Eighty grams of standard butter is replaced by 64 g oil for an exact fat match. With vegetable oil, 64 g is about 70.5 ml, slightly under five US tablespoons.
The calculation
Butter is approximately 80% fat, so multiply its weight by 0.80:
80 g butter × 0.80 = 64 g oil
The butter to oil converter uses the same calculation and can express the answer in millilitres, cups or tablespoons.
Weigh the oil when possible
Because the recipe starts with a weight, the most accurate method is to weigh 64 g oil directly. Converting to volume introduces the density of the chosen oil and the accuracy of the jug or spoon.
Vegetable and canola oil share the same working density in the calculator. Olive oil is slightly lighter, so 64 g occupies a slightly larger volume.
Consider the missing moisture
Eighty grams of butter carries roughly 13 g water. Oil carries none. In a cake or quick bread, up to one tablespoon of milk or water can replace that moisture if the batter appears too stiff.
Do not add liquid automatically in a formula that already contains yoghurt, fruit puree or a generous quantity of egg. Those ingredients may make the adjustment unnecessary.
Where the swap works
Use oil in place of 80 g butter for muffins, brownies, banana bread and simple cakes. Avoid treating it as a direct swap in pastry, shortbread or creamed recipes, where butter’s solid structure and aeration are essential.
The exact quantity is 64 g oil. Whether it is a good substitution depends on the role the butter performs beyond supplying fat.