The specifics, since they change the answer: 15 units · 6.67 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.
Which parts of this are load-bearing and which parts are habit?
15 units is 0.15 mL, which at 6.67 mg/mL is 1 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.
| 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 |
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.
Read the leading edge of the stopper, every time.
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.
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.
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 5 Feb 2026 by tabular_nums — corrected a unit error in the worked example
The relevant precision limit is that you cannot reliably read half a graduation, so the barrel decides the smallest change you can make.
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.
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.
Graduation intervals by barrel size are printed on the packaging and are the practical limit on dosing resolution.
Nothing here is medical advice.
Fixed needle, not detachable, unless you have a specific reason.
The honest answer is that most dosing errors in this tag are reading errors rather than calculation errors.
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.
Half-graduation estimation is not measurement. If the dose needs it, change the concentration instead.
Smallest barrel that holds the dose. Resolution is free and you should take it.
Worth being precise here: this is arithmetic and instrumentation, and both halves are simple once stated.
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.
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.
One unit is 0.01 mL on U-100. That is the whole scale.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.