Vitamin Units: Converting IU to Micrograms for Vitamins D, A, and E
October 29, 2026
Supplement labels sometimes use international units (IU), an older measure based on biological activity, and sometimes micrograms (mcg or µg) or milligrams (mg). The conversion depends on the vitamin and its form, because an IU is defined for each substance. Many current labels in the US show both.
For vitamin D (cholecalciferol or ergocalciferol), 1 mcg = 40 IU. A 600 IU dose is 15 mcg, 1,000 IU is 25 mcg, 2,000 IU is 50 mcg, and 4,000 IU is 100 mcg.
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Vitamin A: 1 IU of retinol = 0.3 mcg RAE (retinol activity equivalents); 1 IU of beta-carotene from supplements = 0.15 mcg RAE (and from food 0.05 mcg RAE) under current US conversions. So 3,000 IU of retinol is 900 mcg RAE, the RDA for men. Vitamin E: 1 IU of natural d-alpha-tocopherol = 0.67 mg; 1 IU of synthetic dl-alpha-tocopherol = 0.45 mg. The RDA of 15 mg corresponds to 22.4 IU of natural vitamin E or 33.3 IU of synthetic. Vitamin B12 and folate use mcg, with folate in dietary folate equivalents (DFE).
| Vitamin | Conversion | Example |
|---|---|---|
| Vitamin D | 1 mcg = 40 IU | 600 IU = 15 mcg; 2,000 IU = 50 mcg |
| Vitamin A (retinol) | 1 IU = 0.3 mcg RAE | 3,000 IU = 900 mcg RAE |
| Vitamin E (natural) | 1 IU = 0.67 mg | 15 mg = 22.4 IU |
| Vitamin E (synthetic) | 1 IU = 0.45 mg | 15 mg = 33.3 IU |
Recommended and Upper Intakes
US recommended dietary allowances for adults include 600 IU (15 mcg) of vitamin D for ages 19 to 70, and 800 IU (20 mcg) above 70. The tolerable upper intake level for vitamin D is 4,000 IU (100 mcg) a day. For preformed vitamin A, the upper level is 3,000 mcg RAE (10,000 IU) a day, because too much can cause toxicity and birth defects. For vitamin E, the upper limit from supplements is 1,000 mg of alpha-tocopherol (1,500 IU natural). Higher doses should be taken only under medical supervision.
Reading Labels
Check whether the label lists the amount in the active form and what percentage of the daily value it provides. The daily values on US labels use mcg for vitamin D (20 mcg = 800 IU). Combination products can add up: a multivitamin, a vitamin D supplement, and fortified foods all contribute. A dose labeled '2,000 IU' of D3 is 50 mcg, which is 250% of the 20 mcg daily value, though still below the upper limit.
Practical Advice
Get nutrients from food where possible, and use supplements to fill gaps identified with your clinician. Blood tests can show whether vitamin D levels are low. This article is educational, not medical advice.
Frequently Asked Questions
How many IU in 1 mcg of vitamin D?
40 IU.
How much is 2,000 IU of vitamin D in mcg?
50 mcg.
How many mcg RAE in 3,000 IU of retinol?
900 mcg RAE.
Is the vitamin E conversion the same for natural and synthetic?
No. Natural is 0.67 mg per IU and synthetic 0.45 mg per IU.
What is the vitamin D upper limit?
4,000 IU (100 mcg) a day for adults.