Reconstitution math

The whole formula

There is one equation and everything else is a consequence of it.

concentration (mg/mL) = mg in the vial ÷ mL of liquid you added

That is it. A 10mg vial with 2mL of bacteriostatic water added is 5 mg/mL. A 10mg vial with 1mL added is 10 mg/mL. The amount of drug in the vial never changes — adding more water does not dilute the dose you end up taking, it changes how much liquid that dose occupies.

That last sentence is the one people trip over, so here it is the other way round: reconstitution volume is a resolution choice, not a strength choice. More water means more syringe markings per dose, which means finer control and less proportional error from dead space. It also means you might not be able to fit a dose into a small syringe.

Units are volume, not mass

An insulin syringe marked U-100 is graduated for insulin at 100 units per millilitre. That makes one "unit" exactly 0.01 mL, and it has nothing whatsoever to do with milligrams of peptide.

units to draw = (dose in mg ÷ concentration in mg/mL) × 100

So on that 10mg-in-2mL vial (5 mg/mL), a 2.5mg dose is 2.5 ÷ 5 = 0.5 mL = 50 units. On the same vial reconstituted with 1mL (10 mg/mL) the same 2.5mg dose is 0.25 mL = 25 units. Same drug, same dose, different number on the barrel.

This is why nobody on this site will answer "how many units should I draw?" without being told the concentration. It is not pedantry, the question is genuinely unanswerable without it.

Calculator

Type your numbers in. Nothing is sent anywhere — this runs entirely in your browser.

Two sanity checks that catch almost everything

1. If your answer is over 100 units, you have probably divided the wrong way round. 100 units is a full 1mL syringe. A single dose rarely fills one.
2. If your answer is under about 5 units, your concentration is too high for the syringe to resolve. Five units is 0.05mL. At that volume, a small error in where you stop the plunger is a large percentage error in dose, and needle dead space becomes a real proportion of what you drew. Reconstitute at a larger volume next time.

A third, softer check: work out how many whole doses the vial contains and see whether that matches how long you expect it to last. If the arithmetic says three doses and you were expecting eight, something is wrong upstream of the syringe.

Dead space

The hub of the needle holds a small volume of liquid that never gets injected. On a low-dead-space insulin syringe with a fixed needle this is small; on a syringe with a detachable luer needle it can be a couple of units, and on a 1mL syringe with a thick needle it can be more.

The important properties of dead space are that it is consistent and proportional. Consistent means once you have measured your own setup you can account for it. Proportional means it matters far more at 8 units than at 60 — the same 2-unit loss is 25% of a small draw and 3% of a large one, which is the strongest practical argument for reconstituting at a larger volume.

Common combinations

Note the 2.5 u entries. Two and a half units is not measurable with confidence on most syringes — those rows are in the table to show you which combinations to avoid, not which to use.

Which water

Bacteriostatic water contains 0.9% benzyl alcohol as a preservative, which is why a multi-dose vial reconstituted with it has a usable in-use window rather than a single-use one. Sterile water for injection has no preservative at all. They are not interchangeable and the packaging looks nearly identical, which is a design failure the whole internet has to work around by reading labels carefully.

The commonly used in-use window after first puncture is 28 days refrigerated. That number comes from the preservative, not from the peptide, and it is a convention rather than a law of nature — see storage and stability for what actually degrades and how fast.

Technique, briefly

• Swab the stopper and let it dry. The drying is the disinfecting step and it is the one everybody skips.
• Add the water slowly, down the inside wall of the vial rather than straight onto the powder cake.
• Swirl or roll, do not shake. Foam is denatured protein. If it foams you shook it.
• Give it a few minutes. A cake that has not fully dissolved looks cloudy and will clear on its own; a solution that is still cloudy after it has had time is a discard, not a debate.
• Rotate the puncture site on the stopper. Hitting the same spot repeatedly cores the rubber and drops fragments into the vial.
• Write the concentration on the vial in marker. Every dosing error in c/reconstitution traces back to two vials reconstituted differently and nobody remembering which was which.

When the arithmetic is right and it still feels wrong

Stop. Do not inject anything while you are unsure. Post the numbers in c/reconstitution with the mg, the mL and the intended dose and somebody will check them within the hour, usually within minutes — this is genuinely the community's favourite kind of question because it is answerable and it matters.

And if the mismatch is between what your arithmetic says and what somebody told you to draw: trust the arithmetic, then go and ask the person who told you.