MCG to IU Converter
Convert mcg ↔ IU for vitamin D and vitamin A, and mg ↔ IU for vitamin E (natural & synthetic) — in both directions, with the exact factor shown for each substance.
Converter
⚠ Educational tool only — not medical advice. Verify against your product label.
Conversion factors used (transparent)
Every factor below is taken from the NIH Office of Dietary Supplements fact sheets. There is deliberately no single “universal” factor — the factor depends on the substance.
| Substance | Mass → IU | IU → mass | Source |
|---|---|---|---|
| Vitamin D (D2/D3) | 1 mcg = 40 IU | 1 IU = 0.025 mcg | NIH ODS — Vitamin D |
| Vitamin A (retinol) | 1 mcg RAE = 3.333 IU | 1 IU = 0.3 mcg RAE | NIH ODS — Vitamin A |
| Vitamin E, natural (d-alpha) | 1 mg = 1.49 IU | 1 IU = 0.67 mg | NIH ODS — Vitamin E |
| Vitamin E, synthetic (dl-alpha) | 1 mg = 2.22 IU | 1 IU = 0.45 mg | NIH ODS — Vitamin E |
Heads-up on old converters: many older sites still use the pre-2020 rule for synthetic vitamin E (1 mg = 1.11 IU). The current NIH ODS / FDA rule is 1 mg synthetic = 2.22 IU (1 IU = 0.45 mg). This converter uses the current rule.
Why one factor doesn’t fit all
An International Unit (IU) measures biological activity — the effect a substance has in the body — not its weight. Each vitamin has its own potency, defined by international agreement, so the “exchange rate” between weight (mcg/mg) and activity (IU) is different for every substance:
- Vitamin D is potent per microgram: just 1 mcg carries 40 IU of activity.
- Vitamin A (retinol) is far less “IU-dense”: 1 mcg carries only 3.333 IU.
- Vitamin E is weighed in milligrams, and the natural form (d-alpha-tocopherol) is more active per mg than the synthetic form (dl-alpha-tocopherol) — hence 1.49 vs 2.22 IU per mg.
That is also why you cannot convert beta-carotene IU to mcg without knowing its source: 1 IU of supplemental beta-carotene = 0.3 mcg RAE, but 1 IU from food = 0.05 mcg RAE.
Quick reference: common label values
| Label says | Equals |
|---|---|
| Vitamin D — 10 mcg | 400 IU |
| Vitamin D — 25 mcg | 1,000 IU |
| Vitamin D — 50 mcg | 2,000 IU |
| Vitamin D — 125 mcg | 5,000 IU |
| Vitamin A — 900 mcg RAE (retinol) | 3,000 IU |
| Vitamin A — 1,500 mcg RAE (retinol) | 5,000 IU |
| Vitamin E natural — 15 mg | 22.35 IU |
| Vitamin E natural — 400 IU | 268 mg |
| Vitamin E synthetic — 15 mg | 33.3 IU |
| Vitamin E synthetic — 400 IU | 180 mg |
Reading supplement labels
- Vitamin D: labels show both mcg and IU (e.g. “25 mcg (1,000 IU)”). Either number converts with the ×40 rule.
- Vitamin A: current labels show mcg RAE. If your bottle still says IU, check whether it is retinol or beta-carotene — the factor differs.
- Vitamin E: current labels show mg of alpha-tocopherol. Look for “d-alpha” (natural) vs “dl-alpha” (synthetic) — it changes the answer by almost 50%.
- Old IU-only bottles are still fine to convert; just pick the matching substance and form above.
Frequently asked questions
How many IU are in 1 mcg of vitamin D?
40 IU. For vitamin D (both D2 and D3), the biological activity of 40 IU equals 1 microgram, so multiply micrograms by 40. For example, 125 mcg × 40 = 5,000 IU.
How do I convert mcg to IU for vitamin D?
Multiply the mcg value by 40: IU = mcg × 40. To go the other way, divide IU by 40: mcg = IU ÷ 40. Example: 2,000 IU ÷ 40 = 50 mcg.
