How do I get 12 units out of a vial reconstituted to 20 mg/mL without guessing?

Stated plainly: 12 units · 20 mg/mL.

Everything I have found on this is either a forum aside or a product page, neither of which I trust.

I am comfortable with the arithmetic; what I am missing is the procedural detail around it.

What is the correct sequence, and where is the step that people usually skip?

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.

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.

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.

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.

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.

The underlying point is that 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.

Read the leading edge of the stopper, every time.

The underlying point is that fixed-needle syringes have far less dead space than detachable-needle ones, which matters more than any other design difference here.

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.

Graduation intervals by barrel size are printed on the packaging and are the practical limit on dosing resolution.

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.

U-100 is a concentration standard meaning 100 units of insulin per millilitre; the syringe graduation is a volume scale expressed in those units.

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