Syringe Units vs mL: A Quick Reference

Comparing syringe units vs mL comes down to one fact. On a U-100 syringe, each unit equals 0.01 mL, one hundredth of a milliliter, which puts exactly 100 units in 1 mL. To go from units to milliliters you divide by 100, and to go the other way you multiply by 100. 

The tables below give sample readings, three barrel sizes and the U-40 scale, followed by a few slips that turn a good reading into a bad one. 

Where the 0.01 comes from 

The U number on a syringe tells you how many units fit in one milliliter. An FDA clearance letter for an insulin syringe gives 100 units in each milliliter for U-100 and 40 for U-40. A school nurse fact sheet from the American Diabetes Association describes strength the same way, as the number of units in 1 mL, and says 100 units in 1 mL is the common one. 

Sources: FDA clearance file K233794 (2024); American Diabetes Association school nurse fact sheet (2022). 

Table 1: units to mL 

These rows assume a U-100 barrel. The last column names the smallest of the three barrel sizes in Table 3 that can hold the reading. 

A reading that is missing from the table works the same way. Divide it by 100 for milliliters, then multiply the milliliters by 1,000 for microliters, so 27 units is 0.27 mL, or 270 microliters. 

Table 2: mL to units 

Anything above 1 mL is more than one of these barrels can take, since the largest barrel in Table 3 holds 1 mL. 

Table 3: the three barrel sizes 

The printed numbers come from the comparison table in the FDA letter, and the two smaller rows describe the earlier syringe the new one was compared with. The small lines between those numbers are where makers differ. MedlinePlus says the small notches on a standard 1 mL syringe are 1 unit apart, while the Nurse Practitioner Healthcare Foundation’s 2010 guide puts 2 units between lines on a 100-unit syringe. Before reading any barrel, subtract one printed number from the next one and divide by the gaps you count in between. 

Sources: comparison table in FDA file K233794; MedlinePlus page 000660 (2024 review); the Nurse Practitioner Healthcare Foundation’s 2010 guide. 

Table 4: when the barrel says U-40 

A U-40 unit takes up two and a half times the space of a U-100 unit, so Tables 1 and 2 fit U-100 barrels only. The school nurse fact sheet gives the rule that keeps this straight: the syringe and the vial should carry the same U number. It also mentions products made at U-200, U-300 or U-500, and notes that in 2022 the one U-500 product sold in a vial had a U-500 syringe made for it. 

Sources: FDA file K233794; ADA Safe at School sheet (2022). 

Slips that the tables cannot catch 

A U-100 table read against a U-40 barrel moves the volume by a factor of 2.5, so look for the U number on the barrel or its box before anything else. 

The letters cc and mL name one volume. In the volume tables of NIST, the US measurement agency, 1 cubic centimeter equals 1 milliliter. Even so, the Institute for Safe Medication Practices (ISMP) asks people to write mL, because a handwritten cc can be mistaken for u, short for units. 

A capital U is risky on paper too. ISMP and The Joint Commission’s “Do Not Use” list both ask for the word unit, since U has been misread as a zero, as the number 4, and as cc. 

Big volumes need more than one fill. Two milliliters reads as 200 units on the U-100 scale, which is two full 1 mL barrels, so no single reading will show it. 

Sources: NIST volume units page (2026); ISMP error-prone abbreviations list (2024); Joint Commission Do Not Use list (2018 page, archived). 

Small readings are the least reliable 

Anything below 5 units is hard to measure, and a 2026 study puts numbers on that. Thirty participants, from novices to experts, measured plain sterile water into syringes, and their smallest target, 0.02 mL, is 2 units on a U-100 scale. Misses of at least 20 percent were common with the smallest volumes, people tended to overfill with larger syringe and needle pairs, and precision was highest when the smallest needle met the smallest volume. 

Source: Henning J and colleagues, JAVMA, 2026 (PMID 41512471). 

Quick answers 

Are cc and mL different volumes? No. A cubic centimeter and a milliliter are two names for one volume, as the NIST tables above show. 

How many units are in half a milliliter? On a U-100 scale, 0.5 × 100 = 50 units. On a U-40 scale, 0.5 × 40 = 20 units. 

What is 30 units in mL? On a U-100 syringe it is 30 ÷ 100 = 0.3 mL, a completely full 0.3 mL barrel. 

Does a smaller barrel change the size of a unit? No. A 0.3 mL barrel and a 1 mL barrel marked U-100 share the same 0.01 mL unit, and the small one simply stops at 30. 

Where can I see more readings? A longer chart for all three barrel sizes, with U-40 and U-500 values set next to the U-100 ones, is at https://www.peptidesvietnam.com/tools/insulin-syringe-units-chart 

This page is a general reference on syringe scales and is not medical advice, and no figure on it is a recommended amount for any person.