How to Read an Insulin Syringe

You can stop squinting at tiny printed lines. This guide shows you how to read insulin syringe markings on a U-100 barrel. On that barrel, 100 units equal 1 mL, and 1 unit equals 0.01 mL, as shown by ADA: Overview of Insulin Concentration for School Nurses (opens in a new tab) and MedlinePlus: Giving an insulin injection (opens in a new tab).

Overhead view of a man writing calculations in a paper notebook beside a calculator and a propped-up phone on a kitchen table

Interactive scale

12 units 0.12 mL

Example reading: 12 units = 0.12 mL on a U-100 syringe. This is a way to read a scale, not an amount to take. Check the numbers printed on your own syringe.

The math

Formula: mL = units / units per mL. A U-100 syringe has 100 units per mL, a U-40 syringe has 40 units per mL.

Math based on your entries. Check it against the print on your syringe. Not medical advice.

Units to mL on a U-100 1 mL syringe

Units	mL
10	0.1
20	0.2
30	0.3
40	0.4
50	0.5
60	0.6
70	0.7
80	0.8
90	0.9
100	1

Read it in 4 steps

Use the printed numbers first. The lines between them are the scale on your own barrel.

Find the numbers printed on the barrel.
Count the lines between two printed numbers.
Divide the difference by that line count.
Line up the plunger tip with the line, and check the U number on your syringe and vial label.

U-100 vial instructions say to use the tip of the plunger at the line for the number of units. Line up the tip of the plunger with that line. Follow your syringe's instructions on the label and in the package insert at FDA: instructions for use on a U-100 insulin vial label (opens in a new tab). If you need help beyond reading the marks and doing the arithmetic, follow the label and ask your pharmacist or prescriber.

Example reading: the tip of the plunger at the 12 line means 12 units, which is 0.12 mL.

Example reading: a plunger tip halfway between the 12 and 13 lines is between 12 and 13 units. Write it down as between the 12 and 13 lines instead of guessing a fraction.

Treat these as examples of reading the marks. You should not read them as an amount to take.

Units to mL chart

Read across one row at a time. You should match the unit number to the mL value for that same concentration.

If you want the same arithmetic with your own numbers, the reconstitution calculator does the conversion on screen and shows the scale without suggesting an amount.

What one line means on each barrel

There is no one rule for every syringe. Count the lines on your own barrel, and let the printed numbers tell you the step.

A larger barrel does not change the volume at a printed unit mark on a U-100 scale. You just see the scale spread out more.

Units are volume, not strength

Units on the syringe describe volume on that syringe. They do not tell you how much of anything is dissolved in the liquid. The mg per mL on your vial label is the part that tells you concentration.

That mix-up matters to check. ISMP says units have been mistaken for milliliters, and that U-100 has also been misread as 100 units per vial at ISMP: Guidelines for subcutaneous insulin use (opens in a new tab). For label math, use a units to mg converter or go the other way with mg to units.

Check the U number too. U-100 means 100 units in 1 mL according to ADA: Overview of Insulin Concentration for School Nurses (opens in a new tab) and MedlinePlus: Giving an insulin injection (opens in a new tab). U-40 means 40 units in 1 mL according to DailyMed: veterinary U-40 insulin label (opens in a new tab). The syringe and the vial label need the same U number.

Print the chart

Use the printable syringe chart (PDF) if you want the scale on paper. You can also Download the free chart or Open the calculator.

Syringe scales drawn to size

A 1 mL U-100 syringe scale with a magnifier on the lines between 18 and 32 units: long numbered lines every 10 units, middle lines every 5, short lines every unit. Example notches at 23 units, 0.23 mL.

A 0.3 mL U-100 syringe drawn empty with its printed scale from 0 to 30 units, numbers every 5 units, mL equivalents below, and an empty field to write your own mark.

A 0.5 mL U-100 syringe drawn empty with its printed scale from 0 to 50 units, numbers every 5 units, mL equivalents below, and an empty field to write your own mark.

A 1 mL U-100 syringe drawn empty with its printed scale from 0 to 100 units, numbers every 10 units, mL equivalents below, and an empty field to write your own mark.

Example numbers only. Check the numbers printed on your own syringe.

Questions

Is 10 units equal to 1 mL?

No. On a U-100 insulin syringe, 100 units equal 1 mL, so 10 units equal 0.1 mL. That matches ADA: Overview of Insulin Concentration for School Nurses (opens in a new tab) and MedlinePlus: Giving an insulin injection (opens in a new tab). On a U-40 syringe, 40 units equal 1 mL, so 10 units equal 0.25 mL, according to DailyMed: veterinary U-40 insulin label (opens in a new tab).

How much is 20 units in an insulin syringe?

On a U-100 syringe, 20 units is 0.2 mL from the U-100 scale of 100 units per 1 mL at ADA: Overview of Insulin Concentration for School Nurses (opens in a new tab). The same mark on a 0.3 mL, 0.5 mL, or 1 mL barrel is the same volume. Barrel size only changes how large the scale looks.

How many mL is in 1 unit of insulin?

On a U-100 syringe, 1 unit is 0.01 mL because 100 units equal 1 mL at MedlinePlus: Giving an insulin injection (opens in a new tab). On a U-40 syringe, 1 unit is 0.025 mL because 40 units equal 1 mL at DailyMed: veterinary U-40 insulin label (opens in a new tab).

Read this first. Information only, not medical advice. This site does not recommend amounts. Talk to your prescriber or pharmacist about your own situation. 18+.

Editorial review against cited sources. Last checked Oct 12, 2026.

Sources

ADA: Overview of Insulin Concentration for School Nurses (opens in a new tab), American Diabetes Association. August 2022.
MedlinePlus: Giving an insulin injection (opens in a new tab), MedlinePlus, U.S. National Library of Medicine (NIH). Reviewed 07/21/2024.
FDA: instructions for use on a U-100 insulin vial label (opens in a new tab), U.S. Food and Drug Administration (Drugs@FDA label; sponsor Eli Lilly). Revised 06/2022.
ISMP: Guidelines for subcutaneous insulin use (opens in a new tab), Institute for Safe Medication Practices (ISMP). Copyright 2017 (PDF posted 2018).
DailyMed: veterinary U-40 insulin label (opens in a new tab), Merck Animal Health, FDA-approved veterinary label, hosted by DailyMed (U.S. National Library of Medicine). Revised 09/2026 (DailyMed effective 09/23/2026).