Every peptide vial label is written in milligrams. Every syringe is written in units. Nothing about that connection is intuitive the first time you see it. This is a short reference for the math that bridges the two — and for why it has to look this way.
Why peptides are dosed in micrograms
A typical peptide dose is on the order of hundreds of micrograms, not milligrams. Semaglutide, for example, ramps from 250 mcg to 2400 mcg over a 12-week titration. Tirzepatide tops out around 15 mg (= 15,000 mcg). BPC-157 lives in the 250–500 mcg/day range.
Vial labels still list the contents in milligrams because that's the natural scale for the powder — "5 mg" of lyophilized peptide is a meaningful, weighable amount. Doses are in micrograms because that's what the body responds to. The first conversion you ever do is just the unit shift.
What an insulin unit measures
A 100-unit insulin syringe holds 1 mL of liquid, marked in 100 equal divisions. Each unit is therefore 0.01 mL, regardless of what's in it. The "unit" is a volume measurement; it has nothing to do with the contents of the syringe.
This is the trick: once you know how many mL you need, multiplying by 100 gives you units on the syringe. That's it.
The four-step conversion
Given a vial label like "5 mg / 2 mL water / 250 mcg dose":
- Total mcg in vial =
mg × 1000 - Concentration =
total mcg ÷ water mL - Volume per dose =
dose mcg ÷ concentration - Units =
volume per dose × 100
For the example above: 5000 mcg total → 2500 mcg/mL → 0.1 mL → 10 units.
Why this gets confusing
Three things trip people up the first time:
- The mg/mcg shift. Vial is in mg, dose is in mcg. You have to multiply by 1000 to get them onto the same scale.
- The volume hiding in "units." Units sound like a count of something, but they're really just 0.01 mL each. It's a volume relabeling.
- The water you added matters. The same 5 mg vial, reconstituted with 1 mL vs 2 mL vs 5 mL, gives you wildly different unit counts per dose.
Once you've done it five times, it's automatic. The calculator on this site is essentially these four lines of arithmetic — useful to verify, not to trust blindly.
A quick reference
For a 5mg semaglutide vial + 2mL BAC water (the most common combo), at standard titration doses:
- 250 mcg → 10 units
- 500 mcg → 20 units
- 1000 mcg → 40 units
- 1700 mcg → 68 units
- 2400 mcg → 96 units
If the calculator gives you a number outside the 0–100 unit range, something's wrong with your inputs — that's a useful sanity bound on a 100u/1mL syringe.