How Reconstitution Calculators Work
A reconstitution calculator answers one question: after I mix this vial with water, how many units do I draw on an insulin syringe to get my target dose? This page explains the inputs and the math behind that answer so the numbers make sense on paper — there is no live calculator here, just the reasoning.
The #1 dangerous mistake
Confusing micrograms (mcg/µg) with milligrams (mg) is a 1000× error and the most common beginner mistake. 1 mg = 1000 mcg. Always confirm which unit your vial and your target dose are in before doing any math.
The three inputs
| Input | What it is | Example |
|---|---|---|
| Vial strength (mg) | Total peptide in the dried (lyophilized) vial | 5 mg |
| BAC water (mL) | Volume of bacteriostatic water you add to dissolve it | 2 mL |
| Target dose (mcg) | How much peptide you want per injection | 250 mcg |
Reconstituting does not change the amount of peptide — a 5 mg vial always contains 5 mg. Adding water only sets the concentration (how much peptide sits in each mL), which in turn sets how many units a given dose occupies.
The formula
Two steps: find the concentration, then convert your dose to syringe units.
Step 1 — Concentration (mcg per mL):
concentration (mcg/mL) = vial strength (mcg) ÷ BAC water (mL)
(Convert the vial’s mg to mcg first: multiply by 1000.)
Step 2 — Units to draw on a U-100 insulin syringe:
A U-100 insulin syringe is marked so that 100 units = 1 mL, i.e. 1 unit = 0.01 mL. So:
volume needed (mL) = target dose (mcg) ÷ concentration (mcg/mL)
units to draw = volume needed (mL) × 100
Combined into one line:
units = ( target dose (mcg) × 100 ) ÷ concentration (mcg/mL)
Worked example
Vial 5 mg, reconstituted with 2 mL BAC water, target dose 250 mcg.
- Vial in mcg: 5 mg × 1000 = 5000 mcg
- Concentration: 5000 mcg ÷ 2 mL = 2500 mcg/mL
- Volume needed: 250 mcg ÷ 2500 mcg/mL = 0.1 mL
- Units: 0.1 mL × 100 = 10 units
So you draw to the 10-unit mark for a 250 mcg dose. The vial holds 5000 ÷ 250 = 20 doses.
Sanity check the concentration
Notice that if you had added only 1 mL of water instead of 2 mL, the same vial would be twice as concentrated (5000 mcg/mL), and the same 250 mcg dose would be just 5 units. More water → more units per dose (easier to measure precisely); less water → fewer units (smaller injection volume). The peptide amount never changes — only how it is spread across the liquid.
The common mcg/mg error
If a chart or calculator is fed the dose in mg while the concentration is in mcg/mL (or vice versa), the result is off by a factor of 1000. In the example above, entering “0.25 mg” where the field expects mcg, or treating the 250 mcg target as 250 mg, produces a wildly wrong unit count. The habit that prevents this: convert everything to the same unit (mcg) first, then divide.
For more worked examples, concentration charts, and the bacteriostatic-vs-sterile-water distinction, see Reconstitution & Dosing Math.
Educational information only — not medical advice. See Disclaimer.