Convert vitamins A, E, and K between metric units (mcg/mg) and International Units (IU). Each vitamin has its own conversion factor, and vitamin E differs by form. For vitamin D, see our dedicated D converter.
Convert
mcg RAE
IU
1 mcg RAE retinol = 3.33 IU vitamin A activity
Conversion factors by vitamin and form
| Vitamin / Form | 1 mcg or 1 mg → IU | RDA reference |
|---|---|---|
| Vitamin A — retinol (animal sources) | 1 mcg RAE = 3.33 IU | Men 900 mcg RAE; Women 700 mcg RAE |
| Vitamin A — beta-carotene (supplement) | 2 mcg = 3.33 IU | 1:2 conversion of beta-carotene to retinol |
| Vitamin A — beta-carotene (food) | 12 mcg = 3.33 IU | 1:12 conversion in dietary food |
| Vitamin E — natural d-alpha-tocopherol | 1 mg = 1.49 IU | 15 mg adults (22.4 IU natural) |
| Vitamin E — synthetic dl-alpha-tocopherol | 1 mg = 2.22 IU | 15 mg adults (33.3 IU synthetic) |
| Vitamin K1 — phylloquinone | 1 mcg ≈ 1 IU | Men 120 mcg; Women 90 mcg (AI) |
| Vitamin K2 — MK-4 | 1 mcg ≈ 1 IU | No separate RDA for K2 forms |
| Vitamin K2 — MK-7 | 1 mcg ≈ 1 IU | Trials use 90-180 mcg/day |
Important: vitamin A is a “RAE” unit, not just mcg
Modern vitamin A labels use mcg RAE (Retinol Activity Equivalents). This accounts for the fact that beta-carotene (plant source) converts to retinol at different efficiencies. The IU system pre-dated RAE and didn’t distinguish source — which is why old IU values can be confusing:
- Pre-2020 label “5,000 IU vitamin A” usually meant 1,500 mcg retinol IF the source was retinol — but could mean far less RAE if from beta-carotene.
- Modern label “900 mcg RAE” is unambiguous: that’s the absorbed retinol activity, regardless of source.
- The Upper Limit (UL) for adults is 3,000 mcg RAE/day from preformed retinol (animal-source supplements). Beta-carotene has no UL because it self-limits absorption.
Vitamin E: natural vs synthetic matters
Vitamin E supplements come in two forms with different conversion factors:
- Natural d-alpha-tocopherol (from sunflower or soybean oil): 1 mg = 1.49 IU. The “d-” prefix is the natural isomer.
- Synthetic dl-alpha-tocopherol (chemically synthesized): 1 mg = 2.22 IU. The “dl-” prefix means a racemic mixture of stereoisomers. Only the d-form has full biological activity; the other isomers are mostly excreted.
- Practical implication: A “400 IU vitamin E” supplement contains either 268 mg natural OR 180 mg synthetic — different amounts on the bottle for the “same” IU dose.
- Modern labels (post-2020 FDA): mg with form clarification (“d-alpha” vs “dl-alpha” or “from natural source”).
Vitamin K1 vs K2 (and why MK-7 is special)
Both K1 and K2 use ~1:1 mcg-to-IU conversion. But the forms have very different bioavailability and half-life:
- Vitamin K1 (phylloquinone): from leafy greens, primarily activates coagulation factors. Short half-life (~1-2 hours).
- Vitamin K2 MK-4 (menatetrenone): short half-life (~1 hour). Used in some bone-health studies at high pharmacological doses (45 mg/day).
- Vitamin K2 MK-7 (long-chain menaquinone, from natto): half-life ~72 hours. The form used in most modern bone-health trials at 90-180 mcg/day. Much more efficient bioavailability than MK-4.