How do I get 30 units out of a vial reconstituted to 5 mg/mL without guessing?

Concretely: 30 units · 5 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 would you do, and what would you check afterwards?

30 units is 0.3 mL, which at 5 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.

Answering this needs to know whether the syringe is U-100 or U-40, since a U-40 syringe used with the U-100 conversion produces a two-and-a-half-fold error.

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.

Concretely, 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.

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.

It helps to be literal here: a dose that lands below about five units on any barrel is being measured too coarsely and the reconstitution should be redone.

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.

On the detail: 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.

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.

The honest answer is that most dosing errors in this tag are reading errors rather than calculation errors.

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.

Put another way, 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.

Check the syringe is U-100 and not U-40 before doing any arithmetic with it.

Fixed-needle syringes have far less dead space than detachable-needle ones, which matters more than any other design difference here.

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.

Read the leading edge of the stopper, every time.

edited 8 Jul 2026 by orla_ferriter — added the method parameters

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.

The 4 mm needle length is supported by injection-technique studies showing adequate subcutaneous delivery across body-mass ranges without increased intramuscular delivery.

Nothing here is medical advice.

Fixed needle, not detachable, unless you have a specific reason.