Details up front: 30 mg · 5 mL.
This should be a straightforward calculation and I keep getting two different answers.
The numbers are arbitrary; the method is what I am after.
Can someone walk through the arithmetic step by step?
The underlying point is that this is arithmetic, so let us do the arithmetic rather than argue about it.
Worked example, because the general form is easier to trust once you have seen it once. Take a 10 mg vial and add 2 mL of diluent: the concentration is 10 ÷ 2 = 5 mg/mL. A 0.5 mg dose is 0.5 ÷ 5 = 0.1 mL. On a U-100 syringe, where 1 unit = 0.01 mL, that is 0.1 ÷ 0.01 = 10 units. Change the diluent to 1 mL and the same dose becomes 5 units — same dose, half the resolution.
| Vial | Diluent | Concentration | 0.25 mg | 0.5 mg | 1 mg | 2.5 mg |
|---|---|---|---|---|---|---|
| 5 mg | 1 mL | 5 mg/mL | 5 u | 10 u | 20 u | 50 u |
| 5 mg | 2 mL | 2.5 mg/mL | 10 u | 20 u | 40 u | 100 u |
| 10 mg | 1 mL | 10 mg/mL | 2.5 u | 5 u | 10 u | 25 u |
| 10 mg | 2 mL | 5 mg/mL | 5 u | 10 u | 20 u | 50 u |
| 10 mg | 3 mL | 3.33 mg/mL | 7.5 u | 15 u | 30 u | 75 u |
Units are U-100 insulin units, where 1 unit = 0.01 mL. Divide dose by concentration for millilitres, then multiply by 100.
The concentration you actually work with is label claim times content fraction divided by actual diluent volume, which is usually not the same as the nominal concentration because content is usually not 100 per cent and you rarely measure the diluent volume to 0.1 mL precision.
The insulin-unit standard U-100 means 100 units per millilitre, so one unit is 0.01 mL — this is the conversion that trips up more people here than any other single piece of arithmetic.
The caveat is that this assumes the vial contains what the label says, and if the content assay has not been done, the arithmetic is precise about an unknown quantity.
Write the arithmetic on the vial label. It costs nothing and removes the step where you reconstruct it from memory.
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 relevant detail is that the common error is getting the concentration right but then misreading the syringe scale, which is why checking the barrel marking rather than your memory matters.
Air bubbles at these volumes are a measurement problem rather than a safety one. A 2 mm bubble in a 0.3 mL syringe is roughly 4 µL, which at 10 units drawn is a four per cent error.
Do not use the same needle to pierce the stopper and to administer. The tip is blunted by the stopper, and the hub now contains a dose you are about to lose to dead space anyway.
The Arrhenius relationship for drawing kinetics means that cold solution takes noticeably longer to draw than room-temperature solution.
Worth noting: the concentration after reconstitution is not the same as the label claim, and most people do not account for the difference.
Do the arithmetic twice, ideally with someone else doing it independently.
The underlying point is that work in the order concentration, then volume, then units, and the arithmetic stops being confusing. Concentration is milligrams per millilitre and comes from the vial contents and the diluent volume. Volume per dose is dose divided by concentration. Units on a U-100 syringe are volume in millilitres multiplied by one hundred.
Room temperature before drawing is worth the ten minutes. Cold solution is more viscous, draws slower, and condensation on a cold barrel makes it harder to read the meniscus.
Dead space quantified: a fixed-needle insulin syringe holds roughly 3 to 5 µL in the hub and needle after the plunger bottoms out. A luer-lock syringe with a detachable needle holds 35 to 100 µL depending on the hub design. At 5 mg/mL that is 15 to 25 µg lost per draw on the insulin syringe and 175 to 500 µg on the luer-lock — which over ten draws is the difference between losing a rounding error and losing half a milligram.
The limitation is that technique reduces risk, it does not remove it, and nothing you can do outside a controlled environment makes a non-sterile preparation sterile.
If in doubt, use more diluent and accept the shorter usable window.
edited 13 Nov 2024 by low_dead_space — updated for the 2026 guidance change
The arithmetic only stops being confusing once you work it through once and see that it is straightforward.
Rotation of injection site is a tolerability measure, not a pharmacokinetic one, but if you are going to do it you might as well do it right.
One qualification: if your arithmetic and someone else's disagree by a factor of ten, one of you has made a unit error, and writing out the units at every step is the diagnostic.
Write the arithmetic on the vial label. It costs nothing and removes the step where you reconstruct it from memory.
This is one of those calculations where checking your work takes two minutes and prevents a very consequential error.
Number of stopper piercings matters less than the gauge doing the piercing. A 30G or 31G needle through a butyl stopper leaves a track that reseals; a 21G or 18G drawing needle punches a core and can drop it into the solution.
Published data on syringe dead space quantifies low-dead-space designs as retaining under 2 µL against 35 µL or more for conventional detachable-needle syringes.
Do the arithmetic twice, ideally with someone else doing it independently.
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.