Why is there no single mcg-to-IU conversion factor?
An International Unit measures biological activity, not weight, and each substance has its own potency. Vitamin D uses 40 IU per mcg, vitamin A (retinol) uses 3.333 IU per mcg, and vitamin E is measured in mg: 1.49 IU per mg for the natural form and 2.22 IU per mg for the synthetic form.
How many IU are in 125 mcg of vitamin D?
5,000 IU. Multiply 125 × 40 = 5,000. This is a very common supplement strength.
How many IU are in 1 mcg of vitamin A?
3.333 IU, when the vitamin A is retinol (preformed vitamin A). The NIH Office of Dietary Supplements states 1 IU retinol = 0.3 mcg RAE, so 900 mcg RAE × 3.333 = 3,000 IU. This factor does not apply to beta-carotene from food, which converts differently.
Why do new supplement labels show mcg instead of IU for vitamin A?
Under the FDA's 2020 labeling rules, vitamin A is listed in mcg RAE (retinol activity equivalents) instead of IU, because a single IU number is misleading when a product mixes retinol and carotenoids. An older label reading '5,000 IU vitamin A (as retinol)' equals 1,500 mcg RAE.
Why do natural and synthetic vitamin E convert differently?
Natural vitamin E (d-alpha-tocopherol) and synthetic vitamin E (dl-alpha-tocopherol) have different biological activity per milligram. Per the NIH Office of Dietary Supplements: 1 mg of natural vitamin E = 1.49 IU, while 1 mg of synthetic vitamin E = 2.22 IU. In reverse: 1 IU natural = 0.67 mg, and 1 IU synthetic = 0.45 mg.
My old supplement bottle says 400 IU of vitamin E. How many mg is that?
It depends on the form. 400 IU of natural vitamin E (d-alpha-tocopherol) = 268 mg. 400 IU of synthetic vitamin E (dl-alpha-tocopherol) = 180 mg. Check the label for 'd-' (natural) or 'dl-' (synthetic) before converting.
Can I convert beta-carotene IU to mcg?
Only if you know the source. Per the NIH Office of Dietary Supplements: 1 IU of supplemental beta-carotene = 0.3 mcg RAE, but 1 IU of beta-carotene from food = 0.05 mcg RAE, and 1 IU of alpha-carotene or beta-cryptoxanthin = 0.025 mcg RAE. Without knowing the source, an exact conversion is impossible, which is why this converter handles retinol only for vitamin A.
Is 1 mcg always equal to 40 IU?
No. The 40 IU per mcg factor applies only to vitamin D. Using it for other vitamins gives wrong results: 1 mcg of vitamin A (retinol) is 3.333 IU, and vitamin E is measured in milligrams, not micrograms. Always match the factor to the substance.
What does IU actually measure?
An International Unit measures the biological effect of a substance, defined by international agreement — not its weight. Two forms of the same vitamin can weigh the same yet have different IU values because the body uses them differently, which is exactly why natural and synthetic vitamin E differ.
Why do some vitamin E labels show IU and others show mg?
The FDA required vitamin E on US labels to be listed in mg of alpha-tocopherol starting in 2020, because IU values hid the natural-versus-synthetic potency difference. Older labels and many non-US labels still use IU. To translate: multiply mg by 1.49 for natural vitamin E or by 2.22 for synthetic to get IU.
Methodology & data sources
Conversions are simple multiplications/divisions using the factors above, rounded to 4 decimals for display. Factors verified October 2026 from:
- NIH Office of Dietary Supplements — Vitamin D fact sheet (40 IU = 1 mcg)
- NIH Office of Dietary Supplements — Vitamin A fact sheet (1 IU retinol = 0.3 mcg RAE)
- NIH Office of Dietary Supplements — Vitamin E fact sheet (1 mg = 1.49 IU natural / 2.22 IU synthetic; 1 IU = 0.67 mg natural / 0.45 mg synthetic)
This is an educational unit converter, not medical advice. Unit translations do not account for absorption, individual needs, or drug interactions — consult a healthcare professional for dosing decisions.
Related tools
Free tool by NRTechStudio