Cakes mixes contain all the necessary dry ingredients, including flour, sweeteners and leaveners, and need only the addition of wet ingredients in the form of eggs, water and oil to create a smooth lump-free batter. Eggs bind the ingredients together, while water supplies moisture and oil enhances the texture of the finished cake. The amount of oil called for also supplements the small amount of fat in the form of soybean and vegetable oils already present in the mix.

As a liquid, pourable fat when melted, margarine can be substituted for vegetable or cooking oil in a cake mix using a 1-to-1 ratio, which means that you should use the same amount of melted margarine as the amount of oil called for in the baking directions. You can melt the margarine in a microwave-safe bowl, which takes about 20 seconds, or in a small pan on the stove placed over low heat, which takes a little longer. Allow the margarine to cool slightly before adding it to the cake mix, as you don't want to "cook" the dry ingredients or the eggs.

Not all margarine is the same, so be sure to use real margarine and not a spread in your cake mix. Like butter, real margarine is 80 per cent fat in the form of oil, and 20 per cent water and solids that include salt. It melts the same as butter and if allowed to harden after melting, the fat solids tend to separate from the liquids the same as they do in butter. Buttery spreads contain significantly less fat and more water along with other ingredients, and some spreads can be composed of as much as 40 per cent water, which makes them unsuitable for use in cake mixes. The lower fat content in margarine spreads also affects baked goods by making them drier and tougher, and interferes with rising.

There is no particular order in which to combine the cake mix ingredients with the eggs, water and melted margarine, as they all go into the bowl all at once. A standard mix for yellow cake instructs you to blend the mix on low speed until all the dry ingredients are moistened, which takes about 30 seconds. Then increase the mixer speed to medium and beat the batter for 2 more minutes or until it's smooth and creamy. Because melted margarine will go back to its solid state if allowed to sit too long at room temperature, be sure to mix all the ingredients immediately.