you choose 2 tablespoons of light mayonnaise, you'll consume 71 calories. The same portion of regular mayonnaise has nearly three times that amount, or 188 calories. Even though it doesn't affect calories, another nutrient to watch for in mayonnaise is sodium. Regular mayonnaise actually has less sodium -- 175 milligrams in 2 tablespoons -- compared with 248 milligrams in light mayonnaise. At the lower amount, mayonnaise contributes 12 percent of your entire day's recommended intake for sodium.

The calories in bread vary more than you might expect from one type to the next, so check the nutrition facts label on the brand you buy. Some calorie differences are obvious, such as cheese bread has significantly more calories than other types, but you will also find a range in healthier types of whole-grain breads. Two slices of commercially prepared white bread contain 132 calories, while two slices of whole-wheat bread have 161 calories, according to the USDA Nutrient Database. One large whole-wheat pita supplies 170 calories.

If you assemble a sandwich using the ingredients with the fewest calories -- light tuna canned in water, light mayo and white bread -- you will consume 302 calories. Using ingredients with the most calories, which includes tuna in oil, regular mayonnaise and whole-wheat pita, produces a sandwich that has 526 calories. You don't need to worry about calories added from vegetables. Whether you add tomatoes, lettuce, grated carrots, sliced cucumbers or onions, you'll gain nutrients, such as vitamins A and C, for very few calories. However, be careful about cheese. Just one slice, or 1 ounce, adds another 79 to 114 calories.