The case in front of me: 30 units · 20 mg/mL.
I am trying to do this correctly the first time rather than learn it by getting it wrong.
I have already made one mistake here that cost me a vial, so I am being deliberately careful.
What does a defensible version of this look like in practice?
30 units is 0.3 mL, which at 20 mg/mL is 6 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.
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.
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.
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.
Published dead-space comparisons put low-dead-space fixed-needle designs under 2 microlitres and conventional detachable-needle designs at 35 microlitres or more.
Smallest barrel that holds the dose. Resolution is free and you should take it.
edited 19 Apr 2025 by Dr_Hanne_Solberg — added the method parameters
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsThe honest answer is that most dosing errors in this tag are reading errors rather than calculation errors.
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.
Stated carefully, 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.
Graduation intervals by barrel size are printed on the packaging and are the practical limit on dosing resolution.
Fixed needle, not detachable, unless you have a specific reason.
More usefully, a dose that lands below about five units on any barrel is being measured too coarsely and the reconstitution should be redone.
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.
Worth being precise here: 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.
U-100 is a concentration standard meaning 100 units of insulin per millilitre; the syringe graduation is a volume scale expressed in those units.
One unit is 0.01 mL on U-100. That is the whole scale.
Concretely, fixed-needle syringes have far less dead space than detachable-needle ones, which matters more than any other design difference here.
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.
The 4 mm needle length is supported by injection-technique studies showing adequate subcutaneous delivery across body-mass ranges without increased intramuscular delivery.
Read the leading edge of the stopper, every time.
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.
Half-graduation estimation is not measurement. If the dose needs it, change the concentration instead.
Check the syringe is U-100 and not U-40 before doing any arithmetic with it.
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