How do I get 5 units out of a vial reconstituted to 1 mg/mL without guessing?

The particulars: 5 units · 1 mg/mL.

I want a method I can write down and repeat, not a rule of thumb.

I would rather over-engineer this than discover a problem later, within reason.

Concretely, what should I do, and how would I know afterwards whether I did it right?

5 units is 0.05 mL, which at 1 mg/mL is 0.05 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.

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.

Reading a lyophilised cake

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.

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.

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.

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.

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

Read the leading edge of the stopper, every time.

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.

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.

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

edited 27 Feb 2025 by zeynep_arslan — removed a claim I could not source

The relevant detail is that a dose that lands below about five units on any barrel is being measured too coarsely and the reconstitution should be redone.

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.

Smallest barrel that holds the dose. Resolution is free and you should take it.

edited 27 Feb 2025 by Dr_Nadia_Farsi — expanded the table to cover the lower concentration

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

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.

Nothing here is medical advice.

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

edited 12 Jan 2025 by mz_4113 — added the placebo-arm figures