400 IU of vitamin D to mg and mcg
400 IU of vitamin D = 10 mcg = 0.01 mg. Vitamin D has 40 IU per microgram, so divide IU by 40 to get mcg and by 40,000 to get mg.
How the conversion works
mcg = IU ÷ 40 → 400 ÷ 40 = 10 mcg mg = mcg ÷ 1,000 → 10 ÷ 1,000 = 0.01 mg
The factor comes from the NIH Office of Dietary Supplements: 1 mcg of vitamin D (cholecalciferol, D3, or ergocalciferol, D2) has 40 IU of activity. It works backwards too: 10 mcg × 40 = 400 IU.
Where you'll see 400 IU
400 IU is the Adequate Intake for babies up to 12 months (10 mcg), and the amount in many infant vitamin D drops. Older labels often printed 400 IU as the adult Daily Value. Since the FDA's label rules changed (compliance from 2020–2021), supplement labels show vitamin D in mcg, often with IU in brackets: look for 10 mcg (400 IU).
This page converts units; it doesn't recommend a dose. NIH sets the adult upper limit at 4,000 IU (100 mcg) a day. Talk to your clinician about what's right for you.
Convert another amount
Nearby doses
- 600 IU to mg = 15 mcg
- 800 IU to mg = 20 mcg
See every common strength in the vitamin D IU to mcg table, or convert vitamin E, A and insulin with the IU to mg converter.
Questions
Is 400 IU the same as 10 mcg?
Yes, for vitamin D (D3 or D2). 400 ÷ 40 = 10 mcg. US labels now list mcg first, for example "10 mcg (400 IU)".
How many mg is 400 IU of vitamin D?
0.01 mg. One milligram is 1,000 mcg, so 10 mcg ÷ 1,000 = 0.01 mg.
Does 400 IU convert the same way for other vitamins?
No. An IU measures biological activity, so every substance has its own factor. 400 IU of vitamin E is about 268 mg (natural) or 180 mg (synthetic), not 0.01 mg. Use the IU to mg converter and pick the substance.