Who is behind the numbers
Oil to Butter is a single-subject reference site. We cover one conversion properly rather than a thousand conversions badly.
Most oil-to-butter answers online are copied from each other. The most repeated of them — that one cup of oil becomes 1⅓ cups of butter — is arithmetic applied in the wrong direction, and it has been reprinted so many times that it now looks authoritative. We built this site because that bothered us.
Butter is roughly 80% fat. Oil is essentially 100% fat. That single difference drives every calculation here. To replace the fat in one US cup of vegetable oil (about 218 g), you need 272.5 g of butter — not a volume conversion, and not the 1⅓ cups figure you will find elsewhere. The calculator applies each oil's own density before matching the fat, which is why olive oil and vegetable oil give slightly different answers.
We also keep the familiar rules of thumb on the site. The quick 1.25 and 0.75 ratios are genuinely fine for most everyday baking, and pretending otherwise would be its own kind of dishonesty. What we will not do is present a rough rule as if it were precise.
Every result on this site can be expanded to reveal the arithmetic behind it — the density used, the fat fraction applied, and the ratio that produced the answer. If you disagree with a number, you can see exactly where it came from.
A cup of oil and a cup of butter are not interchangeable, and a cup is not even a fixed quantity across countries. We lead with grams wherever it matters, because a scale does not care how tightly you packed the cup.
A converter that always says yes is not much use. Laminated pastry, shortbread and creamed cakes depend on solid fat behaving like solid fat, and no ratio fixes that. We say so on the pages where it applies.
Density and fat-content figures are drawn from standard published food composition data, and the same tested conversion engine powers the live calculator, every chart and every worked example on the site. There is one source of truth, covered by an automated test suite, so a chart can never quietly disagree with the calculator sitting above it.
Butter varies. European-style butters can reach 82–84% fat, and salted butter brings its own seasoning to the bowl. We use 80% as the working figure because it reflects the standard butter most people have in the fridge, and we flag the cases where the difference is large enough to matter.
If a number here looks wrong, we would genuinely like to know. Send it to hello@oiltobutter.com or use the contact form, and we will check it against the source data. Corrections are the fastest route to a better site.