How to Read an Insulin Syringe and Convert Units to mL
Learning how to read an insulin syringe takes one sum and one habit. The sum is that 100 units make 1 mL on a U-100 syringe. So one unit equals 0.01 mL, and you divide any reading by 100 to get mL. The habit is checking what each small line on the barrel is worth before you trust the number you see.
Both matter on the ward, because a scale read the wrong way still gives a number that looks right.
The units are a ruler for volume
Most syringes are marked in mL, and the insulin syringe is the odd one out. A health care math textbook from eCampusOntario says it plainly: these syringes measure units, not mL.
A US FDA clearance letter for one syringe defines the two scales. U-100 means 100 units in every mL, and U-40 means 40 units in every mL. MedlinePlus, the health site of the US National Library of Medicine, says most of these syringes are marked for U-100.
On U-100, one unit is 1 ÷ 100 = 0.01 mL, so 35 units is 0.35 mL. Some labels use cc instead of mL. NIST, the US measurement agency, describes the milliliter as a special name for the cubic centimeter. So 1 cc and 1 mL are the same amount.
Sources: LePine, Medical Mathematics 5.6 (eCampusOntario, 2024); FDA 510(k) letter K233794 (2024); MedlinePlus 000660 (reviewed July 2024); NIST, SI Units: Volume (2026).
Check which scale you are holding
U-40 is the scale to watch for. Each U-40 unit is 0.025 mL (1 ÷ 40), so 10 units on a U-40 barrel is 0.25 mL. That is the volume of 25 units on a U-100 barrel. Mix up the two scales and the volume is off by a factor of 2.5.
In the FDA letter, the maker caps its U-40 syringes in red and its U-100 ones in orange, as does the syringe it was compared with. One file is not a rule for every brand, so check the U number printed on the barrel or box. An American Diabetes Association fact sheet for school nurses adds the matching rule: the U number on the syringe should equal the one on the vial.
Sources: FDA letter K233794, device description and comparison table; American Diabetes Association, Safe at School fact sheet (August 2022).
Find what one line is worth
Patient guides do not agree about the small lines. On a standard 1 mL syringe, MedlinePlus says, every small notch stands for 1 unit. A 2010 patient guide from the Nurse Practitioner Healthcare Foundation says each line on a 100-unit syringe marks 2 units. A handout from Learning About Diabetes, Inc. sums it up: “the lines on different syringes may not mean the same thing.”
So measure the line value instead of guessing it. Pick two printed numbers that sit side by side, find the gap between them, and divide that gap by the number of spaces. With a number at every 10 units and 5 spaces between numbers, each line is 10 ÷ 5 = 2 units. With 10 spaces, each line is 1 unit.
Sources: MedlinePlus 000660; Nurse Practitioner Healthcare Foundation, patient guide to syringes and pens (2010); Learning About Diabetes, Inc., Reading a Syringe handout.
Read the plunger at the needle end
The rubber tip of the plunger touches the barrel at two edges, and only one of them marks the volume. The eCampusOntario textbook says to read the edge nearest the tip of the syringe, where the needle is, and not the edge near your fingers. It also warns that bubbles in the barrel make the reading wrong.
Source: Mike LePine, Medical Mathematics, section 5.6, Reading Syringes.
Two worked readings
The first practice barrel is a 0.3 mL syringe, which holds 0.3 × 100 = 30 units. Say it has a number at every 5 units and 5 spaces between numbers, so each line is 5 ÷ 5 = 1 unit. If the needle-side edge of the plunger sits 2 lines past the printed 10, the reading is 10 + 2 = 12 units. That is 12 ÷ 100 = 0.12 mL.
The second is a 1 mL size, which takes 100 units. Say it has a number at every 10 units and 5 spaces between numbers, so each line is 2 units. If the edge sits 3 lines past the printed 70, the reading is 70 + (3 × 2) = 76 units, or 0.76 mL. Count those 3 lines as 1 unit each and you get 73 units, which is 3 units short.
Small volumes are the hardest to get right
Any reading below 5 units, or 0.05 mL, is hard to measure well. A 2026 study shows how rough they get. Thirty people, grouped as novices, beginners and experts, filled 180 syringes with sterile water. They used three syringe and needle pairs: a 0.5 mL insulin syringe and two 1 mL syringes with larger needles. Their targets were 0.02 mL, 0.12 mL and 0.43 mL, which are 2, 12 and 43 units on a U-100 scale.
A miss of 20 percent or worse happened often, mostly at the smallest volumes and mostly with the larger needles. People tended to overfill with the bigger syringe and needle pairs. Experience was no promise of accuracy. The beginners were nearly twice as likely as the experts to land within 20 percent (odds ratio 2.29).
A 20 percent miss on 0.02 mL is 0.02 × 0.2 = 0.004 mL. That is 0.4 of a unit, less than half the space between two 1-unit lines. The study used plain water, so it tests hands and syringes, not any medicine.
Source: Henning J, Boshammer E, Herndon A. J Am Vet Med Assoc. 2026;264(5):599 to 606. PMID 41512471.
Write the reading so no one misreads it
A correct reading can still become an error on paper. The Institute for Safe Medication Practices (ISMP) keeps a list of abbreviations and number styles that were misread in harmful or potentially harmful medication errors. Three items on its 2024 list apply to every syringe reading.
First, write unit or units in full. ISMP says U has been read as a zero or a 4, so 4U can look like 40. Second, write mL, with a small m and a capital L, instead of cc, because cc can be mistaken for u. Third, guard the decimal point. Put a zero in front of it when the amount is under one, so 0.12 mL and never .12 mL. Never add a zero after a whole number, so 5 mL and never 5.0 mL. If the point in .12 mL fades, the note reads 12 mL, a hundred times the volume.
ISMP marks the U rule and both zero rules as also on The Joint Commission’s official “Do Not Use” list. That list allows a trailing zero only to show how precise a reported value is, such as a lab result or a tube size. It never allows one in medication orders.
Sources: ISMP list of error-prone abbreviations and symbols (2024 edition); The Joint Commission, official “Do Not Use” list (page dated September 14, 2018, archived).
Summary
Every 100 units on a U-100 syringe is 1 mL, so dividing a reading by 100 gives mL. Check the U number, find the line value, and read the plunger edge nearest the needle. Count readings under 5 units as rough, and record them with units in full, mL rather than cc, and a leading zero. For a longer table, Peptides Vietnam keeps a units to mL chart covering the three barrel sizes, plus U-40 and U-500 marks.
This is general education about reading and writing down a printed scale. It is not medical advice, and none of the readings is a suggestion for anyone’s use.
