mcg ↔ IU Converter

Use this mcg to IU Converter to convert supported vitamins and selected reference substances between mass units and International Units (IU).

Important: IU is a measure of biological activity, not a fixed unit of mass. There is no universal mcg-to-IU conversion factor.

Conversion Method

The general calculation is:

IU = mass × substance-specific conversion factor

Mass = IU ÷ substance-specific conversion factor

Common vitamin relationships include:

• Vitamin D: 1 mcg = 40 IU
• Natural vitamin E (d-alpha-tocopherol): 1 mg ≈ 1.49 IU
• Synthetic vitamin E (dl-alpha-tocopherol): 1 mg ≈ 2.22 IU
• Vitamin A: conversion depends on whether the source is retinol, supplemental beta-carotene, dietary beta-carotene, or another carotenoid.

Example

To convert 25 mcg of vitamin D to IU:

25 × 40 = 1,000 IU

Therefore:

25 mcg vitamin D = 1,000 IU

For the reverse conversion:

800 IU ÷ 40 = 20 mcg vitamin D

Important Limitations

Always select the exact vitamin or substance and form shown on the label. Natural and synthetic vitamin E use different factors, and vitamin A cannot be converted correctly without knowing its source form.

Insulin, hCG, and other prescription or biologic products use product- or activity-specific relationships. Do not use a general IU converter to change injection doses or prescription instructions.

Sources

• NIH Office of Dietary Supplements — Vitamin D Fact Sheet
• NIH Office of Dietary Supplements — Vitamin E Fact Sheet
• NIH Office of Dietary Supplements — Vitamin A and Carotenoids Fact Sheet