Units of Measure (mcg, mg, mL)
Micrograms, milligrams, millilitres and syringe units — what each one actually measures, how to convert between them, and the one mix-up that causes most dosing-math errors.
The short answer
- 1 mg = 1,000 mcg — and 1 g = 1,000 mg
- mcg = µg = ug — three ways to write micrograms
- mg is mass, mL is volume — not interchangeable
- U-100 syringe: 100 units = 1 mL, so 1 unit = 0.01 mL
Units of measure are the labels that tell you what kind of quantity a number represents. In peptide and supplement tracking, four show up constantly — and they fall into two completely different families:
- Mass (how much substance there is): micrograms (mcg), milligrams (mg), grams (g).
- Volume (how much liquid there is): millilitres (mL), and insulin-syringe units.
Converting within a family is simple arithmetic. Converting between families is impossible without a third number — the concentration. That single distinction explains most of the confusion below.
Quick mass converter
Type an amount and pick its unit — every equivalent updates instantly.
Need volume and syringe units too? Use the full unit converter or the dosage calculator.
Micrograms (mcg) and milligrams (mg)
Both measure mass, and they differ by a factor of exactly 1,000: 1 mg = 1,000 mcg. To go from mg to mcg, multiply by 1,000. To go from mcg to mg, divide by 1,000.
Vials are usually labelled in milligrams (a "5 mg vial"), while individual amounts are often written in micrograms (a "250 mcg" amount). That mismatch is why mg → mcg is the conversion people reach for most. Because the gap is 1,000×, getting it backwards is not a rounding error — it is a thousand-fold one.
mcg to mg conversion chart
| Micrograms | Milligrams | Grams |
|---|---|---|
| 100 mcg | 0.1 mg | 0.0001 g |
| 250 mcg | 0.25 mg | 0.00025 g |
| 500 mcg | 0.5 mg | 0.0005 g |
| 750 mcg | 0.75 mg | 0.00075 g |
| 1,000 mcg | 1 mg | 0.001 g |
| 2,500 mcg | 2.5 mg | 0.0025 g |
| 5,000 mcg | 5 mg | 0.005 g |
| 10,000 mcg | 10 mg | 0.01 g |
Every row is the same quantity written three ways — only the unit changes.
Why "mcg" and not "µg"?
Micrograms can be written mcg, µg, or ug — all three mean the same thing. Labels generally prefer "mcg" because a handwritten or poorly rendered Greek "µ" can be misread as an "m", which would turn micrograms into milligrams and introduce a 1,000× error.
Millilitres (mL) — and why mg ≠ mL
A millilitre measures volume, not mass. A vial containing 5 mg of peptide might hold 1 mL, 2 mL, or 3 mL of liquid after reconstitution — the mass of peptide never changed, only the amount of water it is dissolved in.
The bridge between mass and volume is concentration:
concentration (mg/mL) = total mg in vial ÷ mL of water addedvolume to draw (mL) = amount (mg) ÷ concentration (mg/mL)Only once you know the concentration does a mass become a volume. This is exactly what the dosage calculator automates.
Vial size → total micrograms
| Vial label | Total mcg | × 250 mcg | × 500 mcg |
|---|---|---|---|
| 2 mg | 2,000 mcg | 8 | 4 |
| 5 mg | 5,000 mcg | 20 | 10 |
| 10 mg | 10,000 mcg | 40 | 20 |
| 15 mg | 15,000 mcg | 60 | 30 |
| 20 mg | 20,000 mcg | 80 | 40 |
The last two columns show how many amounts of that size a vial's total mass contains — arithmetic only, independent of how much water you add.
Insulin-syringe units
"Units" on an insulin syringe are a volume marking, not a measure of peptide. On a standard U-100 syringe the barrel is divided so that 100 units = 1 mL, which makes 1 unit = 0.01 mL. A "10 unit" draw is simply 0.1 mL of whatever is in the vial — how much peptide that contains depends entirely on the concentration.
| U-100 units | Volume (mL) |
|---|---|
| 5 units | 0.05 mL |
| 10 units | 0.10 mL |
| 20 units | 0.20 mL |
| 25 units | 0.25 mL |
| 50 units | 0.50 mL |
| 100 units | 1.00 mL |
units (U-100) = volume in mL × 100Not every syringe is U-100 — U-40 and U-50 barrels exist and are marked differently. Always read the barrel before converting. See syringe units for the full breakdown.
Putting it together: a worked example
A 5 mg vial, reconstituted with 2 mL of bacteriostatic water, measuring a 250 mcg amount:
- Convert to one unit family: 5 mg = 5,000 mcg.
- Concentration: 5,000 mcg ÷ 2 mL = 2,500 mcg/mL.
- Volume: 250 mcg ÷ 2,500 mcg/mL = 0.1 mL.
- Syringe units: 0.1 mL × 100 = 10 units on a U-100 syringe.
Add 1 mL of water instead and the concentration doubles to 5,000 mcg/mL, so the same 250 mcg becomes 0.05 mL = 5 units. Same amount, different volume — which is precisely why unit counts are never transferable between vials.
Mistakes worth avoiding
PeptideWiz keeps mass and volume distinct everywhere — you choose preferred units in your profile, and they stay consistent across schedules, logs, the calculator, and exports.
Not medical advice. PeptideWiz is a tracking and organization tool. These definitions are general education only — not medical advice, a recommendation, or instructions for use. Always work with a licensed medical provider.
Frequently asked questions
How many mcg are in a milligram?
There are 1,000 micrograms in 1 milligram, so 1 mg = 1,000 mcg. To go the other way, divide micrograms by 1,000 to get milligrams — for example, 250 mcg = 0.25 mg.
What is the difference between mcg and mg?
A milligram (mg) is 1,000 times larger than a microgram (mcg), so 1 mg = 1,000 mcg. For example, 250 mcg is the same as 0.25 mg. Because the gap is 1,000×, mixing the two up is a large error, not a small one.
How do I convert mcg to mg?
Divide the number of micrograms by 1,000. For example, 500 mcg ÷ 1,000 = 0.5 mg, and 1,500 mcg = 1.5 mg. To convert mg to mcg instead, multiply by 1,000. The converter does this for you.
What is the difference between mg and mL?
Milligrams (mg) measure mass — how much peptide there is. Millilitres (mL) measure volume — how much liquid there is. They are not interchangeable and can only be linked once you know the concentration in mg/mL.
How do I convert mg to mL?
You cannot convert mg to mL directly, because one measures mass and the other volume. You need the concentration: volume (mL) = amount (mg) ÷ concentration (mg/mL). For example, 2 mg from a 5 mg/mL solution is 2 ÷ 5 = 0.4 mL.
Is mcg the same as µg or ug?
Yes — mcg, µg and "ug" all mean micrograms. Labels usually prefer "mcg" because the Greek letter µ can be misread as an "m", which would imply milligrams and a 1,000× error.
How many units is 1 mL on an insulin syringe?
On a standard U-100 insulin syringe, 1 mL = 100 units, so 1 unit = 0.01 mL. Units measure the volume drawn on that barrel — they are not a measure of peptide mass, and are only meaningful alongside the concentration.
Which units are used for peptides?
Amounts are usually described in micrograms (mcg) or milligrams (mg) for mass, and millilitres (mL) for the volume of reconstituted liquid. Insulin-syringe units are also common for measuring a draw. Vials are typically labelled in mg while amounts are often written in mcg.