Vitamin D: IU to mcg converter
US labels print both units and nobody agrees on which one to use. Type either, get the other, and see the reference figures beside it.
50 mcg
One microgram of cholecalciferol is 40 IU, so this is a straight divide or multiply by 40.
The adult tolerable upper intake is 4,000 IU (100 mcg) a day.
How it works
- One microgram of cholecalciferol is 40 IU, so converting is a division or a multiplication by 40. There is no rounding trick: 400 IU is exactly 10 mcg, and 1,000 IU is exactly 25 mcg.
- The reference intake for adults up to 70 is 15 mcg (600 IU) a day, and 20 mcg (800 IU) over 70.
- The tolerable upper intake for adults is 100 mcg (4,000 IU) a day, set because vitamin D is fat soluble and accumulates. Going above it for months can raise blood calcium to a harmful level.
Before you use the number
- If a clinician has set you a dose for a diagnosed deficiency, theirs wins over any calculator.
- High-dose weekly or monthly boluses have not performed like daily dosing in the pooled trial data, so a large occasional dose is not a shortcut.
- Vitamin D supplements interact with some medicines, including thiazide diuretics and digoxin. Ask a pharmacist if either applies.
This is general information, not medical advice, and it does not replace your prescriber or a dietitian who knows you.
Questions people ask
How many micrograms is 1000 IU of vitamin D?
25 mcg. Divide the IU by 40. The same conversion runs the other way: 25 mcg is 1,000 IU.
Is 4000 IU of vitamin D too much?
4,000 IU is 100 mcg, which is the tolerable upper intake for adults. It is a ceiling for daily long-term use rather than a target, and the reference intake is far lower at 600 to 800 IU.
Which is better, IU or mcg?
Neither; they are the same measurement in different units. Micrograms are the scientific convention and IU are what US labels historically print, which is why both appear on the same bottle.
Sources
- NIH Office of Dietary Supplements, Vitamin D fact sheet (RDA and UL in mcg and IU)
- Martineau et al., vitamin D supplementation to prevent acute respiratory tract infections, BMJ
Figures checked against the sources above on 21 September 2026.
Vitamin D in winter: the quiet overachiever
Vitamin D has the strongest evidence of anything in the bottle, and it is not close. What 25 trials and 11,000 people showed about winter dosing, and why daily beats a single big dose.
Read itHow to read a supplement label
How to read a supplement label in ninety seconds: serving sizes, % Daily Value, ingredient forms, and where the useful numbers live.
Read itVitamin D3
Cholecalciferol, the strongest evidence in the bottle. Daily or weekly dosing lowers respiratory infection risk, most in people who start out low.
Read itEating and supplementing well on GLP-1 medications
The hub for this topic: protein, fibre, electrolytes, and the nutrients that slip when appetite drops.
Read itHow much protein do I need on a GLP-1?
A free protein calculator for people on Ozempic, Wegovy, Mounjaro or Zepbound. Your daily range and per-meal target, worked out the way the 2025 GLP-1 nutrition advisory describes.
Work it outHow much fiber do I need a day?
A free fiber calculator that scales to the calories you actually eat, which matters when a GLP-1 has shrunk your appetite. Your daily target, the standard adequate intake, and how to build up gently.
Work it outHow much water should I drink a day?
A free water intake calculator based on the National Academies figures, adjusted for eating smaller portions on a GLP-1. How much to drink in litres, cups and ounces.
Work it outPercent Daily Value calculator
Work out the percent Daily Value for any vitamin or mineral amount, or the amount that matches a percent you have seen on a label.
Work it outProtein per meal planner for small appetites
Turn a daily protein target into per-meal amounts that fit an appetite a GLP-1 has shrunk, with what each amount looks like on a plate.
Work it outSupplement label reader
Paste a Supplement Facts panel and get the serving size, the amounts, the percent Daily Values and the other ingredients pulled out line by line.
Work it out
FDA-registered facilityGMP certifiedEvery dose printedNine good thingsThird-party tested