Reading a U-100 insulin syringe: units, millilitres and graduations

Last reviewed 26 August 2026

A U-100 scale is calibrated at one hundred units to the millilitre. One unit is therefore 0.01 ml and one millilitre is 100 units, and nothing about the liquid in the barrel changes that. What does change, barrel to barrel, is how finely the scale is printed — which is the part most conversion pages leave out. The companion tool is the reconstitution calculator.

Quick conversions

Both directions of the identity, for a U-100 barrel. For any value not in the tables: units = ml × 100 and ml = units ÷ 100.

The three questions this page is most often reached for, answered for a U-100 scale unless stated: how many units in 1 ml — 100 (40 on a U-40 barrel, 500 on U-500); what is 0.25 ml on a 100-unit syringe — 25 units; how many units is 1 mg — there is no fixed answer, because a milligram is a mass and a unit mark is a volume: the conversion depends entirely on the concentration of the solution, and it is worked through in the concentration section further down this page.

The identity, and where it comes from

“U-100” is not a syringe size. It is a concentration, and the syringe is named after it. The National Patient Safety Agency’s 2010 rapid response report on insulin states the position plainly: Current UK licensed insulin products are manufactured to have 100 units per ml where units are a standardised measure of activity. A barrel calibrated for that concentration is a U-100 barrel.

The international standard governing these devices, ISO 8537, Sterile single-use syringes, with or without needle, for insulin, covers two calibrations — 40 units per millilitre (U-40) and 100 units per millilitre (U-100) — and requires that an individual syringe be graduated and labelled for one concentration only. Its second edition was withdrawn in March 2016 and superseded by ISO 8537:2016.

From the calibration, the arithmetic is one division:

1 unit = 1 ml ÷ 100 = 0.01 ml = 10 µl

And in the other direction, 1 ml = 100 units. This is a definition rather than a measurement. It does not depend on what is in the barrel, on the manufacturer, or on the capacity of the particular syringe.

Units and millilitres, both directions

The table is a convenience. The method is ml = units ÷ 100 and units = ml × 100, which takes any value including the ones no table lists.

What a graduation actually is

Every mark on the barrel is a volume mark. The barrel is a cylinder of fixed bore, so volume is linear in plunger travel and the marks are equally spaced for that reason and no other. The number printed beside a mark is not itself a volume: it is the quantity of biological activity the calibrating liquid — insulin at 100 units per millilitre — would carry in that volume.

The NPSA report draws the same distinction from the opposite side. Insulin syringes, it says, are unlike general-purpose syringes because their markings are based on units, not volume, whereas on a millilitre-graduated syringe the graduations are in volume, not units of activity.

The unit in question is an activity unit fixed against a WHO International Standard, not a mass. Knopp and colleagues (2019) set out how its mass equivalence has been revised — 8 international units per milligram in the 1920s, 24 per milligram under the fourth International Standard of 1959, 26 per milligram from 1986. A figure redefined three times in a century is not a mass, and the number on the barrel never was one.

The clearest demonstration available: the U-500 barrel

In October 2016 Eli Lilly issued an Important Prescribing Information letter recording that errors had been reported where U-100 insulin syringes, or volumetric — for example tuberculin — syringes, had been used with a vial of a 500 unit per millilitre product. A syringe calibrated for that concentration was introduced in response.

Its specification is the interesting part. The dedicated barrel holds 0.5 ml, is numbered to 250 units, and its smallest printed mark is 5 units. Five units at 500 units per millilitre is 0.01 ml — the same volume as the 1-unit mark on a 0.5 ml U-100 barrel. Fifty marks over the same nominal capacity in both cases; only the printing differs, and between the two printings one mark means a fivefold different quantity.

That is the whole of the point. On a liquid other than the one a scale was calibrated for, the numbers stop tracking activity and the marks remain exactly what they always were: volume.

