How to calculate vitamin A IU?

January 3, 2026 •

3 min read

According to the National Institutes of Health, vitamin A is now primarily listed on nutrition labels in Retinol Activity Equivalents (RAE), but many still encounter the older International Units (IU) on supplements. This difference can cause confusion, as 1 IU does not represent a constant amount of mass but rather a measure of biological activity, which varies depending on the source.

Quick Summary

Understanding vitamin A conversion involves knowing its source, such as retinol or carotenoids, and applying specific conversion factors. The IU system accounts for varying biological potency, meaning the calculation differs based on whether the vitamin A is from preformed retinol or provitamin A compounds like beta-carotene.

Key Points

Source Matters: The correct calculation depends on whether the vitamin A is from preformed retinol or provitamin A carotenoids like beta-carotene.
RAE is the Standard: Newer nutrition labels use Retinol Activity Equivalents (RAE) to account for varying bioavailability, making IU a less accurate measure.
Retinol Conversion: For preformed vitamin A (retinol), multiply mcg RAE by 3.33 to get the IU value.
Supplemental Beta-Carotene: Use the same conversion factor of 3.33 when calculating IU from mcg RAE of supplemental beta-carotene.
Dietary Carotenoid Factors: Dietary beta-carotene has a conversion factor of 20, while other dietary carotenoids like alpha-carotene use a factor of 40.
Combined Sources: If a product lists multiple vitamin A sources, calculate the IU for each source separately and then add them together for the total.
Consult a Health Professional: Always confirm precise dosing with a healthcare provider, especially when using high-potency supplements.

Understanding the Vitamin A Unit Conversion

Calculating vitamin A in International Units (IU) requires understanding that the conversion is not uniform and depends on the vitamin A source: preformed vitamin A (retinol) or provitamin A carotenoids (like beta-carotene). Retinol, found in animal products, is readily used by the body, while carotenoids from plants need conversion to retinol, a less efficient process.

Historically, vitamin A was measured in IU, but the Retinol Activity Equivalents (RAE) unit was developed to better reflect biological activity from various sources. Below are the factors needed to convert from micrograms (mcg) RAE back to IU, useful for comparing different product labels.

Conversion Factors for Different Vitamin A Sources

The conversion factor varies based on the source of vitamin A, such as preformed vitamin A (retinol), supplemental beta-carotene, and dietary beta-carotene and other carotenoids.

For Preformed Vitamin A (Retinol): This form is from animal sources and is easily used by the body.

1 mcg RAE = 3.33 IU (retinol)
Formula: Amount in IU = Amount in mcg RAE × 3.33

For Supplemental Beta-Carotene: This applies to beta-carotene from supplements, which differs from food sources.

1 mcg RAE = 3.33 IU (supplemental beta-carotene)
Formula: Amount in IU = Amount in mcg RAE × 3.33

For Dietary Beta-Carotene (from food): Found in plants, dietary beta-carotene has lower bioavailability.

1 mcg RAE = 20 IU (dietary beta-carotene)
Formula: Amount in IU = Amount in mcg RAE × 20

For Dietary Alpha-Carotene or Beta-Cryptoxanthin: These are other plant-based provitamin A carotenoids with less efficient conversion than beta-carotene.

1 mcg RAE = 40 IU (dietary alpha-carotene or beta-cryptoxanthin)
Formula: Amount in IU = Amount in mcg RAE × 40

Step-by-Step Calculation Examples

Let's apply these formulas to practical scenarios.

Example 1: Converting Retinol to IU

Scenario: A supplement contains 300 mcg RAE of retinol.
Calculation: 300 mcg RAE × 3.33 = 999 IU

Example 2: Converting Supplemental Beta-Carotene to IU

Scenario: A supplement has 500 mcg RAE from supplemental beta-carotene.
Calculation: 500 mcg RAE × 3.33 = 1665 IU

Example 3: Converting Dietary Beta-Carotene to IU

Scenario: A carrot provides 120 mcg RAE of dietary beta-carotene.
Calculation: 120 mcg RAE × 20 = 2400 IU

Comparison Table: IU to RAE Conversions

This table summarizes conversions between IU and mcg RAE, helpful for understanding nutritional information.


Source of Vitamin A	Conversion from IU to mcg RAE	Conversion from mcg RAE to IU
Retinol (Preformed)	`IU × 0.3 = mcg RAE`	`mcg RAE × 3.33 = IU`
Supplemental β-Carotene	`IU × 0.3 = mcg RAE`	`mcg RAE × 3.33 = IU`
Dietary β-Carotene	`IU × 0.05 = mcg RAE`	`mcg RAE × 20 = IU`
Dietary α-Carotene / β-Cryptoxanthin	`IU × 0.025 = mcg RAE`	`mcg RAE × 40 = IU`

What if the label lists both retinol and beta-carotene?

If a product lists multiple vitamin A sources, calculate the IU for each source separately and then add the results. For example, if a supplement has 450 mcg RAE from retinol and 450 mcg RAE from supplemental beta-carotene:

Retinol IU: 450 mcg RAE × 3.33 = 1498.5 IU
Beta-Carotene IU: 450 mcg RAE × 3.33 = 1498.5 IU
Total IU: 1498.5 + 1498.5 = 2997 IU

This method is essential for an accurate total, showing why RAE is a more precise measure than IU.

Conclusion: Navigating Vitamin A Measurements

Accurately calculating vitamin A IU requires knowing its source and applying the correct conversion factors. The RAE system provides a more accurate measure of biological activity by accounting for differing bioavailability. While IU is still used, understanding these conversions helps in comparing nutritional data and making informed choices. For official guidelines, consult resources like the National Institutes of Health.

Frequently Asked Questions

IU (International Unit) measures a substance's biological effect, so its mass equivalent varies depending on the specific form of the vitamin. Mcg RAE (micrograms of Retinol Activity Equivalents) standardizes the measurement based on the body's actual utilization of vitamin A, making it a more consistent metric.

No, 5,000 IU can represent different amounts of vitamin A depending on the source. For example, 5,000 IU of retinol is equivalent to a different RAE value than 5,000 IU of dietary beta-carotene, due to differences in bioavailability.

The switch was made to provide a more accurate representation of vitamin A's biological activity. Because the body converts provitamin A carotenoids less efficiently than it uses preformed retinol, RAE accounts for these differences, providing a clearer nutritional picture.

You must know the source of the vitamin A listed on the label. If it's preformed vitamin A (retinol) or supplemental beta-carotene, multiply the mcg RAE by 3.33. If it's dietary beta-carotene, multiply by 20.

Unlike preformed vitamin A (retinol), which can be toxic at very high doses, the body regulates the conversion of beta-carotene into vitamin A. This makes it difficult to reach toxic levels from food sources alone.

Preformed vitamin A, or retinol, is found in animal-based foods. Good sources include liver, eggs, and fortified milk.

Provitamin A carotenoids, like beta-carotene, are abundant in many fruits and vegetables. Excellent sources include carrots, spinach, sweet potatoes, and mangoes.