IU To mg Converter Calculator

IU to mg Converter

For Vitamin D, A, E and C — with live results and reference charts

What Is an IU, Anyway?

If you have ever looked at a vitamin supplement bottle and wondered why the dose says ‘5,000 IU’ instead of something more familiar like milligrams, you are not alone. IU stands for International Unit — and the short answer is that it was invented because not all forms of a substance are equally potent.

Here is the key thing to understand: an IU is a measure of biological effect, not weight. Two different forms of Vitamin A, for example, can produce the same effect in your body but weigh completely different amounts. The IU system lets manufacturers (and doctors) talk about ‘how much this does’ rather than ‘how much this weighs’.

The World Health Organization (WHO) and the US Pharmacopeia (USP) set the official definitions for each substance. This is why converting IU to mg is not a single formula — it depends entirely on which substance you are dealing with.

 

Important: There is no universal IU-to-mg formula. You must know the substance first. 1,000 IU of Vitamin D is a completely different mass from 1,000 IU of Vitamin E.

 

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The Vitamins — What They Are and Why IU Matters

Vitamin D

Vitamin D is probably the most common reason people look up IU conversions. It is produced naturally in your skin when you are exposed to sunlight, and it plays a central role in calcium absorption, bone health, and immune function. Most adults in office jobs or northern climates are deficient.

Vitamin D supplements come in two forms: D2 (ergocalciferol, from plant sources) and D3 (cholecalciferol, from animal sources or lichen). Both are measured in IU and share the same conversion factor.

Conversion: 1 IU of Vitamin D = 0.000025 mg (25 nanograms). That means 1,000 IU = 0.025 mg.

Common doses range from 400 IU (basic daily reference value) to 50,000 IU (therapeutic doses prescribed for deficiency). Despite the large IU numbers, the actual weight of Vitamin D in a tablet is tiny.

 

Vitamin A

Vitamin A supports vision (especially night vision), immune function, and skin health. It comes in two main dietary forms: retinol (from animal products like liver and dairy) and beta-carotene (from colourful vegetables like carrots and sweet potato). Your body converts beta-carotene into retinol, but less efficiently.

Because of this efficiency difference, the two forms have different IU conversion factors:

• Retinol: 1 IU = 0.0003 mg
• Beta-carotene: 1 IU = 0.0006 mg

Vitamin A is one of the few vitamins where overdosing is genuinely possible — taking very high doses of retinol long-term can be toxic. This is why supplement labels have shifted toward using RAE (Retinol Activity Equivalents) in newer products, though IU is still widely seen.

 

Vitamin E

Vitamin E is a fat-soluble antioxidant that protects cell membranes from oxidative damage. It is found naturally in nuts, seeds, and vegetable oils. Like Vitamin A, it comes in forms with different potencies, so there are two IU conversion factors:

• Natural Vitamin E (d-alpha-tocopherol): 1 IU = 0.67 mg. This form is more bioavailable.
• Synthetic Vitamin E (dl-alpha-tocopherol): 1 IU = 0.45 mg. Cheaper to produce, less active per mg.

The ‘d-‘ prefix means natural; ‘dl-‘ means synthetic. Worth checking on the label if you care about bioavailability.

Note: this article uses the modern FDA/NIH biological-equivalence factors above, which match current US supplement labels. Some older references and pre-2016 labels instead use a mass-equivalence convention for synthetic Vitamin E (~0.90 mg/IU) — if a number you see elsewhere looks about double ours, that is almost certainly why.

 

Vitamin C

Vitamin C (ascorbic acid) is water-soluble, meaning your body does not store it — you need a regular supply from food or supplements. It is an antioxidant, supports collagen production, helps with iron absorption, and is well-known for immune support.

Vitamin C is often still expressed in IU on older or international labels, though milligrams are now more common in most countries. The conversion is straightforward: 1 IU = 0.05 mg.

So 1,000 IU of Vitamin C = 50 mg. Very different from Vitamin D, which illustrates perfectly why the substance always matters.

 

How to Convert IU to mg — Step by Step

The formula is simple once you have the right conversion factor for your substance:

 

mg = IU × (mg per IU for that substance)   |   IU = mg ÷ (mg per IU for that substance)

 

Worked Example 1: 1,000 IU of Vitamin D to mg

Vitamin D conversion factor: 1 IU = 0.000025 mg

• 1,000 × 0.000025 = 0.025 mg
• So 1,000 IU of Vitamin D = 0.025 mg = 25 mcg

 

Worked Example 2: 400 IU of Vitamin E (natural) to mg

Natural Vitamin E conversion factor: 1 IU = 0.67 mg

• 400 × 0.67 = 268 mg
• So 400 IU of Vitamin E (natural) = 268 mg

 

Worked Example 3: Converting mg back to IU

Say you have 0.05 mg of Vitamin D and want to know the IU value:

• 05 ÷ 0.000025 = 2,000 IU
• So 0.05 mg of Vitamin D = 2,000 IU

 

Converting to mcg and g

Once you have the mg value, the rest is straightforward metric:

• mg × 1,000 = mcg (micrograms)
• mg ÷ 1,000 = g (grams)

For example, 5,000 IU of Vitamin D = 0.125 mg = 125 mcg = 0.000125 g.

 

Quick Reference Conversion Charts

The tables below cover the most requested IU values for each vitamin. All values are calculated from WHO/USP standard conversion factors.

