How to Convert IU to mcg for Vitamins and Supplements

December 12, 2025 •

3 min read

An International Unit (IU) is a measure of a substance's biological activity, not its mass, which is why converting it to a mass unit like micrograms (mcg) is not a simple, universal calculation. Instead, the conversion factor is unique for each specific vitamin and its form, as determined by international agreements.

Quick Summary

The conversion from International Units (IU) to micrograms (mcg) requires a specific, substance-dependent conversion factor, as IU measures biological activity while mcg measures mass. This guide details the different formulas for converting vitamins like A, D, and E to ensure accurate dosing.

Key Points

IU vs. mcg: IU (International Unit) measures biological activity, while mcg (microgram) measures mass; they are not interchangeable and require different conversion factors for each substance.
Specific Conversion Factors: The conversion factor is unique for each vitamin and its form, such as retinol versus beta-carotene for vitamin A.
Vitamin D Formula: To convert vitamin D from IU to mcg, divide the IU amount by 40 (mcg = IU / 40).
Vitamin A Formula: Vitamin A conversion depends on the form; for retinol, divide IU by 3.33 (mcg RAE = IU / 3.33).
Vitamin E Formula: Conversion for vitamin E is typically done from IU to mg first, using a factor that depends on whether the form is natural (d-alpha-tocopherol) or synthetic (dl-alpha-tocopherol).
Labeling Changes: The FDA has shifted toward using metric units like mcg on supplement labels, making it important for consumers to understand these conversions.
Importance of Accuracy: Accurate conversion is crucial for proper dosing, especially for fat-soluble vitamins like A and D, to avoid deficiency or toxicity.
Consult a Professional: Always consult a healthcare provider or nutritionist for personalized guidance on dosage, as they typically work with metric units.

Understanding the Difference Between IU and mcg

Before diving into the calculations, it's crucial to understand what IU and mcg represent. An International Unit (IU) is a standardized measure used for certain biologically active substances, such as vitamins, hormones, and vaccines. It is not a fixed unit of mass but is based on the biological effect of a substance. This is why 1 IU of vitamin D is a different mass than 1 IU of vitamin A.

In contrast, a microgram (mcg or µg) is a standard metric unit of mass, equal to one-millionth of a gram. The U.S. Food and Drug Administration (FDA) has transitioned to using metric units like mcg and mg on supplement labels to improve clarity and precision for consumers. This change makes understanding the conversion more important than ever.

Why Different Vitamins Have Different Conversion Factors

The reason there is no single conversion factor is due to the inherent difference in biological activity and chemical structure of each vitamin. For example, different forms of vitamin A, such as retinol and beta-carotene, have varying levels of activity in the body. The conversion factor reflects this biological potency. The same applies to vitamin E, which exists in several forms with different activities.

Step-by-Step Conversion for Common Vitamins

Here are the specific formulas for converting IU to mcg for some of the most common vitamins found in supplements.

Vitamin D

For both vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol), the conversion is straightforward:

Formula: mcg = IU / 40
Example: To convert 400 IU of vitamin D to mcg, you would calculate 400 / 40, which equals 10 mcg.

Vitamin A

For vitamin A, the conversion depends on the form of the nutrient.

Retinol (animal sources):
- Formula: mcg RAE = IU / 3.33 (RAE stands for Retinol Activity Equivalents).
- Example: For 5,000 IU of vitamin A as retinol, the calculation is 5,000 / 3.33 ≈ 1,500 mcg RAE.
Supplemental Beta-Carotene:
- Formula: mcg RAE = IU / 3.33.
Dietary Beta-Carotene:
- Formula: mcg RAE = IU * 0.05.

Vitamin E

Vitamin E has multiple forms, with d-alpha-tocopherol being the natural form and dl-alpha-tocopherol being the synthetic form. The conversion is typically from IU to mg, but since 1 mg = 1,000 mcg, you can easily convert to mcg.

Natural Vitamin E (d-alpha-tocopherol):
- Conversion to mg: mg = IU * 0.67.
- Example (mg to mcg): 30 IU * 0.67 = 20.1 mg, which is 20,100 mcg.
Synthetic Vitamin E (dl-alpha-tocopherol):
- Conversion to mg: mg = IU * 0.45.
- Example (mg to mcg): 30 IU * 0.45 = 13.5 mg, which is 13,500 mcg.

