Syringes and units: U-100, mL and mg explained

What "U-100" means

Insulin syringes sold in the US are marked for U-100 insulin: 100 units in each milliliter ². On these syringes every unit mark is a volume, 0.01 mL. When the liquid is not insulin, the unit marks still measure only volume. The amount of peptide in each unit depends on how the vial was mixed.

The numbers on the barrel are units of volume: 100 units = 1 mL. Drawn here filled to 25 units (0.25 mL).

Common sizes

Syringe	Holds	Volume per unit	Best for
0.3 mL (3/10 cc)	30 units	0.01 mL	Small volumes; marks are farthest apart and easiest to read
0.5 mL (1/2 cc)	50 units	0.01 mL	Middle range
1 mL (1 cc)	100 units	0.01 mL	Larger volumes; marks are closer together

Health guides advise using the smallest syringe that holds the full amount, because the marks are farther apart and easier to read ¹. Smaller sizes (such as 0.25 mL) also exist ¹.

Reading the marks

On a 1 mL syringe, small marks are usually every 2 units and numbered lines every 10 units. On 0.3 and 0.5 mL syringes, small marks are usually every 1 unit. Some 0.3 mL syringes add half-unit marks. Check the barrel of the syringe in hand.
Read the volume at the edge of the plunger's rubber seal closest to the needle ¹.
Air bubbles take up space and make a measured volume smaller than it looks.

Mistakes that cause large errors

Reading tenths of a mL as units. Safety reports describe large overdoses where 0.1 mL marks were read as single units, a tenfold error ².
Mismatched syringe and concentration. Measuring a concentrated U-500 insulin with the wrong syringe type has led to fivefold errors ³. The same logic applies to any liquid: units on the barrel only mean something once the concentration is known.
U-40 syringes. Veterinary insulin syringes are marked for 40 units per mL. A unit on a U-40 syringe is 0.025 mL, 2.5 times a U-100 unit. Never mix the two.
Confusing mg and mcg. 1 mg is 1,000 mcg. A slipped decimal is a thousandfold error.

Quick conversions

Units (U-100)	mL
1	0.01
5	0.05
10	0.10
25	0.25
50	0.50
100	1.00

To see what a given number of units contains for a particular vial, use the reconstitution calculator.

Units vs mL: read your syringe right

The numbers on an insulin syringe are units, not milliliters. On a standard U-100 syringe, 100 units = 1 mL (1 mL is the same as 1 cc).

Syringe line	Volume
10 units	0.1 mL
25 units	0.25 mL
50 units	0.5 mL
100 units (one full 1 mL syringe)	1 mL
300 units (three full syringes)	3 mL

Example: "300 into the vial, 25 per dose" means 3 mL of water in the vial, then draw to the 25-unit line (0.25 mL). In a 5 mg vial that line holds about 417 mcg; in a 10 mg vial, about 833 mcg.

Syringe units measure volume only. They are not the IU (international units) that some drugs, such as hGH, are dosed in. Practice it in the simulator.