"Units" is a syringe scale, not a fixed volume
A "unit" on an insulin-style syringe is simply 1/100th of the syringe's stated capacity scale — it was standardised decades ago for insulin concentration, and the research peptide community borrowed the same syringes because they're cheap, widely available, and precise at small volumes. The number printed next to a marking only means something once you know which syringe scale it belongs to.
U-100 — the common one
100 units = 1 mL, so 1 unit = 0.01 mL. This is the syringe scale almost every EU research-peptide vendor ships, and the one PeptideCompare's reconstitution calculator assumes when it shows a result in "units."
U-50
Same underlying scale as U-100 (1 unit = 0.01 mL) but on a smaller-capacity barrel (0.5 mL), so the printed lines are physically further apart — easier to read precisely for small draws. The numbers mean the same thing as U-100; only the barrel size differs.
U-40 — a different scale entirely
An older, mostly veterinary insulin standard: 40 units = 1 mL, so 1 unit = 0.025 mL — 2.5× more liquid per "unit" than on a U-100 syringe. Reading a U-40 syringe as if it were U-100 (or vice versa) is a real, well-documented source of measurement error.
⚠ Always check which scale is printed on the syringe barrel itself before drawing anything. Never assume — packaging from different suppliers is not consistent, and "units" without a stated scale is meaningless.
mg, mcg, mL and IU — what each one actually measures
These get mixed up because they answer different questions, not because the conversion itself is hard:
mg (milligram) / mcg (microgram) — mass of the actual peptide. 1 mg = 1,000 mcg. This is what's printed on the vial label as the total content.
mL (millilitre) — volume of liquid, after reconstitution. A vial's mg content doesn't change; only how much liquid it's dissolved into does, which is what sets the concentration (mg per mL).
IU (international unit) — a measure of biological activity, not mass. It's specific to a handful of hormones (e.g. hCG, growth hormone) where potency, not weight, is the historical reference. mg and IU are not directly convertible without a compound-specific potency value — never assume a generic mg-to-IU ratio.
Where this connects to our calculators
The Standard reconstitution calculator outputs a draw volume in both mL and "units" — that units figure assumes a U-100 syringe, because that's overwhelmingly what's available from EU vendors. If you've been supplied a U-40 or another scale, use the mL figure directly and read your syringe's own printed scale, not the unit number.
✓ When in doubt, the mL number is universal across every syringe type. The "units" number only works if you know — for certain — which syringe is in your hand.
Back to the calculators
Standard, Reverse, Blend, Titration and COA Check — all on one page.