IU to mg Converter
International Units (IU) measure the biological activity of vitamins and medications, whilst milligrams (mg) measure actual mass. The conversion between these varies by substance because each has different potency levels. This converter helps you translate IU values to mg for vitamins A, D, E, and insulin with precision.
Quick Conversions
Conversion Result
Recent Conversions
IU to mg Conversion Table
Here’s a quick reference showing common vitamin conversions from IU to milligrams. These values are based on standardised conversion factors established by the World Health Organisation.
| Substance | IU Value | mg Equivalent | mcg Equivalent |
|---|---|---|---|
| Vitamin A (Retinol) | 1000 IU | 0.3 mg | 300 mcg |
| Vitamin A (Retinol) | 5000 IU | 1.5 mg | 1500 mcg |
| Vitamin A (Retinol) | 10000 IU | 3.0 mg | 3000 mcg |
| Vitamin D (D2/D3) | 400 IU | 0.01 mg | 10 mcg |
| Vitamin D (D2/D3) | 1000 IU | 0.025 mg | 25 mcg |
| Vitamin D (D2/D3) | 2000 IU | 0.05 mg | 50 mcg |
| Vitamin D (D2/D3) | 4000 IU | 0.1 mg | 100 mcg |
| Vitamin E (d-alpha) | 100 IU | 67 mg | 67000 mcg |
| Vitamin E (d-alpha) | 400 IU | 268 mg | 268000 mcg |
| Vitamin E (d-alpha) | 800 IU | 536 mg | 536000 mcg |
| Insulin (Human) | 10 IU | 0.347 mg | 347 mcg |
| Insulin (Human) | 100 IU | 3.47 mg | 3470 mcg |
Conversion Formulas & Steps
Converting IU to mg requires knowing the specific conversion factor for each substance. Follow these steps for accurate conversions.
Vitamin A (Retinol)
Example: 5000 IU × 0.0003 = 1.5 mg
For beta-carotene, the factor differs: 1 IU = 0.6 mcg (0.0006 mg)
Vitamin D (Cholecalciferol/Ergocalciferol)
Example: 2000 IU × 0.000025 = 0.05 mg (50 mcg)
Both vitamin D2 and D3 use the same conversion factor.
Vitamin E (d-alpha-tocopherol)
Example: 400 IU × 0.67 = 268 mg
The synthetic form (dl-alpha-tocopherol) uses 0.9 mg per IU.
Insulin (Human)
Example: 10 IU × 0.0347 = 0.347 mg
This applies to human insulin; animal-sourced insulin may vary slightly.
Why IU and mg Differ
International Units measure biological activity rather than weight. Two vitamin A preparations might weigh the same but have different activities. IU standardises this by measuring the effect each substance has on the body.
The World Health Organisation sets these standards. They create reference preparations and assign IU values based on biological testing. This means 1000 IU of vitamin D from any source should produce the same effect in your body, regardless of the specific chemical form.
Mass measurements like milligrams tell you the actual weight of a substance. They’re straightforward but don’t account for potency differences. That’s why supplement labels increasingly show both IU and metric units—you get the biological activity measure plus the actual weight.
Everyday Scenarios
Checking Supplement Labels
You’ll spot both IU and mg on vitamin bottles. A vitamin D supplement might say “1000 IU (25 mcg)”. Knowing the conversion helps you compare products and avoid taking too much or too little.
Following Medical Advice
Your GP might recommend “2000 IU of vitamin D daily”. If you’ve got a supplement labelled in micrograms, you’ll know that’s 50 mcg. This prevents confusion and helps you stick to the recommended amount.
Comparing International Products
UK labels increasingly use metric units, whilst American products often stick with IU. When ordering supplements from abroad, you’ll need to convert to check you’re getting equivalent doses.
Managing Prescriptions
Insulin prescriptions use IU because it’s clearer for dosing. Understanding the mg equivalent helps if you’re reading research papers or comparing different insulin products.
Units of Vitamin Measurement Conversions
Beyond IU and mg, vitamins appear in several other measurement units. Here’s how they relate to each other.
| Vitamin | Alternative Unit | Conversion to IU |
|---|---|---|
| Vitamin A | 1 mcg RAE (Retinol) | 3.33 IU |
| Vitamin A | 1 mcg Beta-Carotene | 1.67 IU |
| Vitamin D | 1 mcg (D2 or D3) | 40 IU |
| Vitamin E | 1 mg d-alpha-tocopherol | 1.49 IU |
| Vitamin E | 1 mg dl-alpha-tocopherol | 1.1 IU |
FAQs
Can I use the same conversion for all vitamins?
No, each vitamin has its own conversion factor. Vitamin D converts at 0.025 mcg per IU, whilst vitamin E converts at 0.67 mg per IU. Always check which vitamin you’re converting before doing the maths.
Why do some supplements show both IU and mg?
Manufacturers include both to help consumers understand dosage. IU shows biological activity whilst mg shows actual weight. UK regulations encourage metric units, so you’ll see both during the transition period.
Does the vitamin source affect the conversion?
Yes, particularly for vitamins A and E. Natural vitamin E (d-alpha-tocopherol) converts differently than synthetic (dl-alpha-tocopherol). Always note which form you’re working with.
Are IU and mg interchangeable in prescriptions?
They measure different things, so you can’t simply swap one for the other. Prescriptions specify which unit to follow. If you’re uncertain, ask your pharmacist to clarify the dosage.
How accurate are these conversions?
The conversion factors come from WHO standards and are highly accurate for standard supplement forms. Specialised preparations might have slightly different factors—check the manufacturer’s information.
Why isn’t vitamin C measured in IU?
Vitamin C is chemically stable with consistent potency, so weight measurements work fine. IU is reserved for substances where biological activity varies significantly between preparations.
Can I overdose by confusing IU and mg?
Yes, it’s possible with fat-soluble vitamins like A and D. Taking 10,000 mg instead of 10,000 IU would be extremely dangerous. Always double-check your units before taking supplements.
Do children need different conversions?
The conversion factors remain the same, but recommended doses differ. Children need smaller amounts of most vitamins. Always follow age-specific guidance from your healthcare provider.