How to Read an Insulin Syringe

What "U-100" means

Insulin syringes are marked in units instead of millilitres. On a U-100 syringe, 100 units equal 1 mL, so each unit is exactly 0.01 mL. The markings measure volume, whatever liquid is inside. The same 20-unit mark always means 0.2 mL, whether the syringe holds insulin, a peptide or water.

The three common sizes

A smaller syringe is easier to read for small draws, because the same volume spreads over a longer part of the barrel. The unit itself does not change: 10 units is 0.1 mL on all three.

Where to read the plunger

The plunger ends in a rubber stopper with a flat edge facing the needle. Read the volume where that front edge lines up with a marking, not at the back of the stopper or at any cone-shaped tip. Hold the syringe at eye level, needle up, and tap out any air bubbles before you read it; a bubble takes up volume that should be liquid.

The U-40 trap

Some syringes, mostly used for veterinary insulin, are U-40: 40 units per mL. A unit on a U-40 syringe holds 0.025 mL, two and a half times more than on a U-100 syringe. U-40 syringes are usually marked with a red cap or "U-40" on the barrel. Every calculator on this site gives units for U-100 syringes, plus the volume in mL. If you have any other syringe, go by the mL.

Turning units into an amount of peptide

A unit is a volume. How much peptide is in it depends on the concentration of the solution. At 5 mg/mL, one unit (0.01 mL) holds 0.05 mg, or 50 mcg. The syringe units converter does this for any concentration.