Vitamin Units Explained: IU, mcg, mg, RAE and DFE and How to Convert Them
What mg, mcg, IU, RAE, DFE and NE mean on vitamin labels, the published conversion factors for vitamins A, D, E and folate, and why one IU number does not fit every vitamin.
Content last updated . This page has not been medically reviewed.
Mixing up mg and mcg is a 1,000-fold mistake. If a unit on a product is unclear, ask a pharmacist before using it. This page explains units. It does not tell anyone how much to take; that is a decision for a clinician or pharmacist who knows your health and medicines. See talking with a health professional.
The metric units
| Unit | Written as | Size | Source |
|---|---|---|---|
| Milligram | mg | One thousandth of a gram | NHS, UK [1] |
| Microgram | mcg or µg | One millionth of a gram; 1,000 mcg = 1 mg | NHS, UK [1] |
IU: why the conversion changes with the vitamin
An International Unit measures biological activity rather than weight, so the weight that equals "1 IU" differs between vitamins. The factors below are the ones published by regulators and health bodies.| Vitamin | Published conversion | Source |
|---|---|---|
| Vitamin D | 1 mcg = 40 IU, so 10 mcg = 400 IU [2][3]. FDA treats vitamins D2 and D3 as bioequivalent for this conversion. [3] EFSA gives the same 40 IU per microgram. [4] | NHS, FDA, EFSA |
| Vitamin A | No direct IU-to-RAE factor for a mixed product. [3] FDA's worked example multiplies IU of preformed vitamin A (retinol) by 0.3 to get mcg RAE. [3] | FDA guidance, 2019 |
| Vitamin E | FDA's guidance multiplies IU by 0.67 for natural vitamin E and 0.45 for synthetic vitamin E to get mg. [3] | FDA guidance, 2019 |
For more on vitamin D amounts see vitamin D intake levels in IU and mcg and vitamin D2 vs D3.
RAE, DFE, NE and "label claim" mg
Several nutrients are now listed in "equivalent" units that account for the form of the vitamin.- RAE (retinol activity equivalents) for vitamin A. The US rule says 1 mcg RAE equals 1 mcg retinol, 2 mcg supplemental beta-carotene or 12 mcg dietary beta-carotene. [5]
- RE (retinol equivalents). The NHS describes vitamin A in retinol equivalents. [6] EFSA's retinol equivalent uses different carotenoid ratios (6 mcg of beta-carotene per RE in its text). [4] So mcg RAE and mcg RE are not interchangeable for foods rich in carotenoids.
- DFE (dietary folate equivalents) for folate. One DFE equals 1 mcg naturally occurring folate or 0.6 mcg folic acid. [5] Put the other way, FDA multiplies folic acid by 1.7. [3] EFSA uses the same 1.7 factor. [4] See folate vs folic acid and methylfolate.
- NE (niacin equivalents). One mg NE equals 1 mg niacin or 60 mg tryptophan. [5]
- Vitamin E "label claim". On US labels 1 mg of vitamin E equals 1 mg RRR-alpha-tocopherol (natural) or 2 mg all-rac-alpha-tocopherol (synthetic). [5] See vitamin E forms.
Worked examples
These show the arithmetic using the published factors. They are not suggested amounts.| Starting figure | Calculation | Result | Source |
|---|---|---|---|
| 100 IU vitamin D | 100 × 0.025 | 2.5 mcg | FDA example [3] |
| 500 IU preformed vitamin A | 500 × 0.3 | 150 mcg RAE | FDA example [3] |
| 200 mcg folic acid | 200 × 1.7 | 340 mcg DFE | FDA example [3] |
| 20 mg synthetic vitamin E | 20 ÷ 2 | 10 mg label claim | FDA example [3] |
Which unit will you see?
US rules require vitamin D in mcg; IU may be added in parentheses. [5] FDA updated its label units in 2016, which is why vitamins A, D and E and folate appear in newer units. [3] Labels and health pages in other countries can use other units, for example the NHS uses retinol equivalents for vitamin A. [6] A product's own table of daily values is covered on reading a vitamin label.Frequently asked questions
How many IU are in 1 microgram of vitamin D?
Forty. The NHS says 1 microgram of vitamin D equals 40 IU, so 10 micrograms equals 400 IU. [2] FDA and EFSA give the same factor. [3][4]
Can I use the same IU-to-mcg number for every vitamin?
No. The NHS states that the conversion depends on the type of vitamin. [1] Vitamin D, vitamin A and vitamin E each have different published factors, and vitamin E also differs between natural and synthetic forms. [3]
What is RAE and why do labels use it for vitamin A?
RAE stands for retinol activity equivalents. It counts retinol and carotenoids on one scale, using ratios such as 12 mcg of dietary beta-carotene to 1 mcg RAE. [5] US vitamin A reference values are listed in mcg RAE. [5]
What does DFE mean on a folate label?
DFE means dietary folate equivalents. It adjusts for the fact that folic acid is used differently from food folate: 1 mcg DFE equals 1 mcg food folate or 0.6 mcg folic acid. [5]
Is it safe to convert units myself and choose an amount?
This page cannot say. Conversions show what a label figure means, not what is right for you. Ask a clinician or pharmacist, especially in pregnancy, for children, or if you take medicines. [9]
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
Why units matter for safety
- Pregnancy: the NHS says not to take supplements containing vitamin A (retinol) in pregnancy, because too much can harm a baby's development. [7]
- Children: the NHS warns that too much of some vitamins can be harmful. [8] If a child swallows many vitamins, call Poison Help at 1-800-222-1222 (US) or 911 if the child is unwell; see poisoning: what to do first.
- Medicines, kidney or liver conditions, and surgery: a clinician or pharmacist should check any supplement against your medicines and conditions. See medication interactions.
References
- NHS (UK). Vitamins and minerals. Page last reviewed 3 August 2020. Read 2026-10-23.
- NHS (UK). Vitamin D. Page last reviewed 3 August 2020. Read 2026-10-23.
- US Food and Drug Administration. Converting Units of Measure for Folate, Niacin, and Vitamins A, D, and E on the Nutrition and Supplement Facts Labels: Guidance for Industry. August 2019. Read 2026-10-23 (text extracted from the PDF; some Greek letters were lost in extraction).
- European Food Safety Authority. Dietary Reference Values for nutrients: Summary report. EFSA Supporting Publication 2017:e15121, approved 4 December 2017. Read 2026-10-23 (text extracted from the PDF; the table layout and some symbols such as the micro sign were reconstructed or lost, and bold type that marks PRIs was not preserved).
- US Code of Federal Regulations. 21 CFR 101.9, Nutrition labeling of food, eCFR text dated 2025-01-02. Read 2026-10-23.
- NHS (UK). Vitamin A. Page last reviewed 3 August 2020. Read 2026-10-23.
- NHS (UK). Pregnancy vitamins and supplements. Page last reviewed 3 June 2026. Read 2026-10-23.
- NHS (UK). Vitamins for children. Page last reviewed 30 May 2024. Read 2026-10-23.
- US Food and Drug Administration. Information for Consumers on Using Dietary Supplements (questions and answers). Content current as of 08/27/2026. Read 2026-10-23.