How do I convert IU to mcg for Vitamin D?

For Vitamin D: 1 IU = 0.025 mcg (micrograms). To convert IU to mcg, multiply by 0.025. To convert mcg to IU, multiply by 40. For example, 600 IU of Vitamin D = 600 × 0.025 = 15 mcg, which matches the US RDA for adults under 70.

What is 5000 IU of Vitamin D in mcg?

5000 IU of Vitamin D equals 5000 × 0.025 = 125 mcg. This is a commonly available supplement dose. The US RDA for adults under 70 is 600 IU (15 mcg), and the tolerable upper intake level (UL) is 4000 IU (100 mcg) per day — so 5000 IU exceeds the UL and should only be taken under medical supervision.

What is the difference between mg and mcg?

mg = milligram = 1/1,000 of a gram. mcg (or μg) = microgram = 1/1,000,000 of a gram. Therefore 1 mg = 1,000 mcg. Vitamins like C and E are dosed in mg (tens of milligrams), while very potent vitamins like B12, D, and folate are dosed in mcg (micrograms) because the body requires only tiny amounts.

Vitamin Dosage Units: mg vs mcg, IU Conversions & RDA Reference Table

Understanding Vitamin Measurement Units

Supplements and lab reports use three types of units for vitamins:

mg (milligrams) — 1 mg = 0.001 g. Used for Vitamins C, E, B1, B2, B3, B6, calcium, iron, magnesium.
mcg or μg (micrograms) — 1 mcg = 0.001 mg = 0.000001 g. Used for Vitamins B12, D, K, folate, biotin.
IU (International Units) — a biological activity unit. The mass per IU differs for each vitamin because different forms have different potencies.

The key conversion: 1 mg = 1,000 mcg. This means 400 mcg of folate = 0.4 mg of folate.

IU Conversion Factors by Vitamin

IU is a measure of biological effect, not mass. Because different chemical forms of a vitamin have different potencies, there is no single universal IU-to-mg conversion. The table below gives the conversion factors established by the WHO and adopted by the US Pharmacopeia.

Vitamin	Form	1 IU Equals	Common Dose (IU)	= mcg or mg
Vitamin A	Retinol	0.3 mcg retinol	5,000 IU	1,500 mcg (1.5 mg)
Vitamin A	Beta-carotene	0.6 mcg beta-carotene	5,000 IU	3,000 mcg (3 mg)
Vitamin D	D2 or D3	0.025 mcg (= 40 IU/mcg)	1,000 IU	25 mcg
Vitamin D	D2 or D3	0.025 mcg	5,000 IU	125 mcg
Vitamin E	Natural (d-alpha-tocopherol)	0.67 mg	400 IU	268 mg
Vitamin E	Synthetic (dl-alpha-tocopherol)	0.45 mg	400 IU	180 mg
Vitamin C	Ascorbic acid	N/A — always in mg	—	—

Adult RDA Reference Table

The Recommended Dietary Allowance (RDA) is the daily intake sufficient to meet the needs of 97–98% of healthy adults. Values below are for adults aged 19–50 from the US National Institutes of Health (NIH) Office of Dietary Supplements, with male/female differences noted where applicable.

Nutrient	RDA (Men)	RDA (Women)	Unit	Tolerable Upper Limit
Vitamin A	900 mcg RAE	700 mcg RAE	mcg	3,000 mcg/day
Vitamin C	90 mg	75 mg	mg	2,000 mg/day
Vitamin D	600 IU (15 mcg)	600 IU (15 mcg)	IU / mcg	4,000 IU/day (100 mcg)
Vitamin E	15 mg	15 mg	mg	1,000 mg/day
Vitamin K	120 mcg	90 mcg	mcg	Not established
Vitamin B12	2.4 mcg	2.4 mcg	mcg	Not established
Folate	400 mcg DFE	400 mcg DFE	mcg	1,000 mcg/day (folic acid)
Iron	8 mg	18 mg	mg	45 mg/day
Calcium	1,000 mg	1,000 mg	mg	2,500 mg/day

Why IU Was Replaced on US Labels

In 2020, the FDA updated Nutrition Facts label rules to phase out IU for Vitamins D and E, requiring labels to report these in mcg and mg respectively. IU remains common on supplement bottles but is being replaced by mass units for clarity. When you see both listed (e.g. "Vitamin D 25 mcg (1,000 IU)"), they refer to the same amount.

Quick Conversion Reference

Vitamin D: 1,000 IU = 25 mcg | 2,000 IU = 50 mcg | 4,000 IU = 100 mcg | 5,000 IU = 125 mcg
Vitamin A (retinol): 3,333 IU = 1,000 mcg RAE | 5,000 IU = 1,500 mcg RAE
Vitamin E (natural): 400 IU = 268 mg | 200 IU = 134 mg

Use our weight converter to convert between milligrams, micrograms, and grams for supplement dosage calculations.