It really just depends upon how one makes the sandwich. For example; one cup of tuna salad has 383 calories -- of course it depends upon what they have put in the salad -- mayonnaise, boiled eggs, onions, etc. . Two slices of bread has about 200 calories or 100 calories per slice. This totals about 583 calories for the average tuna salad.
But it depends on how you make the Tuna. Canned Tuna in water has about 40 cal. per serving. A whole can is 2.5 servings. So a Whole can is 100 calories. If you use Low fat mayo it 40 calories for 1 tbsp which is really all you need to mix one whole can of tuna. Which makes it 140 calories. The Bread is where alot of the calories come from. But if you use low fat bread it wont be so bad. Or you can do what I do and Wrap the tuna in a leaf of Romaine Lettuce which is almost no calories! Its so Good!