Convert micrograms (mcg, µg) to milligrams (mg) and back for supplement labels and prescriptions, with Daily Values, an IU converter for vitamins D, A and E, and the label-reading mistakes that cause 1,000-fold errors.
1 mg = 1,000 mcg · 1 mcg = 0.001 mg
This converts units. It doesn't tell you what dose to take. Don't change a prescribed dose or swap products based on a conversion. Ask your pharmacist if a label and a prescription seem to disagree.
A microgram is a millionth of a gram and a milligram is a thousandth, so there are exactly 1,000 mcg in 1 mg. To get mg, divide mcg by 1,000, which moves the decimal point three places left: 125 mcg becomes 0.125 mg. To go back, multiply by 1,000.
"mcg" and "µg" are the same unit. Scientific writing uses µg, the SI symbol. US pharmacy and supplement labels use mcg, because a handwritten or badly printed µ can be read as an m.
On US Supplement Facts and Nutrition Facts labels, these nutrients are listed in micrograms. The Daily Value is what the %DV column compares against. The upper limit is the most adults should take daily from all sources, unless a doctor advises otherwise.
| Nutrient | Daily Value | In mg | Adult upper limit |
|---|---|---|---|
| Vitamin D | 20 mcg | 0.02 mg | 100 mcg |
| Vitamin B12 | 2.4 mcg | 0.0024 mg | None set |
| Folate | 400 mcg DFE | 0.4 mg DFE | 1,000 mcg (folic acid) |
| Biotin | 30 mcg | 0.03 mg | None set |
| Vitamin K | 120 mcg | 0.12 mg | None set |
| Vitamin A | 900 mcg RAE | 0.9 mg RAE | 3,000 mcg (preformed) |
| Iodine | 150 mcg | 0.15 mg | 1,100 mcg |
| Selenium | 55 mcg | 0.055 mg | 400 mcg |
| Chromium | 35 mcg | 0.035 mg | None set |
Very high %DV figures are common for nutrients with no upper limit: a 1,000 mcg B12 tablet is 41,667% DV, and a 10,000 mcg (10 mg) biotin capsule is 33,333% DV. High-dose biotin can make some blood tests read falsely high or low, including thyroid and troponin tests, so mention it before blood work.
A bottle reads "Vitamin D3 50 mcg (2,000 IU) — 250% DV". Your doctor's note says 0.025 mg a day. Are they the same?
Step 1: Put the note in micrograms
Step 2: Compare with the softgel
Step 3: Check the IU and %DV figures agree
The bottle is 0.05 mg, not 0.025 mg. A 25 mcg (1,000 IU) product matches the note. Both are below the adult upper limit of 100 mcg (4,000 IU), but the point of the conversion is to match what was advised.
Since the FDA's 2016 label rule (required on most products from 2020–2021), vitamins A, D and E are listed by mass instead of International Units. Older bottles, foreign products and many prescriptions still use IU. An IU measures biological activity, so its weight differs for each vitamin and form:
IU factors only work within one nutrient and form. There is no general IU-to-mg conversion.
Folate is labelled in micrograms of dietary folate equivalents (mcg DFE), because the synthetic folic acid in supplements is absorbed better than the folate in food. 1 mcg of folic acid taken with food counts as 1.7 mcg DFE, so a label reading "400 mcg DFE (240 mcg folic acid)" describes one amount two ways, not two ingredients.
That matters for converting to mg: the prenatal advice of 0.4 mg folic acid a day is 400 mcg of folic acid, which is 680 mcg DFE, not 400 mcg DFE. Check which figure a recommendation refers to.
Levothyroxine, a thyroid hormone, is a common example. Its tablets come in 12 strengths, and US labels print both units, for example "88 mcg (0.088 mg)". Pharmacy systems and older prescriptions may use either one, so the same tablet can appear as two very different-looking numbers.
| Tablet strength | mg |
|---|---|
| 25 mcg | 0.025 |
| 50 mcg | 0.05 |
| 75 mcg | 0.075 |
| 88 mcg | 0.088 |
| 100 mcg | 0.1 |
| 112 mcg | 0.112 |
| 125 mcg | 0.125 |
| 137 mcg | 0.137 |
| 150 mcg | 0.15 |
| 175 mcg | 0.175 |
| 200 mcg | 0.2 |
| 300 mcg | 0.3 |
Other medicines commonly written in mcg include inhaled steroids (per puff), fentanyl patches (mcg per hour) and vitamin B12 injections. For infusions dosed in mcg/kg/min, use the pharmacy's calculation, not a unit converter.
The Institute for Safe Medication Practices lists these as recurring causes of 1,000-fold and 10-fold dose errors:
If a converted number is a thousand times bigger or smaller than you expected, assume a unit mix-up until it's been checked.
Yes. 400 ÷ 1,000 = 0.4 mg. For folate, check whether the label means 400 mcg DFE or 400 mcg folic acid; they are different amounts.
5 mg. That is about 16,667% of the 30 mcg Daily Value.
25 mcg (0.025 mg). Vitamin D converts at 40 IU per microgram.
Micrograms of retinol activity equivalents. It accounts for beta-carotene being less potent than retinol, so the same IU figure can mean different weights.
Related Tools
Convert decagrams to grams.
Convert bushels of grain to pounds, tons and tonnes.
Convert grams to ounces.
Convert kilograms to pounds.
Combine pounds and ounces into one weight.
Convert pounds to kilograms.