This is the shortest guide on the site and the most important one. Nearly every peptide calculation error comes back to the same slip: milligrams and micrograms look alike, sound alike, and differ by a factor of one thousand. Learn this conversion properly and most of the danger in the rest of the site disappears with it.
Try the calculator with your own vial size and water volume.
Two units, one thousand apart.
Peptide vials are labelled in milligrams (mg), but doses are often written in micrograms (mcg). Both measure weight; they differ only in scale. The metric ladder runs gram → milligram → microgram, with each step down dividing by 1,000: 1 g = 1,000 mg, and 1 mg = 1,000 mcg. Micrograms are sometimes written “µg” — that is the same unit as mcg, the official SI symbol; mcg is the spelling used on labels because a handwritten µ can be misread as “m”.
The conversion (exact, by definition — not a measured constant).
1 mg = 1,000 mcg. 1 mcg = 0.001 mg. 1 g = 1,000 mg = 1,000,000 mcg. It holds for every substance and does not depend on density, concentration or temperature.
- mcg = mg × 1,000 (multiply by 1,000; move the decimal point 3 places right)
- mg = mcg ÷ 1,000 (divide by 1,000; move the decimal point 3 places left)
Quick reference tables.
| Milligrams (mg) | Micrograms (mcg) | Milligrams (mg) | Micrograms (mcg) |
|---|---|---|---|
| 0.05 mg | 50 mcg | 2 mg | 2,000 mcg |
| 0.1 mg | 100 mcg | 2.5 mg | 2,500 mcg |
| 0.25 mg | 250 mcg | 5 mg | 5,000 mcg |
| 0.3 mg | 300 mcg | 10 mg | 10,000 mcg |
| 0.5 mg | 500 mcg | 20 mg | 20,000 mcg |
| 1 mg | 1,000 mcg | 50 mg | 50,000 mcg |
Worked examples.
- A dose of 0.25 mg in mcg: 0.25 × 1,000 = 250 mcg.
- A dose of 750 mcg in mg: 750 ÷ 1,000 = 0.75 mg.
- A 10 mg vial in mcg: 10 × 1,000 = 10,000 mcg — the figure the concentration formula starts from.
- Putting it together: a 0.5 mg dose (500 mcg) from a vial mixed to 2,500 mcg/mL needs 500 ÷ 2,500 = 0.2 mL = 20 units.
Sanity check.
For the same amount, the mcg number is always the bigger number. If converting mg → mcg gives you a smaller number, you divided instead of multiplying — the operation was reversed. If mcg → mg gives a larger number, same problem in the other direction. Check this every time; it takes two seconds and catches the most common error on this site.
Why this conversion causes so many errors.
Because the numbers look similar. “250” appears in both worlds — 250 mcg is a small fraction of a milligram, while 250 mg would be fifty whole 5 mg vials. Reading a dose written in mcg as if it were mg (or the reverse) makes the amount 1,000 times too large or too small, and the syringe maths will happily produce a confident-looking wrong answer: either a draw far beyond the syringe’s capacity or one too tiny to see. Always note which unit your dose is written in before touching a calculator, and if a result needs more than a full syringe, suspect an mg/mcg mix-up first.
Writing rules that prevent errors (ISMP).
These come from medication-safety practice, not from this site, and they are worth adopting whenever you write a number down:
- Always use a leading zero before a decimal point when the amount is less than one whole unit: write 0.5 mg, never .5 mg. If the decimal point is missed, .5 mg reads as 5 mg — a 10-fold error.
- Never use a trailing zero after a decimal point for whole-number amounts: write 5 mg, never 5.0 mg. If the decimal point is missed, 5.0 mg reads as 50 mg — a 10-fold error.
- Write mcg, not µg. A handwritten µ can be misread as “m”, so µg is mistaken for mg — a thousandfold error. Safety lists recommend writing mcg instead.
- Write the full unit at every step, including the denominator: “5 mg/mL”, not just “5”. Converting the “5” in “5 mg/mL” and dropping the “/mL” gives a number that is arithmetically correct and meaningless.
- Do not round too early. Small errors multiply across repeated calculations.
What this conversion does not do.
It converts mass to mass only. It does not convert mass to volume (mg → mL), mass to syringe units, or mass to IU. Those need concentration or a product-specific factor. IU (international units) measure biological activity, not weight, and the conversion is different for every substance — there is no universal IU-per-mg value, so this site does not convert IU. If your dose is given in IU, ask your prescriber or pharmacist for the product-specific conversion. And it does not establish a safe amount, an equivalent formulation, or an instruction to change how a product is used.
FAQs
Is mcg the same as µg?
Yes. µg is the scientific symbol for the microgram; mcg is the same unit written out in letters, used on labels and prescriptions to avoid misreading the µ symbol. 1 mcg = 1 µg.
How do I convert IU to mcg?
You cannot with a fixed number. IU measure biological activity, not weight, and the conversion is different for every substance. This site does not convert IU — if your dose is given in IU, ask your prescriber or pharmacist for the product-specific conversion.
Which unit should I enter in the calculator?
Whichever one your dose is written in — the toggle beside the dose field switches between mcg and mg and converts the value automatically. Just make sure the toggle matches the unit on your prescription.
Is 1,000 mcg a lot?
1,000 mcg is exactly 1 mg — whether that is a large or small amount depends entirely on the substance and your prescription, which is a question for your provider, not a calculator. This guide covers the units only.
Related guides
See also the Reconstitution Guide, the Syringe Units Guide and the Peptide Calculator home page.
Educational reference only — not medical advice. This guide describes unit conversions only; it does not recommend any dose. Always confirm your dose and preparation with a licensed healthcare provider or pharmacist.