Milligrams (mg) and micrograms (mcg) measure weight. International Units (IU) measure biological activity, and what one IU weighs is different for every nutrient and sometimes for different forms of the same nutrient. So there is no single IU to mg conversion. To compare two labels, you need to know which nutrient, which form and which conversion applies to it.
If you have ever stood in a pharmacy aisle comparing a vitamin D jar that says "1000 IU" with one that says "25 mcg", you are not alone. They can be the same amount, written two ways. This guide explains the units, gives the common conversions that are fixed by definition and shows where to be careful.
This is about reading labels, not about how much to take. How much of any vitamin is right for you is a question for your doctor, pharmacist or dietitian.
The three units
- mg (milligram): one thousandth of a gram.
- mcg (microgram): one thousandth of a milligram. It is sometimes written µg. 1 mg = 1,000 mcg.
- IU (International Unit): a unit of biological activity, agreed internationally for a particular substance. One IU of one vitamin has nothing to do with one IU of another.
The mg and mcg mix-up is the easiest one to get wrong, because the two abbreviations look alike and the numbers differ by a factor of a thousand. Read twice.
Why IU exists at all
Some vitamins exist in several chemical forms that have different strengths in the body. Before these could be measured easily by weight, scientists measured what a sample did, and defined a standard unit for that activity. The IU stuck on labels for some vitamins long after weight became easy to measure.
Over the past several years, labelling in a number of countries has been moving towards metric units for vitamins that used IU. In the United States, for example, newer supplement labels list vitamins A, D and E by weight, and many brands show IU alongside. Depending on where you live and how old the stock is, you may see one unit, the other or both.
Common conversions
These conversions are fixed by definition for the form named. They are useful for comparing labels, not for working out a dose.
Vitamin D
- 1 mcg of vitamin D = 40 IU
- So 25 mcg = 1,000 IU, and 10 mcg = 400 IU
Vitamin D is the most common reason people search for this. The same conversion applies to vitamin D2 and D3 on labels.
Vitamin E
Vitamin E depends on the form:
- Natural form (d-alpha-tocopherol): 1 IU = 0.67 mg
- Synthetic form (dl-alpha-tocopherol): 1 IU = 0.45 mg
So two jars with the same IU can contain different weights, and two jars with the same mg can have different IU. The form is usually printed in the ingredients list.
Vitamin A
Vitamin A is the hardest to compare. Labels may use IU, mcg or mcg RAE (retinol activity equivalents), and the conversion depends on whether it comes from retinol or from carotenoids such as beta-carotene. If you need to compare vitamin A products, ask your pharmacist to do it with you.
How to compare two jars
- Check that it is the same nutrient and form. "Vitamin D3" against "vitamin D3", or "d-alpha-tocopherol" against "d-alpha-tocopherol".
- Look for both units on the label. Many labels show both. If one does, the brand has done the conversion for you.
- Check "per tablet" or "per serving". A serving may be two capsules. This catches more people than the units do.
- Convert only when you know the form. Use the conversions above where they apply, and ask a pharmacist for anything else.
- Compare like with like. Convert one label into the other's unit, not both into a third.
Where mistakes happen
- Reading mcg as mg. A thousandfold difference.
- Missing "per 2 capsules". The strength on the front may be for a serving, not one capsule.
- Combined products. A multivitamin and a separate vitamin D both count. Add them up, and tell your doctor or pharmacist about both.
- Changing brands. A new jar with the same name can have a different strength or unit. Check it every time.
- Children's products. Often a different strength per tablet or drop than the adult version.
Recording supplements accurately
When you add a supplement to a tracking app, copy the strength exactly as it appears on the label, unit and all. If the label says "25 mcg (1,000 IU)", you can enter either, but stick to one. When you switch brands, update the entry so your record matches what is in your cupboard. That way, the list you show your doctor is accurate.
In Cadence, the Strength field takes whatever is printed on the label, so you can record "1000 IU" or "25 mcg" as it appears. You can track supplements alongside medication, set reminders around meals and keep an eye on supply.
Frequently asked questions
Is 1,000 IU the same as 1,000 mg?
No. For vitamin D, 1,000 IU is 25 mcg, which is 0.025 mg. The units measure different things.
Can I convert IU to mg for any vitamin?
Only if you know the nutrient and its form. There is no single conversion.
Why do some labels show both?
To help people compare, and because labelling rules have changed in some countries. Both numbers describe the same amount.
Who can help me check my supplements?
Your pharmacist. They can check strengths, units and whether anything you take overlaps or interacts.
Cadence is a free medication reminder app for iPhone and Apple Watch. It is not medical software and does not give medical advice. Cadence Pro is a one-time purchase, with no subscription.
