You should use approximately 1 cup of ghee to substitute for 1 cup of butter to achieve a similar fat content in your recipe.
Ghee is essentially butter that has had the milk solids and water removed. This process results in a product that is almost pure butterfat. While both butter and ghee are primarily fat, butter contains some water and milk solids, which slightly reduces its fat content compared to ghee.
Butter is typically around 80-82% fat, while ghee is usually 99-100% fat. Therefore, ghee is more concentrated. However, for most recipes, a 1:1 substitution of ghee for butter will work well and provide a similar richness and flavor. The slight difference in fat content is usually negligible in the final outcome of the dish. Using the same volume ensures the texture and moisture levels are close to the original recipe's intention.
When substituting ghee for butter, consider the flavor profile. Ghee has a nuttier, more intense flavor than butter. If you prefer a milder flavor, you can use slightly less ghee (e.g., 7/8 cup) or use a blend of ghee and a neutral oil.