Vitamin D mcg to IU Converter

Vitamin D can be written as a weight, in micrograms (mcg), or as an amount of activity, in international units (IU). For vitamin D, 1 mcg always equals 40 IU, so the two numbers are the same amount written two ways¹.

What Vitamin D mcg to IU Conversion Measures

This tool changes the unit, not the amount. 25 mcg and 1,000 IU of vitamin D are the same dose on a label². It does not tell you how much vitamin D you need, and it does not convert a blood test result into intake.

The Formula

FDA defines 1 IU of vitamin D as the activity of 0.025 mcg of cholecalciferol (vitamin D3), and uses this one factor for D2, D3 or a mix of both².

IU = mcg x 40

mcg = IU x 0.025 (the same as IU / 40)

How This Calculator Computes Your Result

Type an amount in mcg to get IU, or press swap to go from IU to mcg. The factor is exact, so nothing is rounded: 12.345 mcg shows as 493.8 IU. The result also shows the amount in mg and as a share of the 20 mcg Daily Value on US labels. Amounts above 100 mcg (4,000 IU) add a short note about the NIH upper limit. A separate small panel converts blood test units.

What Your Result Means

Your result is the same amount of vitamin D in the other unit. A quick check: the IU number is always 40 times the mcg number. This table covers common label amounts.

FDA's own label examples are 2.5 mcg (100 IU) in a 240 mL serving of milk and 25 mcg (1,000 IU) in a supplement². If you get a number in the hundreds of thousands, check whether the label said mg rather than mcg.

Why US Labels Switched From IU to mcg

FDA updated the Nutrition Facts and Supplement Facts labels in a final rule published in May 2016⁵. Larger companies had to use the new labels by January 1, 2020, and companies with under $10 million in yearly food sales by January 1, 2021⁴. Vitamin D must now be listed in mcg, for both foods and supplements². Companies may add IU, but only in parentheses after the mcg amount, as in "Vitamin D 25 mcg (1,000 IU)"²,³.

The Daily Value also changed. It was 400 IU (10 mcg) under the old rule⁶ and is now 20 mcg (800 IU) for adults and children 4 years and older¹. Older bottles, studies and doctors' notes often still use IU.

NIH Reference Values in mcg and IU

The NIH Office of Dietary Supplements lists these daily amounts. The RDA meets the needs of nearly all (97% to 98%) healthy people, and infants get an adequate intake instead. The upper limit (UL) is the most per day that is unlikely to cause harm¹.

These are population reference values, not a personal dose. NIH notes that vitamin D toxicity almost always comes from supplements¹, so check with a doctor before taking amounts above the upper limit. NIH rounds two figures: 1,500 IU is exactly 37.5 mcg and 2,500 IU is exactly 62.5 mcg, printed as 38 and 63¹.

Vitamin D2 and D3 Use the Same Factor

The 40 IU per mcg factor applies to both forms on labels², so the form does not change your result. That does not mean they act the same in the body. NIH reports that D3 raises blood levels of vitamin D more, and keeps them higher for longer, than D2¹.

Blood Test Units Are Different

A vitamin D blood test measures 25-hydroxyvitamin D, written 25(OH)D. Labs report it in either ng/mL or nmol/L, and 1 ng/mL equals 2.5 nmol/L¹. These are concentration units, so a blood result cannot be turned into mcg or IU of intake, and a blood number typed into the mcg field gives a meaningless answer. For reference, NIH describes 20 ng/mL (50 nmol/L) or higher as generally adequate for bone and overall health in healthy people¹.

Common Mistakes

• Reading IU as mcg. 1,000 IU is 25 mcg, but 1,000 mcg is 40,000 IU.

• Mixing up mcg and mg. 25 mcg is 0.025 mg, a 1,000 times difference.

• Comparing % Daily Value across old and new labels. The Daily Value moved from 400 IU to 800 IU, so the same 1,000 IU pill shows 250% on an old label and 125% on a new one. Compare amounts, not percentages.