"Substitute oil for butter 1:1" is the most repeated wrong conversion in home baking. Butter is an emulsion — roughly 80% fat, 15–18% water, 1–2% milk solids — and the water is a structural ingredient. The clean swap runs on fat weight, and the texture consequences deserve honesty.
The conversion
Oil weight = butter weight × 0.75
One cup of butter (227 g) replaces with ~170 g of oil (¾ cup + 1 tbsp). The logic: you are replacing the butter's fat only; its water joins the recipe's liquid budget. Recipes that anticipated the swap say so; the rest expect you to know.
What changes
| Aspect | Butter | Oil |
|---|---|---|
| Flavor | Milk solids brown and taste | Neutral (unless olive) |
| Structure | Creaming holds air; solid fat laminates | Liquid — tender, denser crumb |
| Spread (cookies) | Controlled by solid fat | More spread — chill the dough |
| Moisture feel | Rich, milky | Wet-tender, lasts longer |
Where the swap works — and where it cannot
- Works: quick breads, muffins, brownies, carrot cake — recipes built on moist tenderness where oil often beats butter anyway.
- Compromises: cookies (expect more spread; chill thoroughly), pancakes (fine, less browned flavor).
- Cannot: anything creamed (butter's solid fat holds the air), laminated dough (croissants are butter's structural job), buttercream (it is an emulsion, not a wetting).
The weights, at a glance
- Butter 1 cup = 227 g → oil ~170 g (¾ cup + 1 tbsp)
- Butter ½ cup (1 stick) = 113 g → oil ~85 g (6 tbsp)
- Olive oil swap: same weight; expect its flavor in the crumb — mild olive for sweets, robust for savory bakes.
The emulsion is the point: butter brings water, milk solids, and structure — oil brings only fat. Convert on weight, choose recipes that want oil, and both ingredients do their best work.