Comparison Table: IU to mcg Conversion Factors


Vitamin	Form	Conversion Formula	Calculation Example (5000 IU)
Vitamin D	All forms (D2/D3)	`mcg = IU / 40`	`5000 IU / 40 = 125 mcg`
Vitamin A	Retinol (animal)	`mcg RAE = IU / 3.33`	`5000 IU / 3.33 ≈ 1502 mcg RAE`
Vitamin A	Beta-Carotene (dietary)	`mcg RAE = IU * 0.05`	`5000 IU * 0.05 = 250 mcg RAE`
Vitamin E	d-alpha-tocopherol (natural)	`mg = IU * 0.67`	`5000 IU * 0.67 = 3350 mg` (or 3,350,000 mcg)
Vitamin E	dl-alpha-tocopherol (synthetic)	`mg = IU * 0.45`	`5000 IU * 0.45 = 2250 mg` (or 2,250,000 mcg)

Reading Supplement Labels Correctly

With the FDA's new labeling rules, many supplement facts panels now list the amounts of vitamins A, D, and E primarily in metric units, with IU often provided in parentheses as an optional extra. Being able to perform the conversions yourself provides an extra layer of understanding and ensures you can compare dosages accurately, especially when comparing older and newer product formulations.

Importance of Accurate Dosages

Accurate dosage is critical for managing your health. For fat-soluble vitamins like A, D, and E, excessive intake can lead to toxicity, while insufficient intake results in deficiency. By converting IU to mcg correctly, you can make informed decisions and compare dosages between different products more effectively.

Practical Application in Daily Life

Knowing these conversions is useful beyond just deciphering supplement labels. When consulting with a healthcare provider or nutritionist, they will typically use metric units. Being able to translate between IU and mcg allows for more precise communication about your nutritional needs and supplement regimen. It also empowers you to interpret scientific studies or dietary recommendations that use either unit of measurement.

Conclusion

Converting IU to mcg is not a one-size-fits-all calculation. The conversion is substance-specific, relying on the unique biological activity of each vitamin and its specific form. By using the correct conversion factors for vitamins like D, A, and E, you can accurately determine dosages and make more informed health decisions. This knowledge is essential for understanding supplement labels and ensuring you maintain a balanced and safe nutritional intake.

For more detailed nutritional information and daily recommended intakes, a reliable resource is the National Institutes of Health Office of Dietary Supplements. Accessing resources like this can provide you with authoritative data to support your health journey.

Frequently Asked Questions

IU, or International Unit, measures a substance's biological effect, whereas mcg, or microgram, measures its physical mass. They are not equivalent and have different conversion rates depending on the substance.

The conversion factor varies because each vitamin has a different level of biological activity per unit of mass. For example, the activity of vitamin A as retinol is different from its activity as beta-carotene, necessitating different conversion factors.

To convert IU to mcg for vitamin D, you should divide the number of IU by 40. For example, 400 IU of vitamin D is equivalent to 10 mcg.

The conversion for vitamin A depends on the form. For retinol, you divide the IU by 3.33 to get mcg RAE (Retinol Activity Equivalents). The conversion for beta-carotene is different due to its lower biological activity.

No, 1000 IU is not the same as 1000 mcg. Since IU measures biological activity and mcg measures mass, the conversion is not a 1:1 ratio. The specific vitamin and its form dictate the conversion factor.

The FDA mandated the change to standardize vitamin labeling and use metric units (mcg, mg) that are more common worldwide. This helps consumers understand dosages more precisely and compare products more easily.

No, you cannot use a universal conversion formula. You must use the specific conversion factor for each vitamin and its specific form. Using the wrong formula can lead to incorrect dosing.

For vitamin E, you typically convert IU to milligrams (mg) first. The formula depends on whether the vitamin E is natural (d-alpha-tocopherol) or synthetic (dl-alpha-tocopherol). Then, multiply the result by 1000 to get mcg.