What does U-100 mean on a syringe?

It means the barrel is calibrated for a concentration of 100 units per millilitre, so one unit occupies 0.01 mL. It is the single human insulin standard in the United States, and it has been since U-40 and U-80 were phased out around 1979.

Is there such a thing as a U-50 syringe?

No. There is no U-50 insulin concentration. A syringe described as U-50 is almost always a 0.5 mL barrel, which holds 50 units precisely because it is calibrated U-100 and half a millilitre is half of 100 units. The barrel size and the calibration are different things.

How many units is each line on an insulin syringe?

On a 0.3 mL and a 0.5 mL barrel, one unit per line. On a 1 mL barrel it depends on the manufacturer: some mark every unit and some mark every two. Check your own barrel, because counting 2-unit lines as one unit each draws exactly half the intended dose.

Which syringe size should I use?

The smallest one that holds the whole draw. Smaller barrels carry finer and more widely spaced graduations, so the same dose is easier to read accurately on a 0.3 mL barrel than on a 1 mL one.

What happens if I use a U-40 syringe with U-100 insulin?

You deliver two and a half times the intended dose. The reverse, drawing U-40 into a U-100 syringe, delivers two and a half times too little. U-40 is still standard in veterinary insulin, which is where most people encounter it.

Insulin Syringe Guide

The barrel decides how much you can draw and how precisely you can read it. Neither of those is what the calibration decides, and mixing the two up is where the errors come from.

What U-100 means

U-100 is a concentration, not a size. It means the barrel is printed for insulin at 100 units per millilitre, so one printed unit is 0.01 mL of liquid. Every standard insulin syringe sold for human use in the US is U-100.

That is why a 0.3 mL, a 0.5 mL and a 1 mL insulin syringe all agree that 20 units is 0.2 mL. They are the same scale in different lengths. The syringe units calculator converts either way.

Barrel sizes and what they hold

Barrel	Holds	Printed every	Best for
0.3 mL	30 units	1 unit	Finest barrel, for small doses
0.3 mL	30 units	0.5 units	Half-unit graduations
0.5 mL	50 units	1 unit	The common middle size
1 mL	100 units	1 unit	Some brands mark every unit
1 mL	100 units	2 units	Other brands mark every 2 units

Clinical guidance is to use the smallest barrel that holds the whole dose, because the graduations on a shorter barrel are both finer and further apart on the page. A 12 unit draw on a 0.3 mL barrel sits a third of the way up a well-spaced scale; the same draw on a 1 mL barrel is a sliver near the bottom.

The 1 mL barrel is not marked the same way by every brand

This is the detail worth taking from this page. On a 1 mL, 100 unit barrel, some manufacturers print a line every 2 units and others print one every unit. Both are sold, and the packaging does not always make it obvious.

Counting the lines on a 2-unit barrel as though each were one unit draws exactly half the intended dose. There is no warning, nothing looks wrong, and the error repeats every time. The fix is to read the numbers printed beside the long lines rather than counting the short ones: if the labelled marks are 10, 20, 30 and there are five short lines between them, each short line is 2 units.

Because the interval varies, the calculator offers both 1 mL barrels as separate options, and it will tell you when a dose does not land on a printed line for the one you picked.

U-40, and why it matters

U-40 is the other calibration still in circulation. It is 40 units to the millilitre, so one unit is 0.025 mL, two and a half times the volume of a U-100 unit. It is no longer sold for human use in the US but it remains the standard for veterinary insulin, and both the insulin and the syringes are readily available.

The same physical barrels hold much less when they are printed at U-40:

Barrel	At U-100	At U-40
0.3 mL	30 units	12 units
0.5 mL	50 units	20 units
1 mL	100 units	40 units

Draw to a U-100 figure on a U-40 barrel and you give two and a half times the dose. Draw to a U-40 figure on a U-100 barrel and you give two and a half times too little. Veterinary insulin labels carry an explicit warning about this for exactly that reason.

Reading the barrel

The dose is read at the top edge of the plunger's rubber stopper, with the syringe held at eye level. That is also the practical limit on precision: you can read to the nearest printed line and no finer, which is why a dose that falls between two lines is a problem to solve with the dilution rather than with care.

If a dose needs half a unit, that needs a barrel printed in half units, which is sold in the 0.3 mL size. Otherwise, adding more diluent moves the dose onto a whole number of units, and the diluent volume calculator works out how much.

Questions about insulin syringes

What does U-100 mean on a syringe?: It means the barrel is calibrated for a concentration of 100 units per millilitre, so one unit occupies 0.01 mL. It is the single human insulin standard in the United States, and it has been since U-40 and U-80 were phased out around 1979.
Is there such a thing as a U-50 syringe?: No. There is no U-50 insulin concentration. A syringe described as U-50 is almost always a 0.5 mL barrel, which holds 50 units precisely because it is calibrated U-100 and half a millilitre is half of 100 units. The barrel size and the calibration are different things.
How many units is each line on an insulin syringe?: On a 0.3 mL and a 0.5 mL barrel, one unit per line. On a 1 mL barrel it depends on the manufacturer: some mark every unit and some mark every two. Check your own barrel, because counting 2-unit lines as one unit each draws exactly half the intended dose.
Which syringe size should I use?: The smallest one that holds the whole draw. Smaller barrels carry finer and more widely spaced graduations, so the same dose is easier to read accurately on a 0.3 mL barrel than on a 1 mL one.
What happens if I use a U-40 syringe with U-100 insulin?: You deliver two and a half times the intended dose. The reverse, drawing U-40 into a U-100 syringe, delivers two and a half times too little. U-40 is still standard in veterinary insulin, which is where most people encounter it.