Convert vitamin D between micrograms (mcg) and International Units (IU). Works for both D3 (cholecalciferol) and D2 (ergocalciferol) — they share the same conversion factor. Math, not medical advice.
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Reference table — common vitamin D amounts
| Mcg | IU | Context |
|---|---|---|
| 5 | 200 | Old US RDA (pre-2010) |
| 10 | 400 | Old multivitamin standard; minimum to prevent deficiency |
| 15 | 600 | RDA for adults age 1-70 (IOM 2010) |
| 20 | 800 | RDA for adults 71+ (IOM 2010) |
| 25 | 1,000 | Common OTC supplement dose |
| 50 | 2,000 | Common higher-dose OTC supplement |
| 100 | 4,000 | Tolerable Upper Intake Level (UL), adults (IOM 2010) |
| 125 | 5,000 | Common D3 supplement strength (above UL — discuss with prescriber) |
| 250 | 10,000 | High-dose D3 (above UL) |
| 1,250 | 50,000 | Prescription D2 (Drisdol), often weekly |
D3 vs D2 — same conversion, different sources
The 1 mcg = 40 IU conversion is fixed for both forms. The forms differ in source and bioavailability:
- Vitamin D3 (cholecalciferol): animal-derived (lanolin from sheep wool, fish liver oil) or made in skin from cholesterol via UVB exposure. Studies suggest D3 raises serum 25(OH)D somewhat more efficiently than D2.
- Vitamin D2 (ergocalciferol): plant-derived (UV-treated mushrooms, fungal sources). Vegan-suitable. The standard prescription “high-dose” formulation (50,000 IU) is D2.
For supplement-grade conversion math, the unit conversion is identical. For dose-response math (how much your serum 25(OH)D rises per IU), see our vitamin D dose calculator.
Why the unit confusion exists
Pre-2020, US supplement labels used IU (legacy from the 1930s when vitamin potency was measured biologically). The 2020 FDA Nutrition Facts and Supplement Facts label updates moved to mcg as the primary unit, with IU as parenthetical. Many older bottles, prescription labels, international products, and clinical reference ranges still use IU — hence the constant need to convert.
