IU to mcg Converter

Converting International Units (IU) to micrograms (mcg or μg) is essential when managing vitamin supplements. The conversion rate varies depending on which vitamin you’re measuring. Vitamin D, vitamin A, and vitamin E each have their own specific conversion factors.

This converter helps you switch between IU and mcg accurately. Whether you’re checking supplement labels or following NHS recommendations, you’ll get precise results for each vitamin type.

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Vitamin Conversion Table

Different vitamins require different conversion factors. Here’s how IU converts to mcg for the most common vitamins found in supplements.

Vitamin IU Value mcg (μg) Equivalent Conversion Factor
Vitamin D 400 IU 10 mcg 1 IU = 0.025 mcg
Vitamin D 600 IU 15 mcg 1 IU = 0.025 mcg
Vitamin D 1000 IU 25 mcg 1 IU = 0.025 mcg
Vitamin D 2000 IU 50 mcg 1 IU = 0.025 mcg
Vitamin A (Retinol) 2500 IU 750 mcg 1 IU = 0.3 mcg
Vitamin A (Retinol) 5000 IU 1500 mcg 1 IU = 0.3 mcg
Vitamin E (Natural) 100 IU 67 mg 1 IU = 0.67 mg
Vitamin E (Natural) 400 IU 268 mg 1 IU = 0.67 mg
NHS Recommendation: Adults and children over 4 years need 10 micrograms (400 IU) of vitamin D daily. This helps maintain healthy bones, teeth, and muscles throughout the year.

Conversion Formulas

You can manually convert between IU and mcg using these established formulas. Each vitamin has its own conversion rate based on its biological activity.

Vitamin D Conversions

IU to mcg: mcg = IU ÷ 40

mcg to IU: IU = mcg × 40

Example: 800 IU ÷ 40 = 20 mcg

Vitamin A (Retinol) Conversions

IU to mcg: mcg = IU × 0.3

mcg to IU: IU = mcg ÷ 0.3

Example: 3000 IU × 0.3 = 900 mcg

Vitamin E (Natural α-tocopherol) Conversions

IU to mg: mg = IU × 0.67

mg to IU: IU = mg ÷ 0.67

Example: 200 IU × 0.67 = 134 mg

Why Conversions Vary

International Units measure biological activity rather than weight. A vitamin’s potency determines how many IU equal one microgram. This system exists because different forms of the same vitamin can have varying effects in your body.

Vitamin D from cholecalciferol (D₃) and ergocalciferol (D₂) both use the same conversion factor. However, vitamin A from retinol converts differently than vitamin A from beta-carotene. Natural vitamin E has more biological activity than synthetic forms, which is why their IU values differ.

Supplement labels in the UK often show both measurements. This helps you compare products and follow dosage guidelines from healthcare providers who might use either unit.

Common Values in Practice

Most vitamin D supplements sold in UK pharmacies and supermarkets contain 400 IU (10 mcg). This matches the NHS daily recommendation for adults. Higher-strength tablets of 1000 IU (25 mcg) or 2000 IU (50 mcg) are available for those with specific needs.

Multivitamins typically include 400-800 IU of vitamin D, 2500-5000 IU of vitamin A, and 10-30 IU of vitamin E. Reading the label in both units helps you track your total daily intake from all sources.

FAQs

What does IU mean for vitamins?
IU stands for International Unit. It measures how biologically active a vitamin is in your body rather than its weight. Scientists use IU because different forms of vitamins can have different potencies even when they weigh the same.
Is 1000 IU of vitamin D the same as 25 mcg?
Yes, exactly. For vitamin D, 1000 IU equals 25 micrograms. You can verify this by dividing 1000 by 40, which gives you 25. Both measurements represent the same amount of vitamin D.
Why do supplement labels show both IU and mcg?
Manufacturers include both units because healthcare advice uses different measurements. NHS guidelines typically reference micrograms, whilst some international research and older recommendations use IU. Showing both helps everyone understand the dosage.
Can I use the same conversion for all vitamins?
No, each vitamin has its own conversion rate. Vitamin D uses 1 IU = 0.025 mcg, vitamin A (retinol) uses 1 IU = 0.3 mcg, and vitamin E uses 1 IU = 0.67 mg for the natural form. Always check which vitamin you’re converting.
How much vitamin D should I take daily in the UK?
The NHS recommends 10 micrograms (400 IU) daily for adults and children over 4 years. This recommendation applies year-round because UK sunlight doesn’t provide enough vitamin D, especially from October to March.
What happens if I take too much vitamin D?
Taking more than 100 micrograms (4000 IU) daily over extended periods can cause calcium to build up in your body. This condition, called hypercalcaemia, may weaken bones and damage your kidneys and heart. Stick to recommended doses unless your GP advises otherwise.
Are natural and synthetic vitamin E measured the same way?
No, they differ. Natural vitamin E (d-alpha-tocopherol) converts at 1 IU = 0.67 mg, whilst synthetic vitamin E (dl-alpha-tocopherol) converts at 1 IU = 0.45 mg. Natural forms have higher biological activity, so they need less weight to achieve the same IU value.
Do I need vitamin D supplements if I eat fortified foods?
Fortified foods like breakfast cereals, spreads, and some dairy products help boost your intake. However, most people in the UK still benefit from a daily supplement, especially during autumn and winter when sunlight exposure is limited.

Related Unit Conversions

When working with vitamin measurements, you might encounter several related units. Here’s how they connect to help you interpret supplement labels and nutrition information.

From To Conversion Typical Use
Milligrams (mg) Micrograms (mcg) 1 mg = 1000 mcg Vitamin C, B vitamins
Grams (g) Milligrams (mg) 1 g = 1000 mg Protein, minerals
Retinol Activity Equivalents (RAE) Micrograms (mcg) 1 RAE = 1 mcg retinol Vitamin A from mixed sources
Nanograms (ng) Micrograms (mcg) 1 mcg = 1000 ng Vitamin B₁₂, blood tests
Milligrams (mg) IU (Vitamin E) 1 mg = 1.49 IU (natural) Vitamin E supplements

References

Office of Dietary Supplements, National Institutes of Health. Dietary Supplement Ingredient Database: Unit Conversions. Available at: https://dsid.od.nih.gov/Conversions.php (Accessed December 2025).
National Health Service. Vitamin D. NHS Digital, 2025. Available at: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/ (Accessed December 2025).
Public Health England. Vitamin D and Health. Scientific Advisory Committee on Nutrition Report, 2016. GOV.UK Publications.
Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes for Calcium and Vitamin D. National Academies Press, Washington DC, 2011.
Food and Agriculture Organization of the United Nations. Food and Nutrition Paper 77: Vitamin A. FAO, Rome, 2001. Available at: https://www.fao.org/4/x6877e/X6877E22.htm
U.S. Food and Drug Administration. Guidance for Industry: A Food Labeling Guide – Converting Units of Measure for Folate, Niacin, and Vitamins A, D, and E on the Nutrition and Supplement Facts Labels. FDA, 2019.

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