mg, mcg, mL and IU: what each unit measures
Labels, prescriptions and dose logs use four units that look alike on paper but measure different things. mg and mcg measure mass. mL measures volume. IU measures biological activity. Mix them up, and a written amount can end up meaning something very different from what was intended.
This guide explains what each unit measures, why you can't swap one for another, and how to keep your own records clear. It explains units only: it doesn't give doses or tell you how to prepare anything. Confirm any amount with your pharmacist or prescriber.
At a glance
| What it measures | How it relates | |
|---|---|---|
| mg (milligram) | Mass | 1 mg = 1,000 mcg, and 1,000 mg = 1 g |
| mcg (microgram) | Mass | 1 mcg = 0.001 mg |
| mL (milliliter) | Volume | 1 mL = 1 cc (cubic centimeter), and 1,000 mL = 1 L |
| IU (international unit) | Biological activity, defined separately for each substance | No fixed conversion to mg, mcg or mL |
| "Units" on an insulin syringe | Volume marks | On a U-100 syringe, 100 units = 1 mL |
mg and mcg: the same thing at two sizes
Milligrams and micrograms both measure mass: how much of a substance there is. Their prefixes are standard metric ones. NIST, the US agency for measurement standards, defines milli as one thousandth and micro as one millionth. A milligram is a thousandth of a gram, a microgram is a millionth of a gram, and so 1 mg = 1,000 mcg.
Converting between them only moves the decimal point three places. That also makes it easy to be off by a factor of 1,000. FDA warns that "mcg" or "µg" misread as "mg" means a 1,000-fold overdose, and its labeling guidance says the µg symbol shouldn't be used at all because it has been mistaken for mg. The abbreviation mcg is the one FDA calls appropriate.
mL: how much liquid, not how much medicine
A milliliter measures volume. NIST defines the milliliter as another name for the cubic centimeter, so 1 mL and 1 cc are the same. Some syringes and older instructions use cc.
A volume tells you how much liquid there is, not how much active ingredient is in it. Stir a spoonful of sugar into a glass of water, and another into a full jug: a sip from each holds a very different amount of sugar. The same is true of any liquid medicine.
Concentration is what links mass and volume
To get from a mass (mg or mcg) to a volume (mL), or back, you need one more number: the concentration, meaning the amount of active ingredient in each mL. It's written as a ratio, such as mg/mL or units/mL.
Concentration belongs to a specific product, and it's printed on the label. FDA's labeling guidance says small-volume injectable products should state their strength as the amount in the whole container, followed by the amount per mL in parentheses. Insulin labels state units per mL, in the form "100 units/mL (U-100)".
Three things follow:
- There's no general mg-to-mL conversion. Without your product's stated concentration, any answer is a guess. The same ingredient can come in more than one concentration: FDA notes, for example, that some insulin pens hold 100 units/mL and others 200 units/mL.
- Read both numbers on the label. FDA describes errors where someone saw the amount per mL and missed the total in the container.
- Keep mg and mL apart in writing. An FDA presentation with the Institute for Safe Medication Practices describes prescriptions entered in mL where mg was meant.
For where the strength appears on a prescription medicine's label, see how to read an FDA drug label.
IU: biological activity, defined substance by substance
An international unit doesn't measure mass or volume. It measures biological activity. WHO establishes International Standards, physical reference preparations for biological substances, and assigns some of them a potency value in International Units. Each substance's IU is defined against its own reference standard.
That's why there's no single conversion between IU and mg. One IU of one substance has no fixed relationship to one IU of another, or to any set mass. Even within one nutrient, the conversion can depend on the form: NIH's Office of Dietary Supplements gives different IU conversions for different forms of vitamin A, and says a conversion is only possible when you know which form it is. FDA has also moved the labels of foods and supplements from IU to mcg or mg for vitamins A, D and E.
The abbreviation causes trouble of its own. FDA's labeling guidance says "IU" shouldn't be used on labels because it has been confused with IV (intravenous).
Syringe "units" are volume marks, not IU
Insulin syringes are marked in "units", and that's where a lot of the confusion starts. The marks measure liquid. They're scaled to insulin of one concentration: a U-100 syringe is made for insulin at 100 units/mL, so its full 100-unit scale holds 1 mL and each unit mark is one hundredth of a mL. U-100 insulin is labeled for use only with a U-100 syringe.
- The marks count insulin units only with the insulin they were made for. With any other liquid, a "unit" mark is just a volume. It says nothing about the mg, mcg or IU of what's inside.
- Scales differ. The vial of a more concentrated insulin labeled U-500 (500 units/mL) is labeled for use only with a U-500 syringe. The same FDA presentation describes errors when a U-100 syringe was used with a concentrated insulin.
