Pie dough and biscuit dough start from the same move — cold fat cut into flour, then just enough liquid to bring it together — which is why the two look almost identical sitting in a bowl. What separates them is which direction the ratio runs. Pie dough carries more fat and less liquid; biscuit dough carries less fat, more liquid, and a chemical leavener the pie crust never touches. Swap one dough for the other and you're swapping what the oven actually does to it.
The ratio each one is built on
Classic pie dough runs 3 parts flour to 2 parts fat to 1 part water, by weight. Biscuit dough runs almost the opposite: 3 parts flour to 1 part fat to 2 parts liquid, usually buttermilk, plus a spoonful of baking powder folded into the flour. Flip the fat and the liquid and you've flipped the dough.
That inversion isn't an accident of two different recipes converging on different numbers. It's the reason the two doughs behave nothing alike once they hit the oven.
What the fat is actually doing
In pie dough, the fat has two jobs at once. Cold pieces of butter, left in flat, unmelted shards, coat the surrounding flour and physically block gluten strands from linking up — that's what keeps a crust tender instead of chewy. Then in the oven, those same shards melt fast, flashing to steam and leaving behind thin gaps where the fat used to sit. Roll the dough thin enough and those gaps stack into visible sheets. It's lamination, but built by chance rather than by folding.
Biscuit dough has less fat per cup of flour, so it can't do as much of that gluten-blocking work on its own. Some of the layering gets built by hand instead — folding the dough over itself a few times before cutting, the same principle as laminating puff pastry, just cruder and faster. The steam from melting fat still helps, but it isn't carrying the whole rise by itself.
Why one has baking powder and the other doesn't
Baking powder makes a dough rise by releasing carbon dioxide into a network that's elastic enough to trap it and stiff enough to hold the shape once it sets. That network is gluten, and gluten needs both liquid and a bit of working to develop. Biscuit dough's extra hydration exists partly to feed that: enough water for the flour to form a dough strong enough to catch the gas the leavener produces, and enough for the baking powder itself to dissolve and start reacting.
Pie dough is built to avoid exactly that structure. A crust that puffed and rose would pull away from the pan, warp out of round, and lose the flat sheets that make it flake instead of bend. There's nothing for a leavener to do in a dough whose whole point is staying thin, tender, and gluten-poor. The small amount of lift a baked crust does show — a bit of puckering at the edges — is just trapped steam finding a way out, not anything a chemical leavener contributed.
The buttermilk in biscuit dough is pulling one more lever pie dough doesn't have: its acid reacts with a portion of baking soda to produce extra gas beyond what the baking powder alone provides, on top of giving the crumb a sharper, tangier flavor pie crust was never asking for.
What happens if you swap them
Bake pie dough as a biscuit and it comes out flat and dense — sandy and tender from all that fat, but with nothing to lift it, because there's no leavener and not enough liquid to build a network that could trap gas even if there were. It reads more like a savory shortbread than a biscuit.
Use biscuit dough for a pie crust and the opposite problem shows up. The extra liquid means more gluten development than a crust wants, so it toughens rather than shatters. Worse, the baking powder keeps working exactly as designed — the dough puffs and domes in the oven instead of holding flat against a filling, and what should have been crisp, flaky layers bakes into something closer to a dense, cakey biscuit wrapped around your pie.