What is the arithmetic to convert 2 mg in 5 mL into units on a U-100 scale?

Details up front: 2 mg · 5 mL.

I can do the algebra. I am not confident about the conversion factors.

If there is a standard way to lay this out, I would rather learn that than invent one.

Can someone show the working rather than just the answer?

Dose arithmetic has three parts: concentration from vial content and diluent, volume from dose and concentration, and units from volume and syringe scale.

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.

Specifically, 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.

The Arrhenius relationship for drawing kinetics means that cold solution takes noticeably longer to draw than room-temperature solution.

Do the arithmetic twice, ideally with someone else doing it independently.

edited 14 Jul 2026 by assay_blank — updated for the 2026 guidance change

Mechanically, this is arithmetic, so let us do the arithmetic rather than argue about it.

The rounding error accumulates if you round too many times — rounding concentration to 5.0, rounding the dose volume to 0.1 mL, rounding the unit reading to 10 — and the safest approach is to work the full precision and round only the final answer.

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.

Write the arithmetic on the vial label. It costs nothing and removes the step where you reconstruct it from memory.

The answer depends on exactly which dose and which vial you are asking about, but the method is always the same.

On filtration: a 0.22 µm syringe filter will remove particulates and organisms, and it will also adsorb a fraction of your peptide onto the membrane — with a low-binding PVDF or PES membrane the loss is typically a few per cent.

Worth noting: the concentration after reconstitution is not the same as the label claim, and most people do not account for the difference.

If in doubt, use more diluent and accept the shorter usable window.

Two people working through the same arithmetic independently should get the same answer, and if they do not, someone has made a unit error.

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.

The content assay results from major testing services show that nominal vial claim and measured content differ by one to ten per cent, making content a driver of dose error.

I would flag the obvious failure mode: people get the concentration right, get the volume right, and then read the syringe against the wrong scale.

Do the arithmetic twice, ideally with someone else doing it independently.

edited 25 Jul 2026 by tyndall_haze — clarified the distinction between purity and content