Fifty grams of standard butter is replaced by 40 g of oil for an exact fat match. With vegetable oil, that is about 44 ml, or just under three US tablespoons.
Why the answer is 40 g
Butter is approximately 80% fat. Multiply 50 g by 0.80:
50 g butter × 0.80 = 40 g oil
That calculation preserves the fat contribution. The quick volume rule uses about three-quarters as much oil and lands in a similar practical range, but Exact mode is clearer when the recipe starts in grams.
Open the butter to oil converter to switch the oil type or result unit.
Convert grams of oil to millilitres
Vegetable and canola oil weigh about 0.908 g per millilitre, so 40 g is roughly 44 ml. Olive oil is slightly lighter and therefore occupies a little more volume for the same weight.
If the recipe is already written in grams, weigh the oil directly into the mixing bowl. It is faster and avoids transferring oil between a jug and bowl.
Adjust the moisture only when needed
Fifty grams of butter contributes around 8 g water. Replacing it with oil removes that moisture. In a small cake or muffin recipe, one to two teaspoons of milk or water can restore it if the batter looks too stiff.
Recipes containing mashed fruit, yoghurt or several eggs may need no adjustment. Use batter consistency rather than adding liquid automatically.
Is the substitution suitable?
The swap works in muffins, banana bread, brownies and simple cakes. It is not a direct replacement in shortbread, pastry or any recipe where solid butter creates structure or is creamed with sugar for aeration.
The quantity is simple: 40 g oil. The recipe method decides whether that quantity will behave as intended.