How this calculator works
Butter is an emulsion of about 81 per cent butterfat, 16 per cent water and a little milk solid. Cooking oil is almost entirely fat. Replacing one cup of butter with one cup of oil therefore adds roughly a quarter more fat and removes the water that butter brings. The usual rule of thumb is to use three quarters of the oil volume in place of the butter; matching the fat exactly gives about 0.845 of the butter volume instead, so the rule slightly under-adds fat. This calculator goes the other way too, so a recipe written for oil can be converted back to butter.
Formula
fat = weight x fat fraction, where the fat fraction is 1.00 for oil and 0.81 for butter. butter weight = fat / 0.81. oil weight = fat. Volumes use 0.921 g/mL for vegetable oil and 0.959 g/mL for butter. One cup of butter weighs 227 g and holds 184 g of fat, which is 200 mL, or 0.845 cup, of oil.
Worked example
A cake recipe asks for 1/2 cup of vegetable oil. That is 118.3 mL of oil, weighing 118.3 x 0.921 = 109 g, all of it fat. The matching butter is 109 / 0.81 = 134.6 g, which is 134.6 / 0.959 = 140.3 mL, or 9.5 tablespoons. That is a little under the two-thirds cup that the rough three-quarters rule suggests.
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Frequently asked questions
Can I swap butter for oil in every recipe?
No. Oil works well in muffins, cakes, quick breads and some cookies. Butter is needed where creaming and flakiness matter, such as pastry, shortbread and buttercream. In cakes, oil usually gives a moister crumb but less flavour and structure.
Why does the calculator not simply use three quarters?
Three quarters is a rounded rule of thumb, not a fat calculation. Butter is about 81 per cent fat and oil is about 100 per cent, so one cup of butter needs about 0.845 cup of oil for the same fat. The difference is small in most recipes but can matter in pastry, where too little fat shortens the texture.
Should I adjust anything else when using oil instead of butter?
Butter carries water, roughly 16 per cent of its weight. When replacing butter with oil, some bakers add a little milk or water, about 1 to 2 tablespoons per cup of butter replaced. This calculator matches fat only, not water or salt.
Does this work for margarine?
Margarines vary from about 35 to 80 per cent fat. A block margarine at 80 per cent fat behaves like butter. A soft spread in a tub has much more water and is a poor substitute in baking, so check the label before using this result.
What about coconut oil?
Coconut oil is also about 100 per cent fat, so the fat-equivalent weight is the same. Its density is close to other oils, about 0.92 g/mL when melted, so the volume result remains a good guide.