Vitamin unit converter

IU to mcg: vitamin D

An international unit (IU) measures what a vitamin does in the body rather than its weight, so each vitamin has its own conversion. The NHS gives vitamin D's: 1 microgram is 40 IU, so 10 micrograms is 400 IU.

The NHS advises everyone to consider 10 mcg (400 IU) a day in autumn and winter, and says adults should not take more than 100 mcg (4,000 IU) a day. For more on who needs it, read vitamin D in the UK winter.

IU to mcg: vitamin A

For vitamin A as retinol, the Institute of Medicine's conversion is 1 mcg of retinol = 3.33 IU, so 1,000 IU is about 300 mcg. That factor only applies to retinol. Beta-carotene is counted differently, so an IU figure for beta-carotene can't be converted this way.

The NHS says adults aged 19 to 64 need 700 mcg a day (men) or 600 mcg (women), and that most people get enough from their diet. If you take a supplement with vitamin A, the NHS says your total from food and supplements should not go over 1.5 mg (1,500 mcg) a day, because research suggests more than that over many years may affect your bones. If you are pregnant, the NHS advises avoiding liver and liver products, which are high in vitamin A. Read more in our vitamin A guide.

IU to mg: vitamin E

Vitamin E comes in a natural form (d-alpha-tocopherol, sometimes written RRR-alpha-tocopherol) and a synthetic one (dl-alpha-tocopherol, or all-rac). They convert differently. The Institute of Medicine's factors are:

• natural: 1 IU = 0.67 mg
• synthetic: 1 IU = 0.45 mg

So 400 IU is about 268 mg of the natural form, or 180 mg of the synthetic form. The NHS says adults need 4 mg a day (men) or 3 mg (women) and should be able to get it from their diet, and that 540 mg (800 IU) or less a day from supplements is unlikely to cause harm. Read more in our vitamin E guide.

mcg to mg

A milligram (mg) is a thousandth of a gram. A microgram (mcg, also written µg) is a thousandth of a milligram. So:

• 1,000 mcg = 1 mg
• 1,000 mg = 1 g
• to turn mcg into mg, divide by 1,000; to turn mg into mcg, multiply by 1,000.

Labels use micrograms for nutrients you need in tiny amounts, such as vitamin D, vitamin B12, folic acid, selenium and iodine, and milligrams for the rest. It is easy to misread one for the other, and a thousandfold mistake matters, so check the unit before you compare two products.

Why labels use different units

UK and EU labels give vitamins by weight, in mg or mcg, along with the percentage of the nutrient reference value (NRV). Products sold elsewhere often use IU, especially for vitamins D, A and E. If you're adding up the same vitamin from two products, convert them to the same unit first. Our guide to supplements to take together covers which combinations overlap.