What the U-100 marking means, how to convert between mL and units, and where the practical precision floor sits on a small insulin syringe.
Insulin syringes are the most common tool for measuring small injectable doses outside a clinical setting. They are labelled in "units" rather than millilitres, which trips people up if they aren't familiar with the convention. This page covers what the U-100 marking actually means, how to convert between units and mL, and how to think about the precision floor of the syringe so you can match the device to the dose you are trying to measure.
This is educational background. It does not recommend any specific dose of any specific medication.
The "U" stands for units per millilitre of insulin in the original insulin labelling convention. A U-100 syringe is calibrated such that 100 units equals 1 mL. The relationship is purely a volume conversion when you're using the syringe for any solution other than insulin — there is no insulin-specific magic in the markings.
1 mL = 100 units (on a U-100 syringe)
Worked conversions:
| Volume in mL | Units on U-100 |
|---|---|
| 0.01 mL | 1 unit |
| 0.05 mL | 5 units |
| 0.10 mL | 10 units |
| 0.25 mL | 25 units |
| 0.50 mL | 50 units |
| 1.00 mL | 100 units |
Other U-scales exist (U-40 is still used in some veterinary insulin contexts), but U-100 is by far the most common in human-use insulin syringes worldwide.
Insulin syringes typically come in three barrel sizes:
Pick the smallest barrel that comfortably contains your usual dose. A 5-unit dose pulled in a 1 mL barrel is much harder to measure accurately than the same dose pulled in a 0.3 mL barrel, because the 0.3 mL barrel spreads the same volume across more physical distance on the scale.
Even with a fine-graduated syringe, the meaningful precision of a manual pull is limited by:
A rough rule of thumb: a careful manual pull on a small-barrel U-100 syringe is reliable to roughly ±1 unit. Pulls of fewer than 3–5 units are dominated by measurement noise; pulls of 20+ units are well within tolerance.
Given a target dose in mg and a concentration in mg/mL:
syringe volume (mL) = target dose (mg) / concentration (mg/mL)
syringe volume (units, U-100) = syringe volume (mL) × 100
DoseCurve does this calculation for you on the dose form when you have entered a concentration — the "syringe volume" field shows both mL and U-100 units.
Worked example: target dose 1 mg, concentration 5 mg/mL.
1 mg / 5 mg/mL = 0.20 mL = 20 units on U-100
Worked example: target dose 250 µg (0.25 mg), concentration 2 mg/mL.
0.25 mg / 2 mg/mL = 0.125 mL = 12.5 units on U-100
A 12.5-unit pull is on the boundary of what a single-unit-marked syringe can resolve. Either accept the rounding (12 or 13 units) or change the reconstitution dilution so the same dose comes out as a more comfortable pull.
The dilution volume you add to a vial during reconstitution is a free choice, and it determines whether your usual dose pulls as a clean whole number of units. A common approach is to pick a dilution that makes your standard dose pull in the 10–25 unit range:
This is purely convenience math. The total mass in the vial does not change; only the syringe pull does.
The numbers after the size — "29G x 12.7 mm", "30G x 8 mm", etc. — describe the needle gauge and length, not the barrel calibration. Gauge runs inversely to thickness: a higher number means a thinner needle. Insulin syringes typically come in 29G–31G gauges and 6–12 mm lengths.
The right gauge and length depend on the injection route (subcutaneous fat usually accepts the shortest needles; intramuscular requires longer), the viscosity of the solution and the user's tissue depth. Your prescriber and pharmacist are the right people to consult on this.