mcg to IU Calculator

IU (International Unit) is not a mass unit. It is a measure of biological activity, defined separately by the US Pharmacopeia for each vitamin and each chemical form of that vitamin. One IU of vitamin D is the activity of 0.025 mcg of cholecalciferol. One IU of vitamin A is the activity of 0.3 mcg of all-trans retinol. The two numbers are completely unrelated because they were calibrated against different reference standards in different decades. That is why this calculator forces you to pick the vitamin first: applying the wrong factor can easily produce a 100x dosing error.

Multiply micrograms by 40. The USP factor is exact: 1 IU vitamin D equals 0.025 mcg of either D2 (ergocalciferol) or D3 (cholecalciferol). So 15 mcg equals 600 IU (the adult RDA), 25 mcg equals 1,000 IU, 50 mcg equals 2,000 IU, and 100 mcg equals 4,000 IU (the NIH tolerable upper intake for adults). To go the other way, multiply IU by 0.025 or divide by 40.

Both are forms of vitamin A, but the body absorbs and converts them differently, so the IU factor is different. The USP standard is 1 IU vitamin A equals 0.3 mcg of all-trans retinol, but 0.6 mcg of beta-carotene from supplements. The two-to-one ratio reflects the fact that beta-carotene must first be cleaved into retinol in the gut, and that conversion is not 100 percent efficient. In other words, you need roughly twice as much beta-carotene by weight to get the same biological vitamin A activity as retinol. Always read the supplement label to see which form is listed before doing the math.

Natural vitamin E is labeled d-alpha-tocopherol, while synthetic vitamin E is labeled dl-alpha-tocopherol. NIH conversion factors use 0.67 mg per IU for natural vitamin E and 0.9 mg per IU for synthetic vitamin E. Therefore, 400 IU equals about 268 mg natural vitamin E or 360 mg synthetic vitamin E. Check the form on the label before converting.

Vitamin C, vitamin K, and all eight B vitamins (B1, B2, B3, B5, B6, B7, B9, B12) are not measured in IU and never have been. Their biological activity does not differ enough across chemical forms to need an activity-based unit, so they have always been dosed by mass: milligrams for C, B1, B2, B3, B5, B6, micrograms for K, B7, B9, B12. Every supplement label, every RDA table, and every clinical guideline for these vitamins uses mg or mcg directly. There is no IU per mcg factor to apply, which is why this calculator rejects them rather than invent one.