Insulin Syringe Units: How Many Units in a mL, U-100 vs U-40

How many units are in a millilitre depends on the syringe, not on what is in it. On a U-100 insulin syringe, the standard in the United States, one millilitre is 100 units and one unit is 0.01 mL; half a millilitre is 50 units, 0.1 mL is 10 units. On a U-40 syringe, a veterinary scale, one millilitre is 40 units and one unit is 0.025 mL. The unit is a volume printed under another name, and the rest of this page is what follows from that for a reconstituted peptide vial.

An insulin syringe is a small syringe with a permanently attached needle whose barrel is graduated in insulin units rather than millilitres. BD's 2019 FDA clearance for its range (510(k) K190054, read 2026-09-18) describes the device as a graduated barrel, plunger rod and needle assembly "intended for subcutaneous injection of U-100 insulins" and lists three capacities: 0.3 mL, 0.5 mL and 1 mL. The open nursing textbook Nursing Skills (2nd edition, Open RN) gives the reason for the scale: insulin syringes are marked in units, not millilitres, because insulin is prescribed in units. Nothing about the scale changes when the barrel holds something else; a peptide solution is read in the same hundredths of a millilitre, and the conversion to micrograms needs the concentration of that particular vial.

U-100, U-40 and what one unit is

The U in each name is the concentration of the insulin the syringe was made for, in units per millilitre, and the scale on the barrel matches it. That is why the two are not interchangeable for insulin: Merck Animal Health's Vetsulin page states that the product contains 40 IU per mL, that U-40 syringes only are to be used with it, and that using a U-100 syringe "would result in a dog receiving two and a half times less insulin than required" (read 2026-09-18). For a peptide solution the scale is only a volume, and a U-40 syringe reads 2.5 times fewer units for the same draw.

The three barrels and their markings

BD's clearance lists the barrel sizes, the cannula gauges and lengths, and the graduation steps (K190054, read 2026-09-18).

The graduation is the practical difference between the barrels. A draw of 5 units sits on a printed line on a 0.3 mL barrel and between the 4 and 6 lines on a 1 mL barrel. A draw of 60 units needs the 1 mL barrel, because the others end at 30 and 50. The MOUNJARO and ZEPBOUND labels (revised 08/2026 and 8/2026, read 2026-09-18) put that choice into label language for their vials: a syringe capable of measuring a 0.5 mL or 0.6 mL dose, which on a U-100 scale is 50 or 60 units. The syringes page covers gauge, length and dead space in full.

Reading the scale

The reading is taken where the leading edge of the plunger's rubber tip meets the scale, the flat edge nearest the needle, not the pointed centre of the tip. On a U-100 barrel every printed number is a hundredth of a millilitre: 5 units is 0.05 mL, 10 units is 0.1 mL, 30 units is 0.3 mL, 50 units is 0.5 mL. The searches "0.5 mL in units" and "how to read 5 units on an insulin syringe" have the same answer on every U-100 barrel; only the fineness of the lines differs between barrel sizes.

Micrograms to units at each concentration

The unit marks measure volume, so the number of units that holds a given quantity of peptide depends on the concentration of the solution, which is the milligrams in the vial divided by the millilitres of water added. The rule:

units on a U-100 syringe = quantity in mg ÷ concentration in mg/mL × 100

Where the concentrations come from: 1 mg/mL is a 1 mg vial in 1 mL; 2 mg/mL is a 10 mg vial in 5 mL or a 2 mg vial in 1 mL; 2.5 mg/mL is a 5 mg vial in 2 mL; 5 mg/mL is a 10 mg vial in 2 mL or a 5 mg vial in 1 mL; 10 mg/mL is a 10 mg vial in 1 mL. Two readings from the table. Quantities of a few hundred micrograms at 10 mg/mL land on one, two or three units, at or below the 2-unit resolution of a 1 mL barrel, which is why microgram compounds are usually worked at lower concentrations or on a half-unit 0.3 mL barrel. Quantities of milligrams at 1 mg/mL exceed a syringe, which is why milligram compounds are worked at higher concentrations. Neither choice changes the quantity; it changes only how the quantity reads.

Where the concentration comes from

The concentration is set at reconstitution and is the one figure a unit reading cannot do without. The site's reconstitution calculator returns concentration and units for 52 compounds at any vial size and water volume, and the per-compound pages under the reconstitution hub work the common vial sizes in tables. The reported ranges the quantities come from are on the dosage hub, each with its source and date.

What the scale does not include

A syringe's graduations measure the liquid between the plunger tip and the zero line. The liquid left in the needle and hub after the plunger bottoms out, the dead space, is outside the scale. Zule et al. measured 56 needle-and-syringe combinations from programmes in 17 countries and found an average of 3 microlitres of dead space in low-dead-space syringes with permanently attached needles, 13 microlitres in high-dead-space syringes with low-dead-space needles, 45 microlitres in low-dead-space syringes with high-dead-space detachable needles and 99 microlitres in high-dead-space syringes with high-dead-space needles (Harm Reduction Journal 2018, PMID 29334973). Insulin syringes with fixed needles are the first group. At 5 mg/mL, 99 microlitres holds about 500 mcg; 3 microlitres holds 15 mcg. The syringes page sets the figures out with the gauges and lengths.

Related pages

The page on how injections are given reports the procedure as the labels and nursing references describe it, including the syringe reading; the syringes page covers gauge, length, barrel and dead space; the reconstitution hub and dosage hub hold the per-compound arithmetic and the reported ranges.

This page does not recommend a dose, a syringe or a concentration. It reports what a syringe clearance, a veterinary product page, a nursing textbook and one consensus document state about the scales, and it does the arithmetic that turns a quantity into a reading.

Sources and dates

Opened 2026-09-18: FDA 510(k) summary K190054, BD Insulin Syringes (barrel sizes, gauges, cannula lengths, graduation steps, indications for use); Merck Animal Health's Vetsulin syringe-administration page (40 IU per mL, U-40 syringes only, the two-and-a-half-times statement); the MOUNJARO prescribing information (revised 08/2026) and the ZEPBOUND label on DailyMed (revised 8/2026, setid 487cd7e7-434c-4925-99fa-aa80b1cc776b) for the vial-syringe wording; Nursing Skills, 2nd edition, chapter 18 (Open RN, NCBI Bookshelf NBK596739); Frid et al. 2016 (PMID 27594187) and Zule et al. 2018 (PMID 29334973), abstracts via the NCBI E-utilities. Corrections go to the contact page.