IU to mcg and mg converter: vitamin D, A and E from the label

How to use the converter

1. Find the vitamin on the label and the number in IU.
2. Pick the vitamin. For vitamin A, check the form: retinol (retinyl palmitate or acetate) or beta-carotene. For vitamin E, check whether it’s natural (d-alpha) or synthetic (dl-alpha).
3. Enter the IU.
4. The result shows the amount in mcg or mg, with the upper limit for reference.

Why labels use IU instead of milligrams

An IU (international unit) measures biological activity, not weight. So 1 IU of vitamin D and 1 IU of vitamin E are completely different amounts of substance.

US labels have moved to mcg and mg. NIH ODS notes, for example, that vitamin E now appears in milligrams: the new Supplement Facts labels became mandatory in January 2020, and in January 2021 for smaller companies. Many bottles on iHerb still show both units, and older advice often uses IU only. In the UK, the NHS gives vitamin D advice in micrograms.

Vitamin D: IU to mcg

1 mcg of vitamin D = 40 IU, so 1 IU = 0.025 mcg. To convert IU to mcg, divide by 40. To convert mcg to IU, multiply by 40.

Doses and forms are covered in how much vitamin D3 to take; our vitamin D test guide explains blood results.

Vitamin A: retinol and beta-carotene

Vitamin A is now listed in mcg RAE (retinol activity equivalents). This unit reflects that your body gets less vitamin A from carotenoids than from preformed retinol. NIH ODS factors:

• 1 IU retinol = 0.3 mcg RAE;
• 1 IU beta-carotene from supplements = 0.3 mcg RAE;
• 1 IU beta-carotene from food = 0.05 mcg RAE;
• 1 IU alpha-carotene or beta-cryptoxanthin = 0.025 mcg RAE.

The US upper limit for adults is 3,000 mcg RAE a day, and it applies only to preformed vitamin A (retinol). UK advice is stricter. According to the NHS, an average of 1.5 mg (1,500 mcg, or 5,000 IU of retinol) a day or less from food and supplements combined is unlikely to cause harm. Post-menopausal women and older men should stay within it for bone health. If you’re pregnant, could be pregnant or are breastfeeding, avoid high-dose vitamin A supplements above 3,000 mcg RAE (10,000 IU): too much retinol can cause birth defects. High-dose beta-carotene (20 mg a day) raised lung cancer risk in smokers in trials, so smokers should avoid it.

More in our guide to vitamins A and E.

Vitamin E: natural and synthetic

Vitamin E has two factors because the synthetic form is less active:

• natural (d-alpha-tocopherol, RRR): 1 IU = 0.67 mg, or 1 mg = 1.49 IU;
• synthetic (dl-alpha-tocopherol, all-rac): 1 IU = 0.45 mg, or 1 mg = 2.22 IU.

Adults need 15 mg of alpha-tocopherol a day by US figures. The US upper limit is 1,000 mg a day: that’s 1,500 IU of the natural form or 1,100 IU of the synthetic form. In the UK, the NHS sets a lower bar: 540 mg (800 IU) or less a day from supplements is unlikely to cause harm. The ingredients tell you which you have: “d-alpha tocopherol” is natural, “dl-alpha tocopherol” is synthetic.

Reference table: IU to mcg and mg

These limits are US figures. NHS guidance is lower for vitamin A (1,500 mcg a day from all sources) and vitamin E (540 mg a day from supplements).

Not sure which row is yours? Pick the form as it’s written on the ingredients line:

Common conversion mistakes

• Mixing up mg and mcg. 1 mg = 1,000 mcg, so “25 mg of vitamin D” instead of “25 mcg” is a thousandfold error.
• Dividing everything by 40. That factor only works for vitamin D; every other vitamin has its own.
• Holding beta-carotene to the retinol limit. The 3,000 mcg RAE limit covers preformed vitamin A only.
• Counting a single drop when the label gives the dose for a serving of several drops or capsules.
• Forgetting the multivitamin. It often contains D, A and E too, and they count towards the limit.

Who should be careful with high doses

Vitamins D, A and E are fat-soluble and build up in the body. Take vitamin D above 100 mcg (4,000 IU) or retinol above 3,000 mcg RAE only on medical advice. In pregnancy, don’t take retinol supplements without your midwife or GP. To check other combinations, use our vitamin compatibility chart.

How to choose on iHerb with this converter

1. Convert the dose from IU to mcg or mg so you compare bottles in the same units.
2. For vitamin D, compare the total from all supplements with the 100 mcg (4,000 IU) limit.
3. With vitamin A, check the form: retinol or beta-carotene.
4. For vitamin E, look for “d-alpha tocopherol” if you want the natural form.
5. Check the rest of the panel with our guide on how to read a supplement label.