Why the ratio is 0.8
Butter is about 80% fat and 20% water plus milk solids. Oil is 100% fat. To match the fat in one cup of butter, you need 0.8 cups of oil — using a full cup makes the bake greasy and can cause it to collapse.
Where the swap works
- Muffins, quick breads, brownies: excellent — oil keeps them moist for days
- Cookies: works, but expect a cakier texture and less spread
- Pie crust and laminated dough: never — you need butter's solid structure
- Cakes: acceptable, but flavour suffers without butter
Compensating for lost flavour
Oil carries flavour differently. Add a tablespoon of butter for taste even when the fat is oil, or use a flavourful oil (olive for savoury, coconut for sweet). Milk solids also brown; without them, baked goods stay pale.
Weight versus volume
Butter is 227 g per cup; oil is about 218 g per cup. If you are weighing, the 0.8 ratio still applies to fat content but the gram conversion differs — the calculator handles whichever unit you choose.
Frequently Asked Questions
Can I substitute oil for all the butter?
In quick breads and muffins yes; in pastry and cookies, texture will change noticeably.
Does oil make baked goods healthier?
It swaps saturated for unsaturated fat, but the calorie count is similar.
Why did my cake collapse with oil?
Usually too much oil — use 80% of the butter volume, not 100%.
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