Details up front: 15 units · 2.5 mg/mL.
I would like to understand the steps well enough to explain them to someone else.
I have access to a refrigerator with a logger and a freezer without one, which may be relevant.
Which parts of this are load-bearing and which parts are habit?
15 units is 0.15 mL, which at 2.5 mg/mL is 0.375 mg. Work it in that order and there is nothing to guess at: units to volume on the U-100 scale, volume to dose through the concentration. Draw with the barrel vertical, expel the air before you read, and read the leading edge of the plunger stopper rather than the shoulder — the difference between the two is about a unit on a 0.3 mL barrel.
Answer first: on a U-100 syringe one unit is 0.01 mL, so units are simply volume in millilitres multiplied by one hundred. Nothing else about the scale is doing any work.
Fixed-needle insulin syringes retain roughly 3 to 5 microlitres after the plunger bottoms out. Detachable-needle luer syringes retain 35 to 100 microlitres depending on hub design — an order of magnitude more.
| Configuration | Dead volume | Loss at 5 mg/mL | Over 20 draws |
|---|---|---|---|
| Fixed-needle insulin syringe | 3–5 µL | 15–25 µg | 0.3–0.5 mg |
| Low-dead-space, detachable | <2 µL | <10 µg | <0.2 mg |
| Standard luer-lock + 30G | 35–60 µL | 175–300 µg | 3.5–6 mg |
| Luer-lock + 21G drawing needle | 70–100 µL | 350–500 µg | 7–10 mg |
Read the leading edge of the rubber plunger stopper against the graduation, not the tip of the conical shoulder. The difference is roughly one unit on a 0.3 mL barrel, which is a four per cent error on a 25-unit dose.
The 4 mm needle length is supported by injection-technique studies showing adequate subcutaneous delivery across body-mass ranges without increased intramuscular delivery.
Fixed needle, not detachable, unless you have a specific reason.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsThe short version: U-100 only, smallest barrel that fits the dose, read the leading edge of the plunger stopper.
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.
It helps to be literal here: a 2 mm air bubble in a 0.3 mL barrel is about 4 microlitres, which against a 10-unit (100 microlitre) draw is a four per cent error. Flick it to the hub and expel it before reading.
Published dead-space comparisons put low-dead-space fixed-needle designs under 2 microlitres and conventional detachable-needle designs at 35 microlitres or more.
Read the leading edge of the stopper, every time.
Start with the barrel size, because a 0.3 mL barrel graduated in single units and a 1 mL barrel graduated in twos are different instruments for this purpose.
The conversion in full: volume in mL = dose in mg ÷ concentration in mg/mL; units = volume × 100. At 5 mg/mL, a 0.25 mg dose is 0.05 mL, which is 5 units. At the same concentration a 1 mg dose is 0.2 mL, which is 20 units.
On the detail: syringes are single-use. Reusing one blunts the tip against the stopper, degrades the silicone coating and makes the graduation reading no more accurate than it was the first time.
The caveat is that this describes an instrument, not a treatment, and research-use compounds are not approved for human use.
Smallest barrel that holds the dose. Resolution is free and you should take it.
The relevant precision limit is that you cannot reliably read half a graduation, so the barrel decides the smallest change you can make.
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.
U-100 is a concentration standard meaning 100 units of insulin per millilitre; the syringe graduation is a volume scale expressed in those units.
If your arithmetic and someone else's differ by a factor of ten, one of you has misplaced a decimal rather than misunderstood the biology.
Check the syringe is U-100 and not U-40 before doing any arithmetic with it.
edited 29 Jun 2026 by orla_ferriter — added the method parameters
This is arithmetic and instrumentation, and both halves are simple once stated.
Barrel sizes and graduations: 0.3 mL barrels are marked in single units to 30, 0.5 mL barrels in single units to 50, and 1 mL barrels usually in two-unit increments to 100. The smallest barrel that holds the dose gives the finest reading.
Graduation intervals by barrel size are printed on the packaging and are the practical limit on dosing resolution.
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.
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