- Syringe units and IU share a word and nothing else.
Check the scale printed on your syringe or its packaging. If you're not sure which you have, ask your pharmacist.
Keep your own records clear
- Write the unit every time, exactly as it appears on your prescription or label. A number without a unit can't be checked later.
- Write "mcg", not "µg", so it can't be read as mg.
- Use a leading zero and no trailing zero: 0.1, not .1, and 1, not 1.0. FDA asks for this on labels because a missed decimal point turns a number into one 10 times larger or smaller.
- Note the concentration printed on the product you use alongside your records, and note when a new supply shows a different one.
- If two numbers disagree, for example your prescription, the label and your notes, check with your pharmacist or prescriber before relying on any of them.
If you keep a paper record, the printable logs have a column for the amount and unit, as written on your prescription or label.
FAQ
How many mcg are in 1 mg?
1,000. To go from mg to mcg, multiply by 1,000. To go from mcg to mg, divide by 1,000.
Can I convert IU to mg?
Not with one formula. Each substance's IU is defined separately, and some conversions depend on the form of the substance. Ask your pharmacist about a specific product.
Is 1 cc the same as 1 mL?
Yes. NIST defines the milliliter as another name for the cubic centimeter.
Units in Peptimes
Peptimes records each dose in the unit you choose: mg or mcg (IU where a compound is dosed in it). It doesn't log doses in mL or syringe units, and it never converts IU to mg or mcg. Before you save, check that the unit shown is the one on your label, and check the number again if you switch units. You can export your records as CSV or JSON to go over them with your prescriber. Peptimes never suggests an amount.
Sources
- NIST, "Metric (SI) Prefixes" (last updated 13 Aug 2025), checked 25 Oct 2026: milli (m) is one thousandth (10⁻³) and micro (µ) one millionth (10⁻⁶), so 1 mg = 1,000 mcg.
- NIST, "SI Units: Mass" (last updated 9 Dec 2025), checked 25 Oct 2026: mass is how much matter something contains; 1 g = 1,000 mg.
- NIST, "SI Units: Volume" (last updated 2 Mar 2026), checked 25 Oct 2026: the milliliter is a special name for the cubic centimeter (1 cm³ = 1 mL); the liter and its symbol L.
- FDA, "Safety Considerations for Container Labels and Carton Labeling Design to Minimize Medication Errors", Guidance for Industry (May 2022), checked 25 Oct 2026: µg mistaken for mg and mcg as the appropriate abbreviation; IU confused with the intravenous route; small-volume injection strength stated per total volume, then per mL; errors when the per-mL figure was seen and the total missed; insulin strength in units per mL, "100 units/mL (U-100)"; leading and trailing zeros.
- FDA Center for Veterinary Medicine, "A Microgram of Prevention is Worth a Milligram of Cure: Preventing Medication Errors in Animals" (content current as of 05/21/2024), checked 25 Oct 2026: mcg or µg mistaken for mg creates a 1,000-fold overdose; errors from trailing zeros and missing leading zeros.
- FDA, Drug Topics slides, "How to Avoid Medication Errors with Pen Injectors" (FDA and ISMP), checked 25 Oct 2026: some insulin pens come in 100 units/mL and others in 200 units/mL; a U-100 syringe used with a concentrated insulin; prescriptions entered in mL where mg was meant.
- WHO, "Providing international biological reference standards", checked 25 Oct 2026: WHO International Standards are physical reference preparations for measuring biological activity; some are assigned a potency value in International Units.
- NIH Office of Dietary Supplements, "Vitamin A and Carotenoids: Health Professional Fact Sheet" (updated 10 Mar 2025), checked 25 Oct 2026: IU conversions differ by form of vitamin A, and can be made only when the form is known.
- FDA, "FDA Provides Guidance for Industry to Convert Units of Measure for Certain Nutrients on Nutrition and Supplement Facts Labels" (15 Aug 2019), checked 25 Oct 2026: labels declare vitamins A and D in mcg rather than IU, and vitamin E in mg.
- DailyMed (NLM, NIH), prescribing information and carton labeling for a human insulin injection, 100 units/mL (updated 16 Jun 2026), checked 25 Oct 2026: "100 units per mL (U-100)"; "Use only with a U-100 syringe".
- DailyMed (NLM, NIH), prescribing information and carton labeling for a concentrated human insulin injection, 500 units/mL (updated 1 Jul 2026), checked 25 Oct 2026: "500 units/mL (U-500)"; the vial carton says to use it only with a U-500 syringe.
- Peptimes Help Center, "Dose amounts and units: mg, mcg and IU", checked 25 Oct 2026: the units Peptimes records; check the unit before you save.
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