What Are the Differences in Micrograms, Milligrams & IUs?

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What Are the Differences in Micrograms, Milligrams & IUs?
Vitamins may be described in milligrams, micrograms or IUs. (Jupiterimages/Photos.com/Getty Images)

Micrograms, milligrams and international units (IUs) are all ways to measure small amounts. Micrograms and milligrams, however, are based upon the gram, which is a metric unit of mass, while IUs are based upon biological activity, or the potency of the substance, and measure quantity. Organic substances, such as vitamins, are commonly measured using all three units. For example, the amount of vitamin A present in a particular food may be measured in milligrams or micrograms, but expressed as a percentage of a daily value that is based upon IUs.

International Units (IUs)

The international unit, or IU, is an accepted way of describing a quantity of a substance that has a certain biological effect. It is used to describe fat-soluble vitamins, as well as certain enzymes, hormones, antibiotics and vaccines. International units are commonly used to describe daily values, and are found on the nutrition chart on many food products.

Milligrams and Micrograms

Milligrams (mg) and micrograms (μg) are units of mass. One milligram equals 1,000 grams and 1 microgram equals 1,000 milligrams. These units are used to measure very small masses, such as a quantity of medicine or the vitamins and minerals in food. They are part of the International System of Units (SI), which is a globally-recognised system of measurement, but which is distinct from the International Unit (IU).


International units are only used to describe organic substances that affect the processes of the human body. Milligrams and micrograms can describe the mass of any substance. IUs are typically used by pharmacists and biological chemists, but not the general population. For instance, a pharmacist may dispense 500 IUs of an antibiotic, but describe the dosage in milligrams for the average user. Scientists and laymen alike, however, use milligrams and micrograms to describe metric masses.


Since IUs describe quantity instead of mass, there is no single conversion unit for changing IUs into milligrams or micrograms. Each substance is unique. For instance, 1,000 IUs of vitamin D equals 0.025 micrograms, while 1 IU of vitamin E has a mass of 1.5 milligrams. Fortunately, it is seldom necessary to make these conversions. However, if the need arises, your local pharmacist can usually assist you.

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