Reconstitution & syringe calculator
Diluent volume in, concentration and insulin units out — with the dead-space loss you are actually going to incur, which most calculators quietly ignore.
Inputs
The plunger position above is drawn to the computed volume on the selected scale. If the mark you need falls between two graduations, that is the calculator telling you to use more diluent.
The arithmetic, so you can do it without this page
Three steps, and the only one people get wrong is the third.
- Concentration. Vial contents divided by diluent volume. A 10 mg vial in 2 mL is 10 ÷ 2 = 5 mg/mL.
- Volume per dose. Dose divided by concentration. 0.5 mg ÷ 5 mg/mL = 0.1 mL.
- Units. On a U-100 syringe one unit is 0.01 mL, so multiply millilitres by 100. 0.1 mL × 100 = 10 units. On a U-40 syringe one unit is 0.025 mL, so you multiply by 40 instead and get 4 units for the same volume — which is why using the wrong barrel is a 2.5-fold error, not a rounding error.
Why the diluent volume is a resolution decision
The dose is fixed by what you intend to deliver. The diluent volume is free, and what you buy with it is measurement resolution.
| Diluent | Concentration | 0.25 mg | 0.5 mg | 1.0 mg | Resolution at 0.25 mg |
|---|---|---|---|---|---|
| 0.5 mL | 20 mg/mL | 1.25 u | 2.5 u | 5 u | Unusable — you cannot read a quarter unit |
| 1 mL | 10 mg/mL | 2.5 u | 5 u | 10 u | Poor — half-unit reading required |
| 2 mL | 5 mg/mL | 5 u | 10 u | 20 u | Good — whole units throughout |
| 3 mL | 3.33 mg/mL | 7.5 u | 15 u | 30 u | Good, but half units return at 0.25 mg |
| 4 mL | 2.5 mg/mL | 10 u | 20 u | 40 u | Excellent resolution, four weeks of solution to keep cold |
What you pay for the resolution is shelf life and refrigerator space: a larger volume takes longer to use, which means more days in solution and more stopper piercings. The conventional compromise is the volume that puts your usual dose on a whole-unit mark at the lowest total volume that achieves it — which for most 10 mg presentations is 2 mL.
Dead space is not a rounding error
When the plunger bottoms out, the hub and needle still contain liquid. On a fixed-needle insulin syringe that is a few microlitres and genuinely negligible. On a luer-lock syringe with a detachable needle it can be eighty-five microlitres, and at that point you are discarding a meaningful fraction of every draw.
| Configuration | Dead volume | Loss at 5 mg/mL | Over 20 draws | As % of a 10 mg vial |
|---|---|---|---|---|
| Low-dead-space, detachable | 2 µL | 10 µg | 0.20 mg | 2.0% |
| Fixed-needle insulin syringe | 4 µL | 20 µg | 0.40 mg | 4.0% |
| Luer-lock + 30G | 45 µL | 225 µg | 4.50 mg | 45.0% |
| Luer-lock + 21G drawing needle | 85 µL | 425 µg | 8.50 mg | 85.0% |
The last row is not a typo and it is not realistic practice — nobody administers twenty times from one vial with a drawing needle attached — but it makes the point that the configuration matters more than the price per vial. If you are comparing cost per milligram between suppliers while using a syringe that discards eighty-five microlitres per draw, you are optimising the wrong variable by an order of magnitude.
What this calculator will not tell you
- Whether the vial contains what the label says. The arithmetic is exact about a quantity nobody has measured. If you have not had a content assay done, treat the label as a hypothesis.
- Whether the solution is sterile. Bacteriostatic water suppresses growth. It does not sterilise, and neither does an alcohol wipe.
- Whether any of this is a good idea. Research-use-only compounds are not approved for human use. This page does arithmetic; it does not offer an opinion on what you do with the result, beyond suggesting you discuss it with a clinician.
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