U-100 vs U-40 Insulin Syringes

Insulin syringes come in two scales, U-100 and U-40. The "U" number is a concentration — units per millilitre — and it sets what every marking on the barrel means. Reading a draw on the wrong scale changes the number without changing the liquid.

What the U number means

The scale says how many units fit in one millilitre. It comes from insulin strength, but the marks are really just a unit-to-volume ruler, so the same idea applies to any liquid you measure in a syringe.

Syringe scales

U-100100 units = 1 mL · 1 unit = 0.01 mL

U-4040 units = 1 mL · 1 unit = 0.025 mL

U-100 is by far the most common today. U-40 is an older, lower-strength scale still seen on some veterinary insulin syringes, which is usually where the mix-up starts.

Why it matters for a peptide draw

Reconstitution math works in volume: concentration is the amount in the vial divided by the water you add, and a dose is a volume of that solution. The syringe scale only decides which number that volume lines up with. On a U-100 syringe, 0.1 mL is the "10 units" mark; on a U-40 syringe the same 0.1 mL sits at the "4 units" mark. Same liquid, different label — so a figure written in U-100 units and read on a U-40 barrel is the wrong volume.

Converting between the two

Because the real quantity is the volume, convert through millilitres. To land on the same volume, U-40 units are the U-100 units multiplied by 40 ÷ 100 — that is, × 0.4.

Same volume, two scales

Volume0.20 mL

On a U-100 syringe20 units

On a U-40 syringe8 units

The calculator assumes U-100

The reconstitution and dosage calculators report units on the standard U-100 scale, where 10 units equals 0.1 mL. If your syringe is a U-40, work in millilitres from the volume the calculator gives, or read against a U-100 barrel so the units match.

Open the reconstitution calculator

Frequently asked questions

What does U-100 mean on an insulin syringe?

U-100 is a scale of 100 units per millilitre. Each unit is 0.01 mL, 10 units is 0.1 mL, and 100 units fills a full 1 mL. It is the standard insulin-syringe scale in most countries.

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

They use different unit-per-millilitre scales: 100 units per mL for U-100, 40 units per mL for U-40. The same volume of liquid therefore lines up with a different unit number on each barrel, so the two are not interchangeable without converting.

How do I convert U-100 units to U-40 units?

Convert through volume. Multiply a U-100 unit figure by 40 ÷ 100 (that is, by 0.4) to get the U-40 units for the same volume. For example, 20 units on a U-100 syringe is 0.20 mL, which reads as 8 units on a U-40 syringe. Always confirm the volume in millilitres independently.

Which syringe scale does the calculator use?

The reconstitution and dosage calculators report units on the U-100 scale, where 10 units equals 0.1 mL. If you use a U-40 syringe, work from the volume in millilitres rather than the U-100 unit figure.