MCG to IU Converter
Convert between micrograms, milligrams, grams, and international units for vitamins, hormones, and common medications.
How the Mcg to IU Converter Works
International Units (IU) and micrograms (mcg) measure different things depending on the substance. This converter translates between mcg, mg, g, and IU using established reference factors for vitamins, hormones, and medications. Select a substance, enter an amount and unit, and the tool auto-converts to all equivalent units.
Common conversion factors
- Vitamin D (D2/D3): 1 mcg = 40 IU
- Vitamin A (retinol): 1 mcg = 3.33 IU
- Vitamin E (natural): 1 mcg = 1.49 IU
- Human insulin: 1 IU = 38.5 mcg (EP reference)
- hCG: ~0.103 mcg per IU (USP bioassay reference)
Some nutrients — including vitamin C, B vitamins, and folate — are not assigned IU values. The calculator shows mass units only for those substances. For general mass conversions, see our mg to mL Calculator.
Frequently Asked Questions
What is the difference between mcg and IU?
Micrograms (mcg) measure mass. International Units (IU) measure biological activity, which depends on the substance. One mcg of vitamin D equals 40 IU, but one mcg of vitamin A retinol equals about 3.33 IU — the factor is substance-specific.
Why doesn’t vitamin C convert to IU?
Vitamin C (ascorbic acid) is labeled and dosed in milligrams or micrograms, not International Units. IU labeling applies mainly to fat-soluble vitamins (A, D, E, K) and certain biologically standardized drugs.
Is insulin conversion exact?
Human insulin potency is standardized (1 IU = 38.5 mcg per European Pharmacopoeia), but actual vial concentration and syringe markings should always be verified against your prescription and product label.
How do I convert IU to mcg for vitamin D?
Divide IU by 40. For example, 2,000 IU vitamin D ÷ 40 = 50 mcg. Enter the IU value with the vitamin D substance selected and the tool shows all equivalent units automatically.
Are hCG and heparin conversions always the same?
No. Biological preparations vary by manufacturer and assay method. The factors used here are common reference values; clinical dosing should follow product-specific labeling and medical guidance.
Can I use this for supplement labels?
Yes, for comparing label amounts across units — especially vitamin D and vitamin A products that mix mcg and IU. Always confirm with your healthcare provider before changing supplement or medication doses.
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