What Is the Difference Between IU and mg?#
If you have ever compared two Vitamin E supplements and found one labeled in IU and the other in mg, you have encountered one of the most confusing aspects of supplement labeling. Understanding IU vs mg is critical for accurate dosing, safe supplementation, and making sense of your doctor's recommendations.
IU (International Unit) measures the biological activity of a substance — how much of a specific biological effect it produces in the body. mg (milligram) measures weight — one thousandth of a gram (0.001 g). Just as with IU vs mcg, the IU-to-mg conversion factor is substance-specific. One IU of Vitamin E (natural) represents a different weight than one IU of insulin. There is no universal IU-to-mg formula.
The IU system exists because some vitamins come in multiple chemical forms with different potencies. Vitamin E is the prime example: natural Vitamin E (d-alpha-tocopherol) is approximately 50% more biologically active than synthetic Vitamin E (dl-alpha-tocopherol) at the same weight. The IU system accounts for this difference, ensuring that "400 IU" delivers the same biological effect regardless of which form you take — even though the actual weight in milligrams differs.
Which Vitamins and Substances Use mg?#
While mcg (micrograms) is the metric unit for Vitamins D and A, milligrams (mg) are used for substances that are active at higher weights:
Vitamin E (Tocopherol)#
Vitamin E is the most common supplement where IU-to-mg conversion matters. It comes in two main forms:
- Natural Vitamin E (d-alpha-tocopherol) — derived from plant oils; higher bioavailability; labeled with the "d-" prefix
- Synthetic Vitamin E (dl-alpha-tocopherol) — manufactured industrially; lower bioavailability; labeled with the "dl-" prefix
The FDA updated Vitamin E labeling in 2020 to use mg as the primary unit, replacing IU. However, many products, prescriptions, and reference materials still use IU, making conversion knowledge essential.
Insulin#
Insulin is always prescribed and administered in International Units, but its weight equivalent in mg is relevant for pharmaceutical manufacturing and research. For clinical dosing, patients should always follow their prescribed IU dose — never attempt to convert and measure insulin by weight.
Other Substances#
Some enzymes, hormones, and biological products also use IU (e.g., heparin, penicillin, certain vaccines), but these are pharmaceutical products dosed by healthcare professionals rather than consumer supplements.
IU to mg Conversion Factors#
Here are the key conversion factors you need to know:
| Substance | 1 IU Equals | Notes |
|---|---|---|
| Vitamin E (natural) — d-alpha-tocopherol | 0.67 mg | Higher bioavailability; preferred form |
| Vitamin E (synthetic) — dl-alpha-tocopherol | 0.45 mg | Lower bioavailability; cheaper to produce |
| Insulin (human) | 0.0347 mg | Pharmaceutical standard; do not self-convert |
Why Are the Vitamin E Factors Different?#
Natural and synthetic Vitamin E have different molecular structures. Natural d-alpha-tocopherol is a single stereoisomer that your body recognizes and uses efficiently. Synthetic dl-alpha-tocopherol is a mixture of eight stereoisomers, only one of which is the natural form. Your body preferentially absorbs and retains the natural form, which is why it has approximately 1.49 times the biological activity per milligram compared to the synthetic version.
This means:
- 400 IU of natural Vitamin E = 400 x 0.67 = 268 mg
- 400 IU of synthetic Vitamin E = 400 x 0.45 = 180 mg
Both deliver "400 IU" of biological activity, but the natural form requires more mg by weight because each mg is more potent. Conversely, the synthetic form requires fewer mg because each mg is less potent — a counterintuitive result that confuses many people.
