Details up front: 15 units · 10 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 10 mg/mL is 1.5 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.
The short version: U-100 only, smallest barrel that fits the dose, read the leading edge of the plunger stopper.
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.
U-100 is a concentration standard meaning 100 units of insulin per millilitre; the syringe graduation is a volume scale expressed in those units.
The caveat is that this describes an instrument, not a treatment, and research-use compounds are not approved for human use.
One unit is 0.01 mL on U-100. That is the whole scale.
HPLC purity, identity confirmation and quantified content on the vial you actually hold. Reports arrive with the chromatogram attached, not just a number.
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Visit GL BiochemStart 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.
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.
Concretely, 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.
The 4 mm needle length is supported by injection-technique studies showing adequate subcutaneous delivery across body-mass ranges without increased intramuscular delivery.
Smallest barrel that holds the dose. Resolution is free and you should take it.
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.
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.
More usefully, 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.
Half-graduation estimation is not measurement. If the dose needs it, change the concentration instead.
Read the leading edge of the stopper, every time.
edited 7 May 2026 by u100_marks — added a caveat about sampling
A dose that lands below about five units on any barrel is being measured too coarsely and the reconstitution should be redone.
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.
Check the syringe is U-100 and not U-40 before doing any arithmetic with it.
The relevant precision limit is that you cannot reliably read half a graduation, so the barrel decides the smallest change you can make.
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.
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.
Fixed needle, not detachable, unless you have a specific reason.
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