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Most calculators tell you the units for a given water volume. This one works backward: pick the round number of units you want to draw for your dose, and it solves the bacteriostatic water volume to add so your dose lands on that clean mark. Reference math, not medical advice.
| Draw | Add water | Concentration |
|---|---|---|
| 10 units | 2 mL | 2.5 mg/mL |
| 15 units | 3 mL | 1.667 mg/mL |
| 20 units | 4 mL | 1.25 mg/mL |
| 25 units | 5 mL | 1 mg/mL |
| 30 units | 6 mL | 0.833 mg/mL |
| 40 units | 8 mL | 0.625 mg/mL |
| 50 units | 10 mL | 0.5 mg/mL |
Water to add = vial mg times target units, divided by (dose mg times 100), on a U-100 syringe. Doses per vial does not change with water. Do not exceed your vial's capacity, and use sterile technique. Arithmetic only — not medical advice; verify against the product label and a clinician.
Frequently asked questions
What does the reconstitution solver do?
It solves the bacteriostatic water volume to add to your vial so that a chosen dose lands on a clean, round number of units on a U-100 insulin syringe — for example, exactly 20 units instead of 17.4. It is the reverse of a normal reconstitution calculator, which starts from the water volume.
How is the water volume calculated?
Water to add equals the vial strength in milligrams times your target units, divided by your dose in milligrams times 100 (the U-100 factor). The resulting concentration is your dose in milligrams times 100, divided by the target units.
Why draw a round number of units?
Round targets such as 10, 20, or 25 units are easier to read and measure on the fine markings of an insulin syringe, which can reduce the chance of a measuring error compared with an awkward value like 17.4 units.
What if the water volume seems too large or too small?
Very small volumes give a highly concentrated solution and very large volumes may not fit the vial. If the suggested volume is outside a typical 0.5 to 5 milliliter range, pick a different target-unit value from the table, or choose one that fits your vial’s capacity.
Is this medical advice?
No. It is arithmetic and reference only. It does not recommend a dose, a concentration, or a product, and does not assess whether any dose is appropriate. Verify against the product label and with a clinician or pharmacist.