Vitamin D Converter (IU to mcg)
Convert vitamin D between IU and micrograms with the 40 IU factor.
What it does
Vitamin D labels speak two units: micrograms on newer facts panels and IU on older bottles and prescriptions. One microgram is 40 IU either way, so 1000 IU reads 25 mcg everywhere.
The tool above converts either direction and prints the round trip beside it. The default 1000 IU gives 25 mcg with 1,000 IU back, while 600 IU gives the adult RDA of 15 mcg.
The factor rests on NIH supplement guidance with MedlinePlus beside it: RDAs print in both units with 10 mcg at 400 IU, 15 at 600, and 20 at 800. Upper limits run 25 mcg for young infants to 100 mcg for older children and adults.
This page changes units the way a weight converter changes scales; it never doses. Needs vary with sun, diet, age, and blood work, so any supplement move belongs with a clinician.
How to use
- Type the vitamin D amount.
- Pick the unit it carries now.
- Pick the unit you want back.
- Read converted first, round trip beside it.
- Compare against RDA only in matching units.
- Log which unit each entry uses.
Formula
- 40
- IU per microgram of vitamin D.
- RDA
- 10 mcg infants, 15 mcg ages 1 to 70, 20 mcg over 71.
- round trip
- Converting back must return the start.
mcg prints to 2 decimals and IU rounds whole, matching label habits. Mega doses need medical supervision regardless of units.
Worked example
Example: 1000 IU
Defaults: value 1000, from IU, to mcg.
- Divide: 1000 / 40 = 25 mcg.
- Round trip: 25 x 40 = 1,000 IU.
- RDA check: adult 15 mcg is 600 IU.
- Upper check: adult limit 100 mcg is 4,000 IU.
- Tool shows 25 mcg with 1,000 IU back.
Reference table
| Who | mcg | IU |
|---|---|---|
| Birth to 12 months | 10 | 400 |
| Ages 1 to 70 | 15 | 600 |
| Over 71 | 20 | 800 |
| Upper limit 0 to 6 months | 25 | 1,000 |
| Upper limit 9 yrs plus | 100 | 4,000 |
Limits
- Unit math only; no dosing advice.
- D2 and D3 share the factor on labels.
- Blood levels need testing, not math.
- Upper limits are daily caps, not targets.
Common mistakes
- Multiplying IU by 40 instead of dividing.
- Reading mcg RDA against IU bottles.
- Treating the upper limit as a goal.
- Stacking multi plus D3 drops uncounted.
- Quoting decimals the label never prints.
Try it yourself
Quick checks. Answers sit tucked under each question.
600 IU in mcg?
15 mcg, the adult RDA.
25 mcg in IU?
1,000 IU.
Negative input?
Rejected: enter 0 to 10000000.
Questions
What is 1000 IU?
25 mcg.
Why 40?
One microgram of D3 is defined as 40 IU.
Adult RDA?
15 mcg (600 IU) to age 70; 20 mcg (800 IU) after.
Upper limit?
100 mcg (4,000 IU) daily for ages 9 plus.
D2 versus D3?
Same 40 factor on supplement labels.
Dosing help?
No: units only. A clinician sets doses from labs.