Insulin syringes are marked in units, not mL. On a U-100 syringe, 100 units = 1 mL (1 unit = 0.01 mL). Always confirm U-100 vs U-40, mixing the two misreads every draw by 2.5×.
Every reconstitution calculation ends at the same place: a tiny volume read off the tick marks of a syringe. That final step is where most measurement drift is introduced, because syringes are marked in units on more than one scale, and the marks are small. Getting the reconstitution math right and then misreading the barrel undoes all of it.
Units are not milliliters
Insulin syringes are marked in units. On the standard U-100 scale, 100 units equals 1 mL, so a single unit is 0.01 mL. That is why a 0.10 mL draw lands on the “10” mark. If you think in mL but read a barrel marked in units, you are off by a factor of 100, an error too large to miss, but easy to introduce when converting quickly.
Barrel size changes your resolution
U-100 syringes come in several barrel sizes, and the size you choose sets how finely you can read a small volume:
For a small draw like 10 units, a 0.3 mL barrel spreads those units across more physical space, so the mark is easier to hit precisely. Reading 10 units on a 1 mL barrel is possible but visually tighter. When a volume is small, sizing the barrel down improves precision.
The U-100 vs. U-40 trap
Not every insulin syringe is U-100. Older and veterinary U-40 syringes exist, where 40 units equals 1 mL. Mix the scales up and every reading is off by a factor of 2.5, the most dangerous kind of error, because the number still looks reasonable. Before anything else, confirm the barrel is U-100, and record that it was. If you ever switch syringe types, the switch itself is a log-worthy event.
Reading between the marks
When a target falls between two ticks, resist eyeballing a fraction of a mark on a coarse barrel. It is more reliable to change the concentration (via diluent volume) or the barrel size so the draw lands on a whole mark. Clean draws on whole marks are easier to reproduce and easier to document than “halfway between 7 and 8.”
Record the syringe, not just the number
A unit count is meaningless in a log without the scale it was read on. “10 units” on U-100 and “10 units” on U-40 are different volumes entirely. A complete entry pairs the number with the hardware:
- Syringe scale (U-100 vs. U-40), confirmed, not assumed
- Barrel size (0.3 / 0.5 / 1.0 mL)
- The unit mark drawn to
- The mL that corresponds to
This closes the loop with the reconstitution math: the mg on the label flows to a concentration, to a volume, to a unit mark, on a specific, recorded syringe.
Let the tool track the scale
The cleanest way to remove scale mismatch is to have PepSync’s calculator know which syringe you use and show the unit mark for that barrel. Then the number on screen and the number on the plunger are the same thing, every time.
Frequently asked questions
Is an insulin syringe measured in mL or units?
In units. On a U-100 syringe, 100 units equals 1 mL, so each unit is 0.01 mL. To convert a volume in mL to units, multiply by 100.
What is the difference between U-100 and U-40 syringes?
A U-100 syringe holds 100 units per mL; a U-40 syringe holds 40 units per mL. Reading a volume on the wrong scale changes the amount by a factor of 2.5, so always confirm which one you have.
Which insulin syringe size is best for small volumes?
A smaller barrel such as 0.3 mL spreads the same units across more physical space, so the tick marks are easier to read precisely for small draws.