Guide

How to Read a Syringe: U-100, Units, and mL

Syringe markings are the number-one source of dosing errors in peptide research — not because the math is hard, but because "units," "mL," and "mcg" measure three different things. This guide untangles them once.

The three measurements

• mL (milliliters) measure volume — how much liquid.
• Units are just a convenient re-labeling of volume on insulin syringes.
• mcg / mg measure mass — how much actual peptide is in that volume, which depends entirely on how the vial was reconstituted.

A syringe can tell you the first two. Only your reconstitutionReconstitutionThe process of mixing a freeze-dried (lyophilized) peptide powder with bacteriostatic water to create a solution of known concentration.See glossary → math tells you the third.

U-100: the standard

A U-100 insulin syringeU-100 syringeAn insulin syringe calibrated so that 100 units equal 1 mL. The standard syringe referenced in peptide research; each unit equals 0.01 mL.See glossary → is calibrated so that 100 units = 1 mL. That gives you the only conversion worth memorizing:

1 unit = 0.01 mL

Insulin syringes come in 0.3 mL (30-unit), 0.5 mL (50-unit), and 1 mL (100-unit) barrels. Smaller barrels space the markings farther apart, so a 0.3 or 0.5 mL syringe reads more precisely for small research doses. Many 0.5 mL syringes mark half-units — the most precise commonly available option.

U-40 exists, and it will burn you

U-40 syringes — calibrated at 40 units per mL — are still sold, mostly for veterinary insulin. Draw "10 units" in a U-40 syringe while doing U-100 math and you've drawn 0.25 mL instead of 0.1 mL: a 2.5× overdose of whatever you measured. Check the barrel printing before first use; research references standardize on U-100 exclusively.

From units to mcg

The bridge between the syringe and the dose is concentration:

mcg in your syringe = units × 0.01 mL × concentration (mcg/mL)

Ten units of a 2,500 mcg/mL solution carries 250 mcg. Ten units of a 5,000 mcg/mL solution carries 500 mcg — same syringe reading, double the dose. This is why writing the concentration on every vial matters, and why a calculator beats mental math.

Dead space and other fine print

• Dead space is the solution left in the needle hub after a full push. Insulin syringes with fixed (staked) needles have near-zero dead space; syringes with removable needles can trap 0.05–0.1 mL — wasted peptide and skewed dosing.
• Air bubbles displace solution. Draw slowly, flick the barrel, and push bubbles back into the vial before final measurement.
• Needle gauge: research references cite 29–31 gauge, 5/16" (8 mm) needles for subcutaneousSubcutaneous (SubQ)An injection into the fatty tissue just below the skin. The most commonly referenced administration route in peptide research literature.See glossary → administration. Higher gauge = thinner needle.
• One syringe, one use. Reusing needles dulls them and introduces contamination risk into a multi-use vial.

Quick reference