By Tessaro Pty Ltd, the maker of Pepti · Reviewed
The same vial can give you two completely different draws. It depends entirely on how much water you added. Here is the maths, in three steps.
Concentration (mg/mL) = vial strength (mg) ÷ water added (mL) Volume to draw (mL) = target dose (mg) ÷ concentration (mg/mL) Syringe units = volume (mL) × 100 (U-100 syringe)
Most insulin syringes sold for at-home use are U-100, marked 0 to 100 units. If your syringe is marked in units and reaches 100, it is U-100 and the formula above applies. If it is marked in millilitres only, read the volume directly and skip the last step.
A 10 mg vial with 2 mL of bacteriostatic water added gives 5 mg/mL. A 0.5 mg target means 0.5 ÷ 5 = 0.1 mL. 0.1 mL × 100 = 10 units on a U-100 syringe.
| Vial strength | 1 mL | 2 mL | 3 mL | 5 mL |
|---|---|---|---|---|
| 5 mg | 5.0 | 2.5 | 1.67 | 1.0 |
| 10 mg | 10.0 | 5.0 | 3.33 | 2.0 |
| 15 mg | 15.0 | 7.5 | 5.0 | 3.0 |
| 20 mg | 20.0 | 10.0 | 6.67 | 4.0 |
If you change how much water you add, every unit number you had memorised is wrong. The vial has not changed — the concentration has. Work it out again from the top whenever your water volume changes.
Doses per vial = water added (mL) ÷ volume per dose (mL)
2 mL of water added, drawing 0.1 mL per dose, gives 20 doses. Work this out once when you reconstitute and count down from it.
This converts a target you already have. It does not tell you what that target should be.
If you would rather not do this by hand, Pepti’s reconstitution tracker does the maths and tracks what is left in each vial.
Not medical advice. Pepti is a record-keeping and measurement tool, not a medical device.
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