1 cup butter = ¾ cup oil (butter is ~80% fat + 16% water; oil is 100% fat, so you need less). Same swap rule by weight: multiply butter weight × 0.875 to get oil weight.
Substitute with
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Caveats
Texture changes: butter gives flakiness (pastry, biscuits). Oil gives moisture (cakes, muffins). Best for recipes where moisture matters more.
Flavour: use neutral oils (canola, sunflower) for baking; olive oil only when the flavour fits.
Avoid swapping in laminated doughs (puff pastry, croissants), shortbread, or pie crust.