Peptide reconstitution calculator
Powder in a vial, water going in, a dose coming out. Three sums, in order.
This page is written for people filling a prescription — a compounded GLP-1, most often.
Common setups:
The three sums
| Sum | Example |
|---|
| Concentration | vial mg ÷ water mL |
10 ÷ 2 = 5 mg/mL |
| Volume per dose | dose mg ÷ concentration |
2.5 ÷ 5 = 0.5 mL |
| Units to draw | volume mL × 100 |
0.5 × 100 = 50 units |
The ×100 in the last line is the definition of a U-100 syringe: one millilitre is a
hundred units. It's the only place the syringe enters the arithmetic at all.
How much water should I add?
Whatever your pharmacy told you. If they didn't, the drug doesn't care — 5 mg is
5 mg in any volume — so pick the amount that puts your dose somewhere comfortable to
measure.
Same 10 mg vial, same 2.5 mg dose, different mixing water.
| Water | Concentration | Units per dose | How it feels |
|---|
| 0.5 mL | 20 mg/mL | 12.5 | Tight. One mark out is an 8 % error. |
| 1 mL | 10 mg/mL | 25 | Fine. |
| 2 mL | 5 mg/mL | 50 | Comfortable. Easy to read. |
| 4 mL | 2.5 mg/mL | 100 | Fills the whole barrel. No room to titrate up. |
The trap is that last row. More water makes today's dose easier to measure and can leave
you with nowhere to go when the dose doubles. Mix for the dose you'll be on in three months,
not the one you're on this week.
Practical bits
- Bacteriostatic water, not sterile water. Bacteriostatic contains benzyl alcohol,
which is what lets a vial be entered more than once. Sterile water is single-use.
- Down the side of the glass, not onto the powder. Aim the stream at the vial wall
and let it run down.
- Swirl, don't shake. Give it a minute to dissolve on its own.
- Cloudy or full of bits after it's dissolved is a stop sign. Call the pharmacy
rather than injecting it.
- Once it's mixed, it lives in the fridge and it has an expiry. Your pharmacy's
beyond-use date is the one that counts.
This is the general version. If you know which drug you're on, the charts
are quicker:
tirzepatide ·
semaglutide.