The three barrel capacities, and why capacity is not resolution

Capacity in units follows from capacity in millilitres by definition. Resolution does not follow from either — it is a decision the manufacturer made and printed on the glass.

The 1 ml barrel is commonly the coarsest of the three. The NPSA report describes it as commonly graduated in major 10 unit markings and minor 2 unit markings, and manufacturer labelling agrees: UltiMed’s U-100 range is graduated in 1-unit increments at 0.3 and 0.5 ml and in 2-unit increments at 1 ml. So a 1 ml barrel carries the same fifty printed marks as a 0.5 ml barrel, each worth twice the volume; against a half-unit 0.3 ml barrel the difference is a factor of four.

Half-unit marking is a distinct product, not a property of small barrels. embecta’s UK labelling for the BD Micro-Fine range states that a ½ unit scale marking is available on the 0.3 ml barrel, and the US device registry lists 0.3 ml devices whose recorded description includes it. A 0.3 ml barrel without that marking is printed in whole units like any other.

Reading a scale you have not measured

• Count the intervals between two printed numbers and divide. Five gaps between the marks labelled 10 and 20 means each is 2 units; ten gaps means each is 1. It is the only way to establish the interval on an unfamiliar barrel, and a habit formed on one barrel does not transfer to another.
• Read the calibration off the barrel, not off the packaging colour or the size. Capacity and calibration are separate facts and both are printed.
• Read at eye level. The scale is printed on a curved surface and viewed through it, so an off-axis line of sight displaces the apparent position of the seal against the mark.

Barrels that are not U-100

The U-100 assumption is safe in most contexts and it is not universal. The volume of one unit is fixed by the calibration and by nothing else:

U-40 barrels are in the UK supply chain. Caninsulin 40 IU/ml is a veterinary medicinal product authorised in the UK, and its Summary of Product Characteristics, revised February 2026, requires either specific U-40 sterile single-use syringes or the manufacturer’s own pen device, and states of the vial presentation that A 40 IU/ml insulin syringe should be used. On that barrel one unit is 0.025 ml — two and a half times the volume of a U-100 unit at the same printed number.

Colour is a convention, not a specification. The 2016 Lilly letter notes that the U-100 cap is orange while the dedicated U-500 syringe carries a green one. Orange and red are also widely used to distinguish U-100 from U-40. But Vyas and Sharma (2021) argued in the Journal of Diabetes Science and Technology that concentration colour coding across vials and their corresponding delivery devices is not standardised and should be, observing that orange and red caps can read as shades of one colour. Colour is a prompt to check the print, not the check.

Volumetric syringes are a different instrument. A 1 ml tuberculin-pattern syringe is graduated in millilitres, conventionally in 0.01 ml divisions, which coincide numerically with U-100 units — exactly why the two get conflated. The NPSA recorded three deaths and seventeen further incidents between January 2005 and July 2009 in which a syringe graduated in volume was used to measure a quantity expressed in units.

From a reconstituted concentration to a volume in graduations

A barrel measures volume. To use one with a solution of known concentration takes one multiplication and one division. Let C be the concentration in mg/ml, obtained as described in the reconstitution arithmetic: the mass of lyophilised solid stated on the vial label divided by the volume of diluent added.

• Mass in one U-100 graduation: C ÷ 100 milligrams, or equivalently 10 × C micrograms, because one graduation is 0.01 ml.
• Graduations per milligram: 100 ÷ C.
• Graduations for an arbitrary mass m: 100 × m ÷ C, and in millilitres, m ÷ C.

On a 1 ml barrel marked every 2 units the first column doubles, because the finest printed increment there is 0.02 ml. That is the practical reason to look at the barrel before doing the arithmetic rather than after. The relation is inverse and linear in diluent volume: doubling the diluent halves the mass in each graduation and doubles the graduations occupied by the same mass.

