A Dictionary of Units of Measurement. For information on a specific unit, click on the first letter of its name: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z. Answers to the three most frequently asked questions: How many micrograms (µg, ug or mcg) in a milligram (mg)? 1000 micrograms = 1 milligram, and 1000 milligrams = 1 gram. How can I convert from international units (IU) to milligrams or micrograms? Generally speaking, you can't. IU's measure the potency of a drug, not its mass or weight. What countries besides the U.S. have not adopted the metric system? Many U.S. teachers think the answer is Liberia and Burma (make that Myanmar). Let's give Liberia and Myanmar a break! All countries have adopted the metric system, including the U.S., and most countries (but not the U.S.) have taken steps to eliminate most uses of traditional measurements.