Reference
They are all U-100.
The three sizes differ in how much they hold and how finely they read. They do not differ in calibration, and the widely repeated U-50 and U-30 do not exist.
- Sizes
- 3
- Calibration
- U-100
- One unit
- 0.01 ml
- Cost
- Free
The science
Capacity is not calibration
01
Every insulin syringe sold is U-100. That is the calibration, and it means one hundred units equals one millilitre, so a single unit is 0.01 ml. It is a property of the scale, not of the syringe, and it does not change between sizes.
02
What changes between sizes is capacity. A 1 ml barrel holds 100 units, a 0.5 ml barrel holds 50, a 0.3 ml barrel holds 30. All three count in the same units of the same size. There is no such thing as a U-50 or U-30 syringe, and calling them that implies a different scale that does not exist.
03
The practical difference is spacing. The 0.5 ml and 0.3 ml barrels print the same hundred-units-per-millilitre scale across a shorter tube, so the marks sit further apart and individual units become easier to see. A 1 ml barrel typically prints a mark every 2 units for the same reason, since printing every single unit would crowd them.
04
That is the whole decision. A smaller barrel reads more finely and runs out sooner. If your dose is 8 units, a 0.3 ml syringe will show it more clearly than a 1 ml one. If your dose is 60 units, the 0.3 ml syringe cannot hold it at all.
FAQ
Common questions
What size insulin syringes are there?
Three common capacities: 1 ml holding 100 units, 0.5 ml holding 50 units, and 0.3 ml holding 30 units. All three are U-100 calibrated, meaning one unit is 0.01 ml on every one of them. The capacity differs; the scale does not.
Is there such a thing as a U-50 or U-30 syringe?
No. Those are not real designations, though they appear often enough online to be worth correcting. U-100 describes the calibration, which is 100 units per millilitre, and every insulin syringe uses it. A 0.5 ml syringe is a U-100 syringe that holds 50 units.
Which syringe size should I use?
The smallest barrel your dose fits in. A shorter barrel spreads the same hundred-units-per-millilitre scale over less length, so the marks are further apart and a small dose is easier to read accurately. The only constraint is that the dose has to fit.
How many units is 0.1 ml?
Ten units, on any insulin syringe. Because they are all U-100, 0.1 ml is 10 units whether the barrel holds 30, 50 or 100 units in total. This is the point that the U-50 and U-30 confusion obscures.
Why does my 1 ml syringe only have marks every 2 units?
Because printing a hundred individual marks along that length would crowd them to the point of being unreadable. A 1 ml barrel commonly carries a graduation every 2 units. That is worth knowing before you aim for an odd number, since 7 units sits between marks on a barrel like that.
Does syringe size change the dose?
No. It changes only how precisely you can measure it and whether it fits. The same 0.2 ml is 20 units on all three sizes. A smaller barrel does not make a dose smaller, it makes the same dose easier to see.
Sources
Where this comes from
Regulatory status and approved dosing are quoted from the primary source rather than summarised from memory. Each link below says what it is being cited for.
- BACTERIOSTATIC WATER prescribing informationDailyMed, U.S. National Library of MedicineMultiple-dose vial handling, which is what the syringe is drawing from.Link checked 1 October 2026
- DailyMedU.S. National Library of MedicineCurrent FDA-approved prescribing information, which is the authority on any approved dose.Link checked 1 October 2026
VialClock
This page calculates. Your notes app forgets.
Every tool here is stateless by design. What it cannot do is remember that your vial was mixed eleven days ago, or that a step-up is due this week.
This tool performs arithmetic on figures you enter, or presents a manufacturer-published schedule. It does not know your protocol, will never suggest a dose, and is not medical advice. Dosing decisions belong with a qualified prescriber.