mg, mcg, mL, and units explained
Peptide calculator questions usually involve four different measurement types at once: mg, mcg, mL, and syringe units. Each answers a different question.
Key takeaways
- mg and mcg measure peptide amount.
- mL measures liquid volume.
- Syringe units are markings on a specific syringe scale.
Metric conversion video for dosage math
Source: Level Up RN
Video notes
- Metric conversions should be solved before any syringe-unit conversion.
- mg-to-mcg mistakes can be 1,000-fold errors, so the units need to be written out.
- Peptide calculator inputs should keep mass and volume separate until concentration is known.
mg and mcg measure mass
Milligrams and micrograms describe how much peptide is present. One mg equals 1,000 mcg.
Vials are often labelled in mg, while smaller target amounts may be discussed in mcg.
- 1 mg = 1,000 mcg
- 0.25 mg = 250 mcg
- 2.5 mg = 2,500 mcg
Common conversions for peptide calculators
These conversion patterns show up often in peptide calculator searches. They are examples only; always apply the actual vial amount, diluent volume, and concentration from your own setup.
- 10 units equals how many mL? On a U-100 syringe, 10 units = 0.1 mL.
- 50 units to mg depends on concentration. On a U-100 syringe, 50 units = 0.5 mL, then mL x mg/mL gives mg.
- 5 mg vial with 2 mL BAC water = 2.5 mg/mL = 2,500 mcg/mL.
- 10 mg vial with 2 mL BAC water = 5 mg/mL = 5,000 mcg/mL.
mL measures liquid volume
Millilitres describe how much liquid is in the syringe or vial. The same mL amount can contain different peptide amounts depending on concentration.
That is why the calculator solves concentration before converting a dose into mL.
Units depend on the syringe
Units are syringe markings, not a universal dose. On a U-100 syringe, 100 units equals 1 mL. On a U-50 syringe, the scale is different.
A worked conversion: from vial label to syringe mark
Take a typical setup: a 5 mg semaglutide vial reconstituted with 2 mL of bacteriostatic water, targeting a 0.25 mg dose on a U-100 syringe. The math runs in three steps.
Step 1 — concentration. The vial holds 5 mg total. Adding 2 mL of water gives 5 mg ÷ 2 mL = 2.5 mg/mL. Equivalently: 2,500 mcg/mL.
Step 2 — dose volume. The target dose is 0.25 mg (250 mcg). Dividing by concentration: 0.25 mg ÷ 2.5 mg/mL = 0.1 mL. The dose occupies 0.1 mL of fluid.
Step 3 — syringe units. On a U-100 syringe, 100 units equals 1 mL. So 0.1 mL = 10 units. The dose is drawn to the 10-unit line.
The math is always the same three steps: vial amount → concentration → dose volume → syringe units. Change any input and the chain re-runs from that point downward.
- Concentration = vial amount ÷ water volume (mg/mL)
- Dose volume = dose ÷ concentration (mL)
- U-100 units = dose volume × 100
Conversion mistakes that cause 1,000-fold errors
The most dangerous error in peptide math is the unit mix-up between mg and mcg. They differ by a factor of 1,000, so getting the wrong one in your calculator produces a dose that is either 1,000 times too small (mostly harmless) or 1,000 times too large (potentially serious).
It happens most often when protocols cite doses in mcg ('500 mcg') but vial labels in mg ('5 mg'). Without converting deliberately, it is easy to enter 500 in a field expecting mg, or to treat a 5 mg vial as 5 mcg.
The defence is simple: write out the unit every time. '0.25 mg' is harder to mis-enter than '0.25' alone. When in doubt, convert to mcg and back to confirm: 0.25 mg × 1,000 = 250 mcg.
Concentration is the bridge between mass and volume
mg and mcg measure mass. mL measures volume. Syringe units measure volume (with a scale that depends on the syringe). The only thing that links mass to volume is concentration — and concentration is determined entirely by how much diluent you add to the vial.
This is why two people drawing the same number of units can be taking very different amounts of peptide. If one reconstituted a 5 mg vial in 2 mL and the other in 1 mL, their concentrations differ by 2× and so does the peptide content of every draw.
When comparing notes with other peptide users, the only meaningful unit to share is mass (mg or mcg). 'I draw 20 units' tells you nothing without the vial setup that produced that draw.
Reading vial labels in mg vs mcg
Lyophilized peptide vials are nearly always labelled in mg of total peptide: a 5 mg vial, a 10 mg vial, a 15 mg vial. The label tells you the total mass inside, not the per-mL amount — that depends on how much diluent you add later.
Some doses, especially for compounds used at small amounts (like ipamorelin, CJC-1295, BPC-157), are described in mcg. A 250 mcg dose from a 5 mg vial is 5% of the vial — manageable to draw on a U-100 syringe if reconstituted appropriately.
For larger-dose compounds (semaglutide, tirzepatide, retatrutide) the doses are typically in mg. The math is identical; only the convention differs based on what is convenient to write.
A second worked example: BPC-157 dose math
Different peptides put the math in different ranges, even though the algorithm is identical. Take BPC-157: a typical setup is a 5 mg vial reconstituted with 2 mL of bacteriostatic water, targeting a 250 mcg dose on a U-100 syringe.
Concentration: 5 mg ÷ 2 mL = 2.5 mg/mL, or 2,500 mcg/mL. Dose volume: 250 mcg ÷ 2,500 mcg/mL = 0.1 mL. Syringe units on U-100: 0.1 mL × 100 = 10 units.
Compare to semaglutide at the same vial setup: a 0.25 mg starter dose (also 250 mcg) gives exactly the same answer — 0.1 mL, 10 units. The same vial and water volume produce the same conversion math regardless of compound, because the math is purely about mass and volume.
Where compounds differ is in the dose range itself. BPC-157 doses are typically in the 200–500 mcg range. Semaglutide doses span 0.25 mg (250 mcg) at start to 2.4 mg at maintenance — a 10× range. Retatrutide doses run higher still. The math scales linearly; only the numbers change.
International units (IU) and other measures
Most peptides are dosed in mg or mcg, but some growth-hormone-related compounds use international units (IU). An IU is not directly convertible to mg — it is defined relative to a biological activity standard, and the conversion factor differs by compound.
For human growth hormone, the commonly cited conversion is roughly 3 IU per mg, but this varies by source and product. If a product is labelled in IU, do not assume a generic IU-to-mg ratio; check the specific product information.
Peptide calculators that handle IU usually require entering the conversion factor for the specific product. PepSync handles this in its HGH reconstitution calculator. For peptides labelled only in mg/mcg, the standard calculator math applies directly.