The line this arithmetic does not cross

C is derived from two facts that can be stated: the mass printed on the vial label, and the volume of diluent added. m is not one of those facts. It is not on the label, not in the catalogue, not in the product data, and not pre-filled anywhere in NovoVita’s calculator, whose 23 vial presets fill in the vial strength field and nothing else. The position is deliberate and it is the same one the calculator page states: the volume is ours to calculate, the target amount is not.

What the barrel will not resolve

Two limits bracket every reading and a third sits outside the barrel entirely. All three are properties of the instrument.

The ceiling is capacity. A volume larger than the barrel cannot be measured on it. At a given concentration the largest mass a barrel will hold is C multiplied by its capacity in millilitres.

The floor is the reading itself. Casella and colleagues (1993) measured delivered volumes gravimetrically from 0.3 and 0.5 ml U-100 syringes, reported individual coefficients of variation between 5 and 33 per cent among the non-professional operators, and concluded that deliveries below 20 µl — two graduations on a U-100 scale — carried an unacceptably large error. Silva and colleagues (1996), using the same gravimetric approach, reported a consistent over-draw averaging 0.22 units across all targets, and that among the caretakers studied, where two or more people shared responsibility for one measurement, the combined variability was approximately 40 per cent greater than for one individual.

Dead space is volume the scale never sees — liquid retained in the hub and needle after the plunger has bottomed. Arnott and colleagues (1982) examined U-100 syringes of differing designs and reported errors exceeding 100 per cent at low volumes, with poor reproducibility, where a dead-space design was used to draw two liquids in sequence. For scale: ISO 7886-1, which governs general-purpose hypodermic syringes rather than insulin syringes, permits a maximum dead space of 0.07 ml below 5 ml nominal capacity (Krisdiyanto and colleagues, 2022) — seven graduations on a U-100 scale. That review found the lowest dead space among the devices it surveyed in a 1 ml insulin syringe, which is the consequence of a permanently attached needle.

The scope of these reports

All three studies measured a device and an operator, gravimetrically, using insulin at 100 units per millilitre as the liquid. They are statements about how accurately a small volume can be drawn from a graduated barrel. They are not statements about any compound, and nothing in them transfers to one.

Writing the figure down

The notation has its own documented failure mode. The NPSA report records that abbreviating ‘units’ to ‘U’ has produced tenfold errors, because a ‘U’ beside a figure can be read as an additional zero; that ‘IU’ has been misread as ‘10’ or ‘0’; and that ‘10units’ closed up has been read as ‘100nits’. It notes that the British National Formulary specifies the word should not be abbreviated, and recommends a lower-case ‘units’ written in full with a space after the figure. Between August 2003 and August 2009 the agency received 3,881 wrong-quantity reports involving insulin, of which one death and one severe-harm incident were attributed to the abbreviation itself.

Nothing about that is specific to a clinical setting: a figure in a laboratory notebook is read later by someone who was not there when it was written, which is the condition under which those errors occurred.

Seven things about the barrel that are commonly misread

1. “One unit is 0.01 ml on any insulin syringe.” Only on a U-100 scale. On U-40 it is 0.025 ml; on U-500, 0.002 ml.
2. “A 1 ml barrel resolves 0.01 ml.” It is commonly printed in 2-unit increments, so the finest mark is 0.02 ml — twice the increment of a 0.5 ml barrel and four times that of a half-unit 0.3 ml barrel.
3. “A unit is a quantity of substance.” It is a volume mark labelled with the activity the calibrating liquid would carry in that volume, and the mass equivalence of the insulin unit has been revised three times since the 1920s.
4. “A tuberculin syringe is an insulin syringe with millilitres printed on it.” It is a volumetric instrument. On a 1 ml, 0.01 ml-division barrel the numbers coincide and the quantities do not, and confusion between the two is documented in the NPSA incident record.
5. “The cap colour tells you the calibration.” Colour is a manufacturer convention, and a 2021 letter in the Journal of Diabetes Science and Technology argued that concentration colour coding is not standardised across vials and devices and ought to be.
6. “The smallest mark is the smallest measurable quantity.” It is the smallest printed quantity. The smallest reliably measurable one is larger — put at two graduations in the gravimetric literature — and dead space removes a further volume the scale never accounts for.
7. “The barrel tells you what is in it.” It measures volume. It says nothing about identity, composition or concentration, and it cannot distinguish one clear solution from another.

