IU (International Unit) is a standardized measure of biological activity used for vitamins, hormones, enzymes, and certain biologic products. It does not directly measure mass (mg or mcg). Each substance has its own IU-to-mass relationship based on its defined potency.

Why do we use IU instead of mg for some vitamins and biologics?

Different formulations of the same vitamin or biologic can have different strengths per milligram. IU measures biological effect rather than simple weight, so 1 IU of a properly standardized substance should deliver the same biological activity regardless of brand or form.

How do I convert IU to mcg or mg with this calculator?

To convert IU to mass you need the potency in IU per microgram (IU/µg). The calculator loads presets for common vitamins like Vitamin D and Vitamin A and lets you edit the IU/µg value for custom products. Once potency is set, enter either IU or mass and the tool instantly calculates the corresponding mcg, mg, or g.

How much is 1 IU to mg for Vitamin D?

For Vitamin D (D₂ or D₃), 1 IU is defined as 0.025 µg, which equals 0.000025 mg. This corresponds to 40 IU per microgram (IU/µg = 40). The calculator includes this as a preset so you can quickly convert common Vitamin D doses.

How many IU are in 1 mg of Vitamin D?

Because 1 µg of Vitamin D equals 40 IU and 1 mg = 1000 µg, 1 mg of Vitamin D is equal to 40,000 IU. Selecting the Vitamin D preset and entering 1 mg in the calculator will show this conversion automatically.

Is the IU to mg / mcg calculator medical advice?

No. This tool is for informational and educational purposes only. It uses standard reference values and user-provided potency, but it cannot verify your product’s labeling or your personal health needs. Always follow your product label and consult a qualified healthcare professional or pharmacist before making any dosing decisions.

IU to mg

How the IU to mg Calculator Works

IU measures biological activity, not weight. To convert between IU and mass, you must know the substance’s potency in IU per microgram (IU/µg).
This tool:

Provides presets for common vitamins (A, D, E) based on widely-used reference values.
Lets you override potency for any custom supplement, hormone, enzyme, or biologic based on its official IU definition.
Converts both ways: IU → µg → mg/g and mg/g → µg → IU.

What is IU?

IU (International Unit) standardizes the activity of vitamins and biologics. It lets different formulations with different weights be compared using the same activity scale.

How do I convert IU to mcg or mg?

Use your product’s potency (IU/µg). Compute micrograms = IU ÷ (IU/µg), then milligrams = micrograms ÷ 1000. The calculator does this automatically as you type.

How do I convert mcg or mg back to IU?

First convert the entered mass into micrograms, then IU = micrograms × (IU/µg). Change the unit picker to mcg, mg, or g and the correct IU will be displayed.

Example: 5000 IU of Vitamin D to mg

Vitamin D uses 40 IU per µg. So micrograms = 5000 ÷ 40 = 125 µg, which is 0.125 mg. Selecting Vitamin D preset and typing 5000 IU will show exactly this.

This calculator is for general information only and does not replace medical, pharmaceutical, or nutritional advice. Always follow product labeling and consult your doctor, pharmacist, or qualified professional for dosing guidance.