The calories in a cheese omelette come from three sources -- eggs, cheese, and butter or oil. A three-egg omelette with 100 g of cheese, fried in butter contains about 721 calories. If you're counting calories, you can reduce this number by cutting back to two eggs and less cheese.

Three large eggs contain 216 calories. To reduce the calories in the omelette, make a smaller omelette. A two-egg omelette contains only 144 calories from the eggs. An omelette made from egg whites alone contains 51 calories. About 1/2 cup of fat-free liquid egg substitute contains 58 calories.

A 100 g slice of cheddar cheese, a bit less than a cup shredded, contains 403 calories, according to the USDA National Nutrition Database. To reduce this number, use less cheese or a different kind of cheese. For instance, a 100 g slice of part skim mozzarella cheese contains 254 calories. And a 100 g slice of American cheese contains 137 calories.

To fry your omelette, you will need some kind of butter or oil. If you fry your omelette in unsalted butter add 102 calories per 1 tbsp. If you use canola oil, add 124 calories per 1 tbsp. A lower calorie alternative is nonstick cooking spray. A three-second spray contains only 2 calories.