The diluent makes no difference to any of this either. Bacteriostatic water and sterile water differ in composition and in container designation, and neither difference enters a volume measurement.

References

1. National Patient Safety Agency. Rapid Response Report NPSA/2010/RRR013: Safer administration of insulin. June 2010. UK safety guidance; the passages quoted above are from the report’s accompanying background document, and are the source of the 100 units/ml statement, the 1 ml barrel graduation description, the abbreviation recommendations and the incident figures.
2. International Organization for Standardization. ISO 8537, Sterile single-use syringes, with or without needle, for insulin. Standards document. Second edition published 2007-10, withdrawn 2016-03-21 and superseded by ISO 8537:2016. Scope covers U-40 and U-100 calibrations.
3. Eli Lilly and Company. Important Prescribing Information letter concerning the dedicated U-500 syringe, October 2016. Regulatory communication; source of the dedicated barrel’s capacity, numbering and 5-unit smallest mark, and of the cap colours.
4. Veterinary Medicines Directorate. Caninsulin 40 IU/ml — Summary of Product Characteristics. AN 00541/2025, revised February 2026. UK regulatory document; source of the 40 IU/ml strength and the U-40 syringe requirement.
5. Knopp JL, Holder-Pearson L, Chase JG. Insulin units and conversion factors: a story of truth, boots, and faster half-truths. J Diabetes Sci Technol. 2019;13(3):597–600. PMID 30318910. DOI 10.1177/1932296818805074. Commentary.
6. Vyas V, Sharma A. An urgent need to color code vials and the corresponding delivery devices for the concentration of insulin. J Diabetes Sci Technol. 2021;15(1):186–187. PMID 32723085. DOI 10.1177/1932296820943871. Letter.
7. Casella SJ, Mongilio MK, Plotnick LP, Hesterberg MP, Long CA. Accuracy and precision of low-dose insulin administration. Pediatrics. 1993;91(6):1155–7. PMID 8502520. Gravimetric measurement of delivered volume by human operators.
8. Silva SR, Clark L, Goodman SN, Plotnick LP. Can caretakers of children with IDDM accurately measure small insulin doses and dose changes? Diabetes Care. 1996;19(1):56–9. PMID 8720535. DOI 10.2337/diacare.19.1.56. Gravimetric measurement of delivered volume by human operators.
9. Arnott RD, Cameron MA, Stepanas TV, Cohen M. Insulin syringes: dangers of dead space. Med J Aust. 1982;2(1):39–40. PMID 7050644. DOI 10.5694/j.1326-5377.1982.tb124209.x. Device comparison.
10. Krisdiyanto, Raja Ghazilla RAB, Azuddin M, Ahmad Hairuddin MKF, Muflikhun MA, Risdiana N, Afifuddin E. The hypodermic syringe performance based on the ISO 7886-1:2017: a narrative review. Medicine (Baltimore). 2022;101(49):e31812. PMID 36626504. PMCID PMC9750608. DOI 10.1097/MD.0000000000031812. Narrative review; source of the ISO 7886-1 dead space limit.
11. embecta. BD Micro-Fine + insulin syringes, UK product information. Manufacturer labelling; source of the ½ unit scale marking on the 0.3 ml barrel.
12. UltiMed, Inc. UltiCare U-100 insulin syringes, product information. Manufacturer labelling; source of the 1-unit increments at 0.3 and 0.5 ml and 2-unit increments at 1 ml.
13. US Food and Drug Administration, AccessGUDID. Device record B2086345030, a 0.3 ml insulin syringe recorded with half-unit marking. Device registry record.