Vitamin D (Cholecalciferol) used to be measured in micrograms (mcg). At one point, international units were introduced (IU). Today, either or both IU and mcg are found on supplement labels. Sometimes, it is beneficial to know the amount in IU and the amount in mcg. Especially when you want to use the correct ratio of several vitamins simultaneously. An important example in this context would be the combination of vitamin D and vitamin K. While Cholecalciferol ensures that there is enough calcium in the blood, vitamin K is involved in bringing calcium to the correct location (bones). Calcification of soft tissues such as cartilage and arteries is definitely something no one wants. Therefore, both vitamins should be used together. How much of each is a discussion for another time. However, once you decide and settle on a ratio between vitamins D and K, you can use the below converter to ensure that you use the correct amounts.