Oil can be replaced with butter in brownies, and the swap often improves flavour. The trade-off is texture: oil promotes a uniformly moist, flexible crumb, while butter adds water and firms as it cools. Whether that is good depends on the brownie you want.
How much butter to use
Match the fat by dividing the oil weight by 0.80. If the recipe calls for 100 g oil, use 125 g butter. For one US cup of vegetable oil, the exact result is 272.5 g butter.
For cup, gram, millilitre or tablespoon amounts, use the main converter. The oil to melted butter guide explains why melted butter is the right form here.
Melt the butter
Brownie recipes that specify oil normally combine it with sugar, eggs and cocoa or melted chocolate. Replace it with melted butter, not creamed butter. Cool the butter until it is warm to the touch before adding it to eggs.
If the recipe blooms cocoa in hot oil, warm melted butter can perform the same job. Avoid aggressively boiling the butter unless you deliberately want browned butter; evaporation changes its water content and makes the conversion less direct.
Fudgy brownies
For dense, fudgy brownies, the substitution works best when the recipe already contains plenty of chocolate and relatively little flour. Butter deepens the chocolate flavour and gives a clean, rich finish. The brownies may set more firmly when chilled, so judge the texture at room temperature.
Do not add extra flour to compensate for a batter that looks slightly different. Butter begins to crystallise as it cools, and the baked texture will firm without that adjustment.
Cakey brownies
In cakey brownies, butter can encourage a more cake-like bite, particularly if it is beaten extensively with eggs and sugar. That may be desirable, but it moves the result further from an oil-based brownie.
For a shinier top, dissolve sugar thoroughly in the warm fat and egg mixture, then beat only as much as the recipe requires. The crackled surface depends more on dissolved sugar and a thin meringue-like layer than on whether the fat began as oil or butter.
When oil remains the better choice
Keep the oil if you want brownies that stay very soft straight from the refrigerator, need a neutral flavour, or must remain flexible for several days. Choose butter when dairy richness and chocolate aroma matter more than maximum softness.
The exact conversion keeps the fat contribution aligned. The final brownie still reflects the physical difference between a liquid fat and a fat that sets when cool.