Why does a U-100 insulin syringe read differently from the volume I thought I drew at 2.5 mg/mL?

Details up front: a U-100 insulin syringe · 2.5 mg/mL.

I have a result I cannot explain, and I would rather diagnose it than guess.

I have checked the obvious explanations and eliminated the two easiest ones.

What is the most likely explanation, and how would I confirm it?

At 2.5 mg/mL a 1 mg dose is 0.4 mL, which on a U-100 insulin syringe is 40 units — and that is the number the barrel is showing you, not the number you were thinking in. Three things separate the two. The scale: a barrel graduated in units is graduated in volume, so every dose has to pass through 2.5 mg/mL to become a mark. The meniscus: read the leading edge of the plunger stopper, not the shoulder — on a narrow barrel the two are about a unit apart. And the dead space: what the barrel reads is what is in the barrel, while what leaves it is that minus whatever stays in the hub and the needle. Write 2.5 mg/mL and the units-per-milligram on the vial at reconstitution and the arithmetic stops happening at the point of use.

The relevant precision limit is that you cannot reliably read half a graduation, so the barrel decides the smallest change you can make.

U-40 syringes still exist, mostly in veterinary supply. A U-40 barrel marked "20 units" holds 0.5 mL, not 0.2 mL. Using one with U-100 arithmetic overdoses by a factor of two and a half.

Needle length for subcutaneous administration is conventionally 4 to 8 mm; the 4 mm length is sufficient for essentially all adults and reduces the chance of an intramuscular injection.

Half-graduation estimation is not measurement. If the dose needs it, change the concentration instead.

One unit is 0.01 mL on U-100. That is the whole scale.

edited 27 Jul 2026 by tare_weight — removed a claim I could not source