Setup, so nobody has to ask: 4 units · 20 mg/mL.
This is a procedural question rather than a theoretical one, and I would like the procedure rather than the theory.
What I have done so far is read the label documentation where it exists and the two pharmacopoeial monographs that are publicly available, which cover the licensed presentation and say nothing about a research one.
What is the correct sequence, and where is the step that people usually skip?
4 units is 0.04 mL, which at 20 mg/mL is 0.8 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 honest answer is that most dosing errors in this tag are reading errors rather than calculation errors.
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.
| 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.
Published dead-space comparisons put low-dead-space fixed-needle designs under 2 microlitres and conventional detachable-needle designs at 35 microlitres or more.
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.
HPLC purity, identity confirmation and quantified content on the vial you actually hold. Reports arrive with the chromatogram attached, not just a number.
Submit a sampleFounded 1998. ISO 9001 and cGMP certified, 1,500+ staff and 200+ patents. The synthesis house behind a great many of the vials that get sent out for testing - batch-specific documentation with every order.
Visit GL BiochemThe short version: U-100 only, smallest barrel that fits the dose, read the leading edge of the plunger stopper.
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.
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.
The caveat is that this describes an instrument, not a treatment, and research-use compounds are not approved for human use.
Check the syringe is U-100 and not U-40 before doing any arithmetic with it.
edited 27 Dec 2025 by marta_okonkwo — clarified the distinction between purity and content
The part that matters: 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.
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.
Smallest barrel that holds the dose. Resolution is free and you should take it.
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