How to Convert IU to mg#
Formula: IU x conversion factor = mg
Formula (reverse): mg / conversion factor = IU
Or equivalently for the reverse:
- Natural Vitamin E: mg x 1.49 = IU
- Synthetic Vitamin E: mg x 2.22 = IU
- Insulin: mg x 28.82 = IU
Common IU to mg Conversion Tables#
Vitamin E — Natural (d-alpha-tocopherol)#
| IU | mg | Common Use |
|---|---|---|
| 100 IU | 67 mg | Low-dose daily supplement |
| 200 IU | 134 mg | Moderate daily supplement |
| 400 IU | 268 mg | Most popular supplement strength |
| 800 IU | 536 mg | High-dose supplement |
| 1,000 IU | 670 mg | Therapeutic dose; near UL |
Vitamin E — Synthetic (dl-alpha-tocopherol)#
| IU | mg | Common Use |
|---|---|---|
| 100 IU | 45 mg | Low-dose daily supplement |
| 200 IU | 90 mg | Moderate daily supplement |
| 400 IU | 180 mg | Common supplement strength |
| 800 IU | 360 mg | High-dose supplement |
| 1,000 IU | 450 mg | Therapeutic dose; near UL |
Insulin (Human)#
| IU | mg | Context |
|---|---|---|
| 1 IU | 0.0347 mg | Single unit dose |
| 10 IU | 0.347 mg | Typical low meal dose |
| 50 IU | 1.735 mg | Moderate daily total |
| 100 IU | 3.47 mg | Standard vial concentration per mL (U-100) |
Important: Insulin dosing should always be discussed with your healthcare provider. Never attempt to measure insulin by weight in a clinical setting — always use calibrated insulin syringes or pens that measure in IU.
Upper Safety Limits for Vitamin E#
The tolerable upper intake level (UL) for Vitamin E in adults is 1,000 mg/day of any form of alpha-tocopherol, as established by the NIH Office of Dietary Supplements. In IU, this equals:
- Natural Vitamin E: approximately 1,493 IU/day
- Synthetic Vitamin E: approximately 2,222 IU/day
Exceeding the UL may increase the risk of bleeding by interfering with blood clotting. High-dose Vitamin E supplementation (above 400 IU/day) has been associated with a slightly increased risk of all-cause mortality in some meta-analyses, though the evidence remains debated.
The Relationship Between mg and mcg#
Understanding the metric hierarchy is helpful when navigating supplement labels:
| Unit | Abbreviation | Equivalent |
|---|---|---|
| Gram | g | 1 g |
| Milligram | mg | 0.001 g (1/1,000 of a gram) |
| Microgram | mcg (or μg) | 0.000001 g (1/1,000,000 of a gram) |
The key relationship: 1 mg = 1,000 mcg
This means:
- Vitamin E dosed at 268 mg = 268,000 mcg
- Vitamin D3 dosed at 125 mcg = 0.125 mg
Vitamins that work at very small amounts (D, A, B12, K) are typically measured in mcg. Vitamins that work at larger amounts (E, C, B-complex) are typically measured in mg. This is purely a matter of convenience — using mcg for Vitamin C would produce unwieldy numbers like "90,000 mcg" instead of "90 mg."
For a detailed comparison of IU and mcg specifically, see our companion guide on IU vs mcg.
Practical Calculation Examples#
Let's walk through real-world scenarios to solidify your understanding.
Example 1: Comparing Two Vitamin E Supplements#
You are choosing between two Vitamin E products:
- Product A: 400 IU (natural, d-alpha-tocopherol)
- Product B: 400 IU (synthetic, dl-alpha-tocopherol)
Both deliver the same biological activity (400 IU). But the actual weight differs:
- Product A: 400 x 0.67 = 268 mg of natural Vitamin E
- Product B: 400 x 0.45 = 180 mg of synthetic Vitamin E
Which is better? Natural Vitamin E is generally preferred because your body retains it more efficiently. Despite containing more mg by weight, Product A is not "stronger" in IU terms — both are 400 IU. The difference is in how your body processes them.
Example 2: Converting a Doctor's Prescription#
Your doctor says: "Take 200 mg of natural Vitamin E daily." Your supplement bottle says "400 IU." How many capsules should you take?
Step 1: Convert 200 mg to IU
200 mg / 0.67 mg per IU = 298 IU
Step 2: Compare to your supplement
Your bottle contains 400 IU per capsule. 298 IU is less than one capsule.
Step 3: Discuss with your doctor
You would need approximately 0.75 capsules, which is impractical. Ask your doctor if 400 IU (268 mg) is acceptable, or find a lower-dose product.
Example 3: Reading an Updated Label#
Your new Vitamin E supplement reads: "Vitamin E — 180 mg (400 IU) as dl-alpha-tocopherol."
Verify: 400 IU x 0.45 mg/IU = 180 mg. The label is correct, and this is the synthetic form.
