Convert micrograms to grams

One gram contains exactly 1,000,000 micrograms. The decimal point moves six places, which is why this conversion is worth checking rather than doing in your head — 25 mcg is 0.000025 g.

For vitamin D3, 1 microgram equals 40 International Units, so a 25 mcg dose is 1,000 IU. The conversion is specific to the substance: IU measures biological activity, not mass, so the factor for vitamin A or vitamin E is different and the two scales cannot be converted in general. Supplement labels commonly print both, and they are describing one dose twice.

Because handwritten µ has killed people. The Greek letter mu, written quickly, reads as an m — turning micrograms into milligrams and the dose into a thousand times itself. The Institute for Safe Medication Practices lists µg on its roster of error-prone abbreviations and recommends mcg, or the word written out, in any handwritten order. It is one of the few places where a typographic choice is a safety control.

Micrograms appear almost entirely in medical and laboratory work, where a factor-of-1,000 error is the difference between a dose and an overdose. Use this to check arithmetic you have already reasoned through — never as the authority for a dose. Confirm against the label, the prescription, or a pharmacist.

The prefix micro is defined in the SI Brochure published by the International Bureau of Weights and Measures (BIPM) as 10⁻⁶, so a microgram is a millionth of a gram. The symbol is µg, but the Institute for Safe Medication Practices lists µg as error-prone in medication orders, because it can be misread as mg, and US Nutrition Facts labels write the unit as “mcg” — the FDA’s 2016 Nutrition Facts rule requires vitamin D in micrograms.