Insulin syringe units, explained

What is a syringe unit?

Insulin syringes were designed around insulin, which is sold at standard strengths such as U-100 (100 insulin units per mL). Printing the barrel in insulin units lets someone using U-100 insulin read the amount directly. The printed numbers are really milliliters multiplied by 100.

Once the syringe is used with anything other than U-100 insulin, the numbers stop meaning "insulin units" and become a plain volume scale. A reading of 20 is 0.20 mL, whatever is in the barrel. To turn that into milligrams you need the concentration:

Small readings are the least precise: nurses measuring 2-unit insulin doses with syringes erred by a median 0.6 units, about 30% (Kondo et al., 2024).

At 5 mg/mL (10 mg in 2 mL), 20 units = 0.2 mL = 1 mg. At 2.5 mg/mL (10 mg in 4 mL), the same 20 units is 0.5 mg.

What sizes do U-100 syringes come in?

U-100 syringes are commonly made in three capacities. The unit is the same on all three; capacity and mark spacing differ. The illustrations below are drawn by the same code as the live calculator.

A 1 mL U-100 syringe holds 100 units, a 0.5 mL syringe holds 50 units and a 0.3 mL syringe holds 30 units. Smaller barrels usually print a mark for every unit (some 0.3 mL barrels also print half units), while 1 mL barrels usually mark every 2 units, so a smaller barrel is easier to read for small volumes.

What is the difference between U-100, U-40 and U-50 syringes?

U-40 syringes exist for U-40 insulin, which today is mainly used in veterinary medicine. U-50 barrels are less common. The danger is that all of them print plain numbers:

• 40 on a U-40 barrel = 1.0 mL
• 40 on a U-50 barrel = 0.8 mL
• 40 on a U-100 barrel = 0.4 mL

Figures copied from a source that assumed U-100 are wrong by 2.5× on a U-40 barrel. The calculator's syringe selector includes U-40 and U-50 under "Other scales" and flags them, and the units to mL converter charts all three.

How do you read the marks on an insulin syringe?

1. Read at the stopper's leading edge. The measurement is where the flat top of the rubber plunger tip meets the scale, not the tip of the cone.
2. Count minor marks from a labeled number. On a 1 mL barrel the minor marks are usually 2 units apart, so the mark after 20 is 22, not 21.
3. Between-mark readings are estimates. If the arithmetic gives 21 units on a 2-unit barrel, the calculator tells you the nearest marks (20 and 22) and suggests either a smaller barrel or a water volume that lands on a mark.
4. Air bubbles take up volume. A bubble in the barrel makes the reading larger than the liquid actually drawn.

Which syringe size fits your reading?

The smallest barrel that holds the whole reading is usually the easiest to read, because its marks are farther apart. This table applies the calculator's barrel model to common readings:

Readings above 100 units don't fit any insulin syringe in one draw. Either split the volume into two draws (and add the readings) or reconstitute with less water so the same amount takes fewer units. The water calculator shows the volume that makes a reading land where you want it.

How do you convert a reading between U-40 and U-100?

To move a reading from one scale to another, go through milliliters: U-100 reading = U-40 reading × 2.5, and U-100 reading = U-50 reading × 2.

Are syringe units the same as international units (IU)?

"Units" means two different things, which is another source of mix-ups. A syringe unit is a volume mark (0.01 mL on U-100). An international unit (IU) measures biological activity and is defined separately for each substance; it's what insulin and some hormones are labeled in. The two only line up for U-100 insulin, where 1 IU is in 0.01 mL by design. For anything else, an IU figure has to be converted to mg or mL using that product's own labeling before it means anything on a syringe. Peptide Vial Math works only in mg, mcg, mL and syringe units, never IU.

Reading checklist

1. Confirm the barrel's scale (U-100, U-50 or U-40) on the packaging or barrel.
2. Confirm its capacity (30, 50 or 100 units on U-100) and the spacing of its minor marks.
3. Convert the target to mL and check the units figure against it.
4. If the reading falls between marks, change the barrel or the water volume rather than estimating.

What measuring errors has FDA reported?

In 2024 FDA alerted health-care providers, compounders and patients to dosing errors with compounded injectable semaglutide in multiple-dose vials. The reports included people told to measure 5 units (0.05 mL) on a U-100 syringe who drew 50 units instead, a tenfold mistake. FDA said unfamiliarity with drawing from a vial, along with confusion between milliliters, milligrams and "units", may have contributed. It encouraged providers and compounders to supply the appropriate syringe size for the intended dose and to show patients how to measure it, and warned that compounded products may come in different concentrations.