If you wanted the natural form at the same IU level, the label would read: "Vitamin E — 268 mg (400 IU) as d-alpha-tocopherol."
Example 4: How Many IU in 1 mg?#
This is a common reverse question. The answer depends on the substance:
| Substance | IU in 1 mg |
|---|---|
| Vitamin E (natural) | 1.49 IU |
| Vitamin E (synthetic) | 2.22 IU |
| Insulin (human) | 28.82 IU |
So 1 mg of natural Vitamin E provides 1.49 IU, while 1 mg of synthetic provides 2.22 IU — again reflecting the higher potency of the natural form per IU.
Try the Calculator#
Skip the manual calculations and use our interactive converter. Select Vitamin E (natural) and enter your IU value for an instant mg conversion:
For other converters, visit our dedicated pages:
- Vitamin E (natural) IU to mg converter
- Vitamin E (synthetic) IU to mg converter
- Insulin units to mg converter
Frequently Asked Questions#
Is 400 IU of Vitamin E the same as 400 mg?#
No. 400 IU of natural Vitamin E equals 268 mg, and 400 IU of synthetic Vitamin E equals 180 mg. IU and mg are different units, and the conversion depends on whether you have the natural or synthetic form. Confusing IU with mg could lead to taking significantly more or less than intended.
How do I know if my Vitamin E is natural or synthetic?#
Check the ingredient list on the label:
- Natural: Listed as "d-alpha-tocopherol" or "d-alpha-tocopheryl acetate" (note the "d-" prefix)
- Synthetic: Listed as "dl-alpha-tocopherol" or "dl-alpha-tocopheryl acetate" (note the "dl-" prefix)
The single letter difference — "d" vs "dl" — has a significant impact on conversion factors and bioavailability.
Can I convert IU to mg for Vitamin D?#
Technically yes, but it would produce extremely small numbers. Vitamin D3 is measured in mcg (micrograms), not mg. Since 1 mg = 1,000 mcg, a 2,000 IU dose of Vitamin D3 equals 50 mcg, which is just 0.05 mg. Using mg for Vitamin D would be impractical and confusing, which is why mcg is the standard metric unit for this vitamin.
Why is Vitamin E measured in mg but Vitamin D in mcg?#
It comes down to the amount needed for biological activity. Vitamin E is active at milligram levels (the RDA is 15 mg/day), while Vitamin D is active at microgram levels (the RDA is 15-20 mcg/day). Using the more appropriate unit keeps the numbers manageable and reduces the risk of decimal-point errors.
Is natural Vitamin E better than synthetic?#
Most evidence suggests natural Vitamin E is superior in terms of bioavailability. Your body absorbs and retains d-alpha-tocopherol roughly 2:1 over dl-alpha-tocopherol. A study published in the American Journal of Clinical Nutrition confirmed that natural Vitamin E produces approximately twice the blood levels compared to the same dose (in IU) of synthetic Vitamin E (Burton et al., 1998). However, synthetic Vitamin E is cheaper and still provides vitamin E activity.
How many IU are in 1 mg of Vitamin E?#
- Natural (d-alpha-tocopherol): 1 mg = 1.49 IU
- Synthetic (dl-alpha-tocopherol): 1 mg = 2.22 IU
To convert mg to IU, divide by the mg-per-IU factor (or multiply by the IU-per-mg factor above).
What about IU to mg for other vitamins like Vitamin C or B vitamins?#
Vitamin C, B vitamins, and most minerals are not measured in IU — they are measured directly in mg or mcg because they do not have the same issue of multiple forms with different bioactivities. Only Vitamins A, D, E, and insulin commonly use International Units. If your supplement label shows mg for Vitamin C, there is no IU conversion needed.
For a comprehensive overview of how IU relates to all measurement units used in supplements, see our guide on What Is an IU (International Unit)?.
Sources:
- NIH Office of Dietary Supplements — Vitamin E Fact Sheet
- FDA — Dietary Supplement Labeling Guide
- WHO International Standards for Biological Substances
- Burton GW, et al. "Human plasma and tissue alpha-tocopherol concentrations in response to supplementation with deuterated natural and synthetic vitamin E." Am J Clin Nutr. 1998;67(4):669-684.
- FDA — Changes to the Nutrition Facts Label (2016-2020)