Syringes

How to read an insulin syringe: units, mL and tick marks

Insulin syringes measure volume, marked in units instead of millilitres. For insulin, a unit is also a dose. For peptides it is not: the same 10 units can hold very different amounts depending on how the vial was mixed. Reading the barrel correctly is the last step where a small slip becomes a large error.

Medically reviewed by Juan Pablo Frías, MD · 3 Oct 2026 · DisclosureUpdated 3 Oct 20264 min read

U-100 syringe: 100 units = 1 mL; 1 unit = 0.01 mL
U-40 syringe: 40 units = 1 mL; 1 unit = 0.025 mL
Common sizes: 0.3, 0.5, 1 mL; 30, 50 and 100 units on U-100
Units ≠ mg: Depends on mg/mL; Recompute for every vial.

What a "unit" is on the barrel

Insulin syringes are labelled with a scale, most often U-100. U-100 means the syringe is designed for insulin that contains 100 units per millilitre, so the barrel is marked so that 100 units equals 1 mL and each unit is 0.01 mL.

When a peptide is drawn into an insulin syringe, the unit markings are simply a volume scale. Ten units on a U-100 syringe is 0.10 mL, whatever is in it. How much peptide that 0.10 mL contains depends on the concentration, which depends on how much water was added to the vial. That is why "how many units is 2.5 mg" has no single answer; the reconstitution calculator works it out from your vial.

Syringe sizes and their markings

U-100 insulin syringe sizes
Barrel	Capacity	Typical marking step	Easiest for
0.3 mL	30 units	1 unit (some show half units)	Small volumes up to 30 units
0.5 mL	50 units	1 unit	Volumes up to 50 units
1 mL	100 units	2 units	Larger volumes up to 100 units

Marking steps vary by brand, so count the tick marks on your own barrel before relying on them. A smaller barrel spreads the same volume over a longer distance, which makes small amounts easier to read. If the volume you need is more than the barrel holds, use a larger syringe rather than drawing twice.

The needle is described separately on the package: gauge (a higher number is a thinner needle) and length. Neither changes the volume scale.

How to read the measurement

Hold the syringe at eye level with the markings facing you.
Find the edge of the plunger's rubber stopper that is closest to the needle. That edge is the reading, not the far end of the stopper and not the plunger rod.
Count the tick marks from zero to that edge, using the step your barrel is marked in.
Check for air bubbles. A bubble takes up space, so the liquid is less than the reading suggests. Tap gently and push the air out before reading again.

If you are unsure what a reading means in mL, the units to mL converter shows it for any syringe scale.

Units to mL on a U-100 syringe

U-100: 1 unit = 0.01 mL
Units	mL
5 units	0.05 mL
10 units	0.10 mL
20 units	0.20 mL
25 units	0.25 mL
30 units	0.30 mL
40 units	0.40 mL
50 units	0.50 mL
100 units	1.00 mL

For any other reading, use the units to mL converter. To go from a dose in mg or mcg to units, you need the concentration as well; use the reconstitution calculator or a peptide-specific one such as the tirzepatide calculator.

U-40 syringes: the 2.5× trap

U-40 syringes are marked so that 40 units equals 1 mL. They exist mainly for U-40 insulins, which today are used mostly in veterinary care. Their barrels can look similar to U-100 syringes, and the cap colour is not a reliable guide.

The same reading on two scales
Reading	Volume on U-100	Volume on U-40
10 units	0.10 mL	0.25 mL
20 units	0.20 mL	0.50 mL
40 units	0.40 mL	1.00 mL

Never mix scales. A unit count calculated for a U-100 syringe, drawn on a U-40 syringe, delivers 2.5 times the intended volume. Check the scale printed on the barrel or package every time you switch boxes.

Mistakes that cause dosing errors

Treating units as milligrams. A unit is a volume (0.01 mL on U-100), not an amount of peptide.
Using a chart made for a different vial size or water volume.
Reading the wrong edge of the plunger stopper.
Counting 1 mL syringe marks as single units when they are 2-unit steps.
Leaving air bubbles in the barrel.
Switching between U-100 and U-40 syringes without recalculating.

Syringes and needles are single-use. Do not share them, and dispose of them in a sharps container.

Common questions

How many mL is 10 units on an insulin syringe?

On a U-100 syringe, 10 units is 0.10 mL. On a U-40 syringe, 10 units is 0.25 mL.

Is 1 unit the same as 1 mg?

No. A unit on an insulin syringe is a volume. How many mg it holds depends on the concentration of the solution, which depends on the vial size and how much water was added.

Which syringe size should I use?

The smallest one that holds the full volume in a single draw. Smaller barrels have finer, more widely spaced markings, which makes small volumes easier to read.

Can I use a U-100 chart with a U-40 syringe?

No. The same unit reading is 2.5 times more volume on a U-40 syringe. Recalculate for the scale printed on your syringe.

Keep going

Tirzepatide · FDA approved (Mounjaro, Zepbound)Semaglutide · FDA approved (Ozempic, Wegovy, Rybelsus)Retatrutide · Investigational · phase 3

Sources

1BASAGLAR (insulin glargine) injection, prescribing information (DailyMed)U.S. Food and Drug Administration / DailyMed (Eli Lilly and Company label) · 2026dailymed.nlm.nih.govAccessed 2 Oct 2026
2Metric (SI) Prefixes (NIST Reference on Constants, Units, and Uncertainty)National Institute of Standards and Technology (NIST) · 2025physics.nist.govAccessed 2 Oct 2026

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