Margarine is a solid fat that's used in place of butter as a condiment and in baking. Margarine itself is 80 percent fat; many vegetable oil products are erroneously called margarine but are only "margarine-like," according to the USDA National Nutrient Database for Standard Reference.
One tablespoon of margarine, whether in stick or spread form, contains about 100 calories.
Margarine contains approximately 2.4 mg of potassium in each tablespoon. The spread also contains trace amounts of calcium and protein per serving.
Margarine and butter contain the same number of calories per tablespoon, according to the USDA National Nutrient Database. You may prefer one over the other, but you won't save any calories by using margarine.