 

Vitamin D (1 IU = 0.000025 mg)

IU	mg	mcg	g
10 IU	0.00025 mg	0.25 mcg	2.5e-7 g
30 IU	0.00075 mg	0.75 mcg	7.5e-7 g
60 IU	0.0015 mg	1.5 mcg	0.0000015 g
250 IU	0.00625 mg	6.25 mcg	0.00000625 g
400 IU	0.01 mg	10 mcg	0.00001 g
1,000 IU	0.025 mg	25 mcg	0.000025 g
2,000 IU	0.05 mg	50 mcg	0.00005 g
5,000 IU	0.125 mg	125 mcg	0.000125 g
10,000 IU	0.25 mg	250 mcg	0.00025 g
50,000 IU	1.25 mg	1,250 mcg	0.00125 g
60,000 IU	1.5 mg	1,500 mcg	0.0015 g

 

Vitamin A — Retinol (1 IU = 0.0003 mg)

IU	mg	mcg	g
10 IU	0.003 mg	3 mcg	0.000003 g
30 IU	0.009 mg	9 mcg	0.000009 g
60 IU	0.018 mg	18 mcg	0.000018 g
250 IU	0.075 mg	75 mcg	0.000075 g
400 IU	0.12 mg	120 mcg	0.00012 g
1,000 IU	0.3 mg	300 mcg	0.0003 g
2,000 IU	0.6 mg	600 mcg	0.0006 g
5,000 IU	1.5 mg	1,500 mcg	0.0015 g
10,000 IU	3 mg	3,000 mcg	0.003 g
50,000 IU	15 mg	15,000 mcg	0.015 g
60,000 IU	18 mg	18,000 mcg	0.018 g

 

Vitamin E — Natural d-alpha-tocopherol (1 IU = 0.67 mg)

IU	mg	mcg	g
10 IU	6.7 mg	6,700 mcg	0.0067 g
30 IU	20.1 mg	20,100 mcg	0.0201 g
60 IU	40.2 mg	40,200 mcg	0.0402 g
250 IU	167.5 mg	167,500 mcg	0.1675 g
400 IU	268 mg	268,000 mcg	0.268 g
1,000 IU	670 mg	670,000 mcg	0.67 g
2,000 IU	1,340 mg	1,340,000 mcg	1.34 g
5,000 IU	3,350 mg	3,350,000 mcg	3.35 g
10,000 IU	6,700 mg	6,700,000 mcg	6.7 g
50,000 IU	33,500 mg	33,500,000 mcg	33.5 g
60,000 IU	40,200 mg	40,200,000 mcg	40.2 g

 

Vitamin E — Synthetic dl-alpha-tocopherol (1 IU = 0.45 mg)

IU	mg	mcg	g
10 IU	4.5 mg	4,500 mcg	0.0045 g
30 IU	13.5 mg	13,500 mcg	0.0135 g
60 IU	27 mg	27,000 mcg	0.027 g
250 IU	112.5 mg	112,500 mcg	0.1125 g
400 IU	180 mg	180,000 mcg	0.18 g
1,000 IU	450 mg	450,000 mcg	0.45 g
2,000 IU	900 mg	900,000 mcg	0.9 g
5,000 IU	2,250 mg	2,250,000 mcg	2.25 g
10,000 IU	4,500 mg	4,500,000 mcg	4.5 g
50,000 IU	22,500 mg	22,500,000 mcg	22.5 g
60,000 IU	27,000 mg	27,000,000 mcg	27 g

 

Putting the Numbers in Context

Now that you have the conversions, here is what typical doses actually look like for the most common vitamins:

 

Vitamin D

• 400 IU (0.01 mg) — Basic daily reference value; found in many multivitamins
• 1,000–2,000 IU (0.025–0.05 mg) — Common maintenance dose for adults
• 5,000 IU (0.125 mg) — Popular higher-dose supplement for people with low sun exposure
• 10,000 IU (0.25 mg) — Sometimes used short-term; long-term use should be supervised
• 50,000–60,000 IU (1.25–1.5 mg) — Therapeutic doses for deficiency, prescribed by doctors

 

Vitamin A

• 2,500–5,000 IU (0.75–1.5 mg retinol) — Typical multivitamin range
• 10,000 IU (3 mg) — Upper limit many health bodies suggest for long-term daily intake
• Above 10,000 IU daily — Potential toxicity risk with long-term retinol supplementation

 

Vitamin E

• 15 mg (roughly 22 IU natural / 33 IU synthetic) — Adult daily reference intake
• 400 IU — Common supplement dose; widely considered safe
• 1,000 IU — Upper limit recommended by most guidelines

 

Frequently Asked Questions

 

Summary

The IU system exists because biological potency matters more than raw weight for certain substances — especially vitamins and hormones. There is no single IU-to-mg formula; every substance has its own WHO/USP-defined factor.

For the vitamins most people care about:

• Vitamin D: 1 IU = 0.000025 mg (1,000 IU = 0.025 mg = 25 mcg)
• Vitamin A (retinol): 1 IU = 0.0003 mg
• Vitamin E (natural): 1 IU = 0.67 mg
• Vitamin E (synthetic): 1 IU = 0.45 mg
• Vitamin C: 1 IU = 0.05 mg

 

Use the charts above as a quick reference, or use our calculator at the top of this page to convert any value instantly.

 

References

• U.S. Food and Drug Administration. Changes to the Nutrition Facts Label — Converting Units of Measure for Folate, Niacin, and Vitamins A, D, and E. FDA.gov, 2020.
• National Institutes of Health, Office of Dietary Supplements. Vitamin D, Vitamin A, and Vitamin E — Fact Sheets for Health Professionals. ods.od.nih.gov.
• World Health Organization & United States Pharmacopeia (USP). International Unit (IU) reference standards for fat-soluble vitamins.

 

All conversion factors in this article follow WHO/USP international standards and current FDA/NIH labeling guidance. This content is for informational purposes only and does not constitute medical advice. Always confirm dosing with a doctor or pharmacist.