Reconstitution Math
How Many Doses Are in a Peptide Vial?
Reference tables for doses per vial by vial size and dose amount, why bac water volume doesn't change dose count, real-world yield vs. theoretical, and GLP-1 vial dose counts.
Informational only. Not medical advice. Consult a licensed healthcare provider before starting, changing, or stopping any protocol.
The formula
Doses per vial = Vial strength (mcg) ÷ Target dose (mcg)
A 5 mg (5,000 mcg) vial at 250 mcg/dose yields 20 doses. At 500 mcg/dose: 10 doses. At 1,000 mcg/dose: 5 doses.
The amount of bacteriostatic water you add does not affect the number of doses — it only affects the concentration and therefore the volume you draw per dose. More bac water = same number of doses, larger draw. Less bac water = same number of doses, smaller draw.
Reference table: doses per vial by dose amount
| Vial size | 100 mcg/dose | 200 mcg/dose | 250 mcg/dose | 500 mcg/dose | 1,000 mcg/dose |
|---|---|---|---|---|---|
| 2 mg (2,000 mcg) | 20 | 10 | 8 | 4 | 2 |
| 5 mg (5,000 mcg) | 50 | 25 | 20 | 10 | 5 |
| 10 mg (10,000 mcg) | 100 | 50 | 40 | 20 | 10 |
| 20 mg (20,000 mcg) | 200 | 100 | 80 | 40 | 20 |
For tirzepatide and semaglutide, doses are in milligrams (not micrograms), and vials are typically pre-mixed at a specific concentration. Use the tirzepatide calculator for those.
How many days will a vial last?
Combine doses per vial with your injection frequency:
- Once daily: 5 mg vial at 250 mcg = 20 doses = 20-day supply.
- Twice daily: Same vial = 10-day supply.
- Once daily + 500 mcg: Same vial = 10-day supply.
Running a multi-compound protocol complicates this further — each compound has its own vial, its own dose, and its own run-out date. My Pep Calc tracks doses remaining per vial in real time and displays days-until-empty so you can order refills before running out.
Why vials don't always yield their theoretical doses
In practice, you often get slightly fewer doses than the theoretical count:
- Dead volume in the needle and syringe hub. Each draw leaves a small amount of solution in the needle (0.01–0.05 mL). Over 20 doses, this adds up to roughly 0.2–1.0 mL lost — equivalent to 1–4 doses from a 2 mL vial.
- Inability to draw the last drop. The last 0.1–0.2 mL in a vial becomes hard to draw without introducing air. Most people discard this rather than risk drawing air into the syringe.
- Vial underfill. Compounded vials occasionally contain slightly less than labeled. Reputable pharmacies include slight overfill to compensate.
A conservative rule: assume 90–95% yield from a multi-dose vial. For a 5 mg vial at 250 mcg/dose, plan for 18–19 reliable doses rather than the theoretical 20.
Tracking vial inventory matters
Running out of a compound mid-protocol is more than inconvenient — for some protocols (especially tirzepatide escalation), an unplanned gap disrupts the pharmacology and may require re-titration. For peptides with physical therapy goals (BPC-157 for injury healing), a 5-day supply gap can break the protocol's continuity.
My Pep Calc deducts from vial inventory with each logged dose and shows you the estimated run-out date. The only prerequisite is that you log your doses — which is the whole point of a tracker.
For GLP-1 vials: doses per pre-mixed vial
Compounded GLP-1 vials typically come pre-mixed at concentrations like 5 mg/mL or 10 mg/mL in a 3 mL or 5 mL vial. Doses per vial:
| Vial size | Concentration | Total peptide | At 5 mg/dose | At 10 mg/dose | At 15 mg/dose |
|---|---|---|---|---|---|
| 3 mL | 5 mg/mL | 15 mg | 3 doses | 1.5 doses | 1 dose |
| 3 mL | 10 mg/mL | 30 mg | 6 doses | 3 doses | 2 doses |
| 5 mL | 10 mg/mL | 50 mg | 10 doses | 5 doses | 3.3 doses |
| 5 mL | 20 mg/mL | 100 mg | 20 doses | 10 doses | 6.7 doses |
For once-weekly GLP-1 dosing, a 3 mL vial at 10 mg/mL gives you a 3-week supply at 10 mg/week. A 5 mL vial at 10 mg/mL gives 5 weeks. Confirm with your pharmacy.
Frequently asked questions
- Does adding more bacteriostatic water give me more doses?
- No. The number of doses depends entirely on the total amount of peptide in the vial and your dose per injection. Bac water volume only changes the concentration — you draw a proportionally larger or smaller volume per dose, but the total peptide available stays the same.
- How do I know when my peptide vial is almost empty?
- Track your doses. If you log every injection in My Pep Calc, the app shows doses remaining and estimated run-out date in real time. Without tracking, you're estimating from the solution level in the vial — unreliable for small-volume draws.
- Can I combine two vials if the first is almost empty?
- Combining vials is generally not recommended due to contamination risk. Each puncture of a new vial with the old vial's needle could introduce contaminants from the older vial into the new one. The safer practice is to use the old vial to its practical limit, then open the new vial fresh.
- What is "dead volume" in a syringe?
- Dead volume (or dead space) is the solution that remains in the needle hub after the plunger is fully depressed. It ranges from 0.01 mL (low dead-space syringes) to 0.05 mL (standard insulin syringes). For high-value peptides or large draw counts, low dead-space syringes reduce waste.
- How many weeks does a tirzepatide vial last?
- It depends on the vial size, concentration, and your weekly dose. A 3 mL vial at 10 mg/mL (30 mg total) at 10 mg/week lasts 3 weeks. At 5 mg/week, the same vial lasts 6 weeks. Use the My Pep Calc tirzepatide calculator with your specific vial and dose.
Sources
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