Reconstitution math basics
How to calculate peptide doses — the three-step formula for converting mg vial size and BAC water volume into accurate insulin syringe units.
Updated May 7, 2026 · 4 min read
Reconstitution math is where most peptide errors happen. The numbers are small, the units shift between mg and mcg, and a single decimal-place mistake produces a 10x or 1000x dose error. The math itself is not hard — it's three multiplications — but the inputs need to be checked every time. This page walks through the formula with worked examples for the most common strength peptides.
The three-step formula
Every reconstitution calculation has the same shape:
Step 1 — Concentration: vial mg ÷ water mL = mg/mL
Step 2 — Volume per dose: dose mg ÷ concentration = mL
Step 3 — Insulin units: mL × 100 = units (on a U-100 syringe)
That's it. Memorize the three steps and the rest is plugging numbers in. The reconstitution calculator does this live with a syringe diagram, and it's the right place to actually do the arithmetic — even experienced users hit calculation errors when doing it by hand.
Worked example 1 — BPC-157 at 250 mcg
You have a 5 mg vial of BPC-157 and want to dose 250 mcg per injection. You'll add 2 mL of bacteriostatic water.
Concentration: 5 mg ÷ 2 mL = 2.5 mg/mL
Dose in mg: 250 mcg = 0.25 mg
Volume: 0.25 mg ÷ 2.5 mg/mL = 0.1 mL
Units: 0.1 mL × 100 = 10 units on a U-100 syringe
A 30-unit insulin syringe is the right pick — 10 units lands at the one-third mark and is easy to read.
Worked example 2 — TB-500 at 2.5 mg
You have a 5 mg vial of TB-500 and want to dose 2.5 mg twice weekly. You add 2 mL of BAC water.
Concentration: 5 mg ÷ 2 mL = 2.5 mg/mL
Volume: 2.5 mg ÷ 2.5 mg/mL = 1.0 mL
Units: 1.0 mL × 100 = 100 units
That fills a 100-unit syringe completely. If you want a smaller syringe per injection, reconstitute with less water — 1 mL of BAC water in the same 5 mg vial gives 5 mg/mL, and a 2.5 mg dose becomes 0.5 mL = 50 units.
Worked example 3 — Ipamorelin at 200 mcg
You have a 10 mg vial of ipamorelin and want to dose 200 mcg per injection. You add 2 mL of BAC water.
Concentration: 10 mg ÷ 2 mL = 5 mg/mL
Dose in mg: 200 mcg = 0.2 mg
Volume: 0.2 mg ÷ 5 mg/mL = 0.04 mL
Units: 0.04 mL × 100 = 4 units
4 units is small. If your syringe markings are spaced for 100-unit total volume, a 4-unit dose is hard to read accurately. Use a 30-unit syringe instead — same 4 units, but the spacing is more than three times wider on the barrel.
Why intuition fails here
Three things make peptide math harder than it looks:
| Problem | What it does |
|---|---|
| mcg vs mg | A 1000x difference. "0.25" can mean 0.25 mg (250 mcg) or 0.25 mcg depending on context |
| Small volumes | Doses in the 0.04–0.2 mL range require precise syringe choice |
| Concentration depends on you | The same vial gives different concentrations depending on how much water you add |
The fix is mechanical: write the units explicitly every time, double-check decimals, and use the calculator instead of doing the math in your head.
How to choose the water volume
The amount of BAC water you add determines the concentration, which determines how many units you'll draw. There's no universally correct answer — it depends on the dose size and the syringe you want to use.
A rough heuristic:
| Goal | Suggested water amount |
|---|---|
| Round-number units (most common) | 2 mL into a 5 mg vial → 2.5 mg/mL |
| Larger doses with smaller syringe volume | 1 mL into a 5 mg vial → 5 mg/mL |
| Very small doses for accuracy | 3 mL into a 5 mg vial → 1.67 mg/mL (more volume per dose) |
The general rule: more water = more volume per dose = easier to measure small doses. Less water = less volume per dose = bigger doses fit on smaller syringes.
Sanity-checking your answer
Before you inject, run two checks:
- Volume check. Is the calculated volume between 0.05 mL (5 units) and 1 mL (100 units)? If under 5 units, the syringe markings are too coarse — reconstitute with more water. If over 100 units, your dose won't fit on a single syringe.
- Order-of-magnitude check. Is your dose realistic for the peptide? BPC-157 at 250 mcg = 10 units is normal. BPC-157 at 250 mg = 100 mL is impossible. If your math gives an absurd answer, you mixed mcg and mg somewhere.
If both checks pass and the calculator agrees, you're good to draw.