Margarine vs shortening
One is roughly a sixth water and the other is none, and that single difference decides which bakes they can and cannot replace butter in.
The short answer
Both are manufactured solid fats and they are not interchangeable, because one contains water and the other does not. Margarine is an emulsion — roughly the same fat-to-water balance as butter — so it browns, it contributes liquid, and it behaves close enough to butter for most substitutions. Shortening is essentially all fat, so it contributes no water at all: pastry made with it is exceptionally short, biscuits spread less and stay taller, and icings made with it hold their shape in heat that would collapse a butter one. What it does not do is taste of anything or brown, which is why the usual professional answer is a blend rather than either alone.
Where they differ
| Dimension | First | Second |
|---|---|---|
| Water content | Roughly a sixth, like butter. | Essentially none. |
| Browning | Browns, though less than butter. | Barely. There is nothing in it to brown. |
| Flavour | Some, and it varies a great deal by brand. | Effectively none. |
| Melting behaviour | Softens over a range like butter. | Higher and wider melting range; holds shape in heat. |
| Cookie spread | Similar to butter. | Markedly less. Biscuits stay taller and softer. |
| Substituting for butter | Usually one for one; the water is already accounted for. | Not one for one — the missing water has to come from somewhere. |
| The low-fat spread trap | A spread below about 70% fat is mostly water and fails in baking. | Not applicable. |
Why substituting shortening for butter needs a liquid adjustment
Butter is roughly four-fifths fat and one-fifth water, and that water is doing real work: it hydrates the flour, it makes steam, and in a pastry it is part of what separates the layers.
Swap in an equal weight of shortening and the fat goes up while the liquid goes down, in a recipe that was balanced on both. The usual result is a dough that will not come together and a bake that is drier than intended, and it is a common enough failure that it has its own entry in BakeHQ's dough diagnosis.
The conventional correction is to use slightly less shortening than the butter it replaces and add a little water — which reconstructs the emulsion in the bowl rather than in the packet.
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Compares
One is roughly a sixth water and the other is none, which decides which of them can stand in for butter.
One is roughly a sixth water and the other is none, which decides which of them can stand in for butter.