How to read a U‑100 insulin syringe

Units vs millilitres

MedlinePlus, the US National Library of Medicine's patient reference, defines U-100 as 100 units in 1 millilitre^[1]. A unit on the barrel is therefore a volume, 0.01 mL, and not an amount of any particular compound: 10 units is 0.1 mL, 50 units is 0.5 mL and 100 units is 1 mL. The name comes from insulin, which is made at 100 units per millilitre, but the syringe itself only measures volume. How much compound a unit holds depends entirely on the concentration of the solution in the vial.

U-100 units to mL, 1 to 100

Every reading on a U-100 barrel, converted: multiply the units by 0.01 to get millilitres, or divide millilitres by 0.01 to get units. The same figures hold on the 0.3, 0.5 and 1 mL barrels; a smaller barrel simply stops sooner.

The three barrel sizes

Syringe makers sell U-100 insulin syringes in 1 mL, 0.5 mL and 0.3 mL capacities^[3]. On the U-100 scale those hold 100, 50 and 30 units. The smaller barrels are usually marked in finer steps, so the smallest barrel whose capacity covers the volume being measured gives the most readable scale. The scale is a similar physical length on all three, so a 0.3 mL barrel spreads 30 units over the distance a 1 mL barrel uses for 100. A common bench setup keeps 1 mL syringes for moving water into a vial and a smaller barrel for reading small volumes. Pharmacies and medical supply shops in the Philippines stock all three sizes.

Tick spacing

UltiMed documents its 0.3 mL and 0.5 mL barrels as graduated in 1-unit increments and its 1 mL barrel in 2-unit increments^[2]. embecta, the former BD insulin syringe business, offers a half-unit scale on its 0.3 mL barrel^[3], and MedlinePlus describes 1 mL barrels on which every small notch is one unit^[1]. The step varies by make, so read the numbers printed on the barrel in hand before counting marks. The diagrams follow UltiMed's documented steps.

Taking a reading

Hold the syringe at eye level with the needle pointing up and tap out any air bubble, since a bubble takes up space the scale counts as liquid. Read the line level with the edge of the plunger's rubber stopper nearest the needle, not the far edge and not the tip of the cone. In the diagram below the stopper edge sits on the 30 line of a 1 mL barrel, which is 0.30 mL.

From units to milligrams

To know what a reading holds, multiply the volume by the concentration. A vial made with 5 mg in 2 mL is 2.5 mg/mL; 10 units on the barrel is 0.1 mL, and 0.1 mL of a 2.5 mg/mL solution holds 0.25 mg, or 250 mcg. Change the water volume and the same reading holds a different amount, which is why the concentration belongs on the vial label. Because one unit is 0.01 mL, micrograms per unit is the concentration in mg/mL multiplied by ten:

The stronger the solution, the more each mark holds, so a small amount lands between marks sooner. Mixing the same vial with more water lowers the concentration and spreads the same amount across more units, which is easier to read. The reconstitution calculator runs this arithmetic both ways for numbers you enter; it never suggests one. For the mixing side, see how to reconstitute a peptide vial.

U-100 vs U-40

Not every insulin syringe is U-100. On a U-40 syringe 40 units fill 1 mL, so one U-40 unit is 0.025 mL, 2.5 times a U-100 unit. Every table and answer on this page assumes U-100. Check the scale printed on the barrel or the box before using any conversion, because the same number of units is a different volume on the two scales.

Unit questions

Volume arithmetic on the U-100 scale, in the words people search for it.

How many units in a 1 ml syringe?

100 units. On a U-100 syringe 100 units fill 1 mL, so a 1 mL barrel is marked from 0 to 100.

What is 10 units on a 1 ml syringe in ml?

0.1 mL. Each unit is 0.01 mL, so 10 units is 10 × 0.01 = 0.1 mL. The volume is the same on any U-100 barrel; only the spacing of the marks changes.

100 units to ml

100 units is 1 mL on the U-100 scale, a full 1 mL barrel.

How many ml is 50 units?

0.5 mL. 50 × 0.01 mL = 0.5 mL, which is a full 0.5 mL barrel.

How many units in 0.5 ml?

50 units. 0.5 mL ÷ 0.01 mL per unit = 50.

How many units in 0.3 ml syringe?

30 units. A 0.3 mL U-100 barrel is marked from 0 to 30, and each unit is still 0.01 mL.