U-100 vs U-50 vs U-40 Syringes Explained

The short version

Two of these are the same scale in different sizes. One is a different scale altogether. Mixing up the first two is harmless. Mixing up U-100 and U-40 changes every volume by a factor of two and a half.

What the U number means

Insulin is sold by strength: U-100 insulin contains 100 units of insulin in each millilitre. An insulin syringe is printed to match. On a U-100 syringe, the scale divides 1 mL into 100 units, so a person drawing 20 units of U-100 insulin pulls the plunger to 20.

The U number on a syringe is therefore a statement about its ruler: how many unit marks fit into one millilitre. It says nothing about needle length, gauge or barrel size.

For anything other than insulin, including a reconstituted peptide, the units are simply a fine ruler for volume. What that volume contains depends on the vial’s concentration, which how to read syringe units covers.

U-100: one scale, three barrels

U-100 is the standard, and it is the scale every calculator on this site displays. It comes in three common barrel sizes.

One unit is 0.01 mL on all three. The smaller barrels do not measure differently. They are narrower, which spreads the same scale further along the barrel, so the lines sit further apart and small volumes are easier to see.

That matters more than it sounds. A hospital pharmacy study of syringe accuracy found that errors above 5% became more likely when less than 20% of a syringe’s labelled capacity was being measured. Ten units is 10% of a 1 mL barrel but a third of a 0.3 mL one.

“U-50”: usually a nickname, not a scale

Here is the source of most of the confusion. In shop listings and forum posts, “U-50” almost always means the 0.5 mL U-100 syringe, named after the 50 units it holds. By the same habit, a 0.3 mL syringe sometimes gets called “U-30”.

These are U-100 syringes. Their units are the same 0.01 mL. A result of 10 units means the same volume on a “U-50”, a “U-30” and a 1 mL U-100 barrel. In the peptide calculator you would simply pick the 0.5 mL barrel.

The way to be certain is to ignore the nickname and read the box. If the packaging says U-100 and 0.5 mL, that is what you have.

U-40: a genuinely different scale

A U-40 syringe is printed for insulin at 40 units per mL. Its scale divides 1 mL into only 40 units.

Each U-40 unit is two and a half times the volume of a U-100 unit. In the United States, U-40 syringes are mostly sold for veterinary insulin, which is why they turn up in pet supply shops and online marketplaces next to U-100 ones.

The danger is that both are “insulin syringes”, both are marked in “units”, and the barrels look alike. The number on the barrel is the only thing that separates them.

Converting between the scales

Never convert units to units directly. Go through millilitres, which mean the same thing on every syringe.

Now read the table sideways to see the mistake. A calculation that says “10 units”, done for U-100, is 0.1 mL. Pull a U-40 plunger to 10 and you have 0.25 mL: two and a half times the intended volume. Going the other way gives 40% of it.

These volumes are arbitrary and illustrate the scales only.

The cap colour is a hint, not proof

Insulin syringes are colour coded. U-100 syringes normally carry an orange needle cap and U-40 syringes a red one. Manufacturers’ FDA filings describe this as colour marking in accordance with ISO 8537, the international standard for insulin syringes, “to distinguish the insulin concentration”.

It is a useful first check and a poor last one. Orange and red are close, caps get separated from syringes, and not every product follows the convention. The scale printed on the barrel and on the box is what counts.

U-500, and why the scale is everything

One more scale shows how seriously this is taken. Concentrated U-500 insulin is five times the strength of U-100. In 2016 the FDA cleared a dedicated U-500 syringe for it: a 0.5 mL barrel with a green shield, on which each line measures 5 units of U-500 insulin.

The reason is in that insulin’s own labelling, which warns that measuring it with a U-100 syringe “can give you a 5 times overdose”. Same liquid, same barrel volume, wrong ruler.

You are unlikely to meet a U-500 syringe. The point is the principle: a unit means nothing until you know which scale it belongs to.

Tuberculin syringes: no units at all

A 1 mL tuberculin syringe is a similar size but is marked in millilitres, in steps of 0.01 mL, with no unit scale. A calculator result in mL can be read on it directly. A result in units cannot, without dividing by 100 first.

What the calculators assume

Every calculator here shows U-100 units, and lets you pick the 0.3 mL, 0.5 mL or 1 mL barrel so the illustration and the capacity check match what is in your hand. They also show the volume in mL, which is the figure to use for any other scale.

• For a volume in mL and U-100 units from an amount you enter, use the peptide dosage calculator.
• For the whole chain from vial to syringe, use the peptide calculator.

Neither suggests an amount. They convert numbers you supply.

Write the syringe down too

A logged mark is only meaningful with its scale. “10 units” in a record means 0.1 mL if the syringe was a U-100 and 0.25 mL if it was a U-40. Noting the syringe type once per box removes the doubt.

Peptrack keeps each vial’s concentration with its log, so an entry can always be traced back from a mark to a volume to an amount. The peptide reconstitution guide puts syringes in context with vial labels, diluents and the math.

Sources

Factual statements in this article come from these primary sources, last checked in September 2026.

• FDA 510(k) K253068: insulin syringes, U-40 (red cap) and U-100 (orange cap), colour marking per ISO 8537
• FDA 510(k) K151870: BD U-500 insulin syringe (2016)
• DailyMed: Humulin R U-500 label (Eli Lilly), Instructions for Use
• Jordan MA et al. Development of guidelines for accurate measurement of small volume parenteral products using syringes. Hospital Pharmacy 2021;56(3):165-171