Vitamin D IU to Mcg Conversion Chart (400 to 10,000 IU)
Informational summary of NIH guidance — not medical advice. A 25(OH)D blood test and your clinician have the final say on your dose.
Short answer: for vitamin D3, 40 IU equals 1 mcg. To go from IU to mcg, divide by 40. To go from mcg to IU, multiply by 40. So 1,000 IU is 25 mcg, 2,000 IU is 50 mcg, and 5,000 IU is 125 mcg. Supplement bottles most often list IU; the Nutrition Facts panel and the NIH's official RDA tables list mcg — the numbers below convert between the two.
Vitamin D3 Dose Conversion Table
Using the 40 IU = 1 mcg conversion factor, here's how common vitamin D3 doses translate between units:
| IU | Mcg | Where it falls |
|---|---|---|
| 400 IU | 10 mcg | NIH RDA for infants 0-12 months |
| 800 IU | 20 mcg | NIH RDA for adults 71+ |
| 1,000 IU | 25 mcg | Conservative maintenance dose |
| 2,000 IU | 50 mcg | Typical maintenance / sweet-spot dose |
| 5,000 IU | 125 mcg | Standard correction dose for deficiency |
| 10,000 IU | 250 mcg | Endocrine Society upper limit for adults |
The conversion is linear, so you can scale it to any dose: divide the IU amount by 40 to get mcg, or multiply the mcg amount by 40 to get IU. A quick way to sanity-check any label: if a bottle lists both units, the mcg number should be almost exactly the IU number divided by 40.
RDA and Upper Limit, in Both Units
The dosage guide lists the NIH's age-based recommendations in IU; here they are in both units side by side:
| Age group | NIH RDA (IU) | NIH RDA (mcg) | Upper limit (IU) | Upper limit (mcg) |
|---|---|---|---|---|
| Infants 0-12 months | 400 IU | 10 mcg | 1,000-1,500 IU | 25-37.5 mcg |
| Children 1-18 years | 600 IU | 15 mcg | 2,500-4,000 IU | 62.5-100 mcg |
| Adults 19-70 years | 600 IU | 15 mcg | 4,000 IU | 100 mcg |
| Adults 71+ years | 800 IU | 20 mcg | 4,000 IU | 100 mcg |
| Pregnancy & lactation | 600 IU | 15 mcg | 4,000 IU | 100 mcg |
The NIH's official tolerable upper limit for adults is 4,000 IU/day (100 mcg/day). The Endocrine Society sets its own clinical upper limit higher, at 10,000 IU/day (250 mcg/day) for adults — the figure used when actively correcting a deficiency under supervision. See the full dosage guide for the complete blood-test-based dosing table and safety notes.
Why Two Units Exist
IU (International Units) measures biological activity rather than mass, and it's the unit most U.S. supplement bottles have used historically for vitamin D3. Mcg (micrograms) is a plain mass measurement, and it's the unit required on the Nutrition Facts panel under current FDA labeling rules, as well as the unit the NIH uses in its official RDA tables. Both describe the same amount of vitamin D3 — they're just different scales, related by the fixed 40 IU = 1 mcg factor. Reading a label in the "wrong" unit for what you're used to is one of the more common sources of dosing confusion, which is why the conversion table above lists both side by side.
Frequently Asked Questions
How do you convert vitamin D IU to mcg?
For vitamin D3 (cholecalciferol), 40 IU equals 1 mcg. To convert IU to mcg, divide by 40. To convert mcg to IU, multiply by 40. For example, 1,000 IU divided by 40 equals 25 mcg, and 2,000 IU divided by 40 equals 50 mcg.
How many mcg is 1000 IU of vitamin D3?
1,000 IU of vitamin D3 equals 25 mcg (1,000 divided by 40). This is a common maintenance dose that sits above the NIH RDA of 600-800 IU (15-20 mcg) and well under the 4,000 IU (100 mcg) upper limit.
How many IU is 50 mcg of vitamin D?
50 mcg of vitamin D3 equals 2,000 IU (50 multiplied by 40). This is the maintenance dose most commonly recommended by integrative and functional-medicine practitioners for adults who are not deficient.
Is vitamin D measured in IU or mcg?
Both units appear on supplement labels and in research. IU (International Units) is the more common unit on U.S. supplement bottles and in most clinical dosing guidance. Mcg (micrograms) is the unit used on the Nutrition Facts panel under current FDA labeling rules and in the metric-based NIH RDA tables. The conversion factor for vitamin D3 is 40 IU per 1 mcg.
Related Guides
- Vitamin D3 Dosage Guide — Full dosing tables by age and blood-test result, in IU
- Is 1,000 IU Enough? — 25 mcg, evaluated against blood-level dosing
- 1,000 vs 5,000 IU — 25 mcg vs 125 mcg, the full comparison
- Best Vitamin D3 Products — Verified picks ranked by cost per dose
- Vitamin D Hub
Sources
- NIH Office of Dietary Supplements. "Vitamin D: Fact Sheet for Health Professionals." ods.od.nih.gov