Research peptides such as retatrutide ship as a lyophilised (freeze-dried) powder. Before the powder can be measured, it has to be dissolved, usually in bacteriostatic water. The maths that follows confuses many people, so this guide walks through it with worked examples.
Want the numbers instantly? Use our free retatrutide dosing calculator. It does all of the arithmetic below and draws the syringe for you.
Retatrutide is an investigational compound without UK marketing authorisation. This article explains laboratory arithmetic and isn’t a dosing recommendation.
The three numbers that matter
- Vial content (mg). What the label says: 10 mg, 20 mg, 30 mg and so on.
- Water added (ml). You choose this. Common choices are 1, 2 or 3 ml.
- Amount wanted (mg). The quantity you need to draw.
Everything else follows from those three.
Step 1: Work out the concentration
Concentration (mg/ml) = vial mg ÷ water ml
| Vial | + 1 ml | + 2 ml | + 3 ml |
|---|---|---|---|
| 10 mg | 10 mg/ml | 5 mg/ml | 3.33 mg/ml |
| 20 mg | 20 mg/ml | 10 mg/ml | 6.67 mg/ml |
| 30 mg | 30 mg/ml | 15 mg/ml | 10 mg/ml |
| 60 mg | 60 mg/ml | 30 mg/ml | 20 mg/ml |
Step 2: Convert mg to ml
Volume (ml) = amount (mg) ÷ concentration (mg/ml)
Example: a 10 mg vial with 2 ml of water is 5 mg/ml. To draw 2 mg: 2 ÷ 5 = 0.4 ml.
Step 3: Convert ml to syringe units
Insulin syringes are marked in units, not ml. On a standard U-100 syringe, 100 units = 1 ml, so:
Units = ml × 100
0.4 ml × 100 = 40 units.
Worked examples
| Vial | Water | Concentration | 2 mg = | 4 mg = | 8 mg = |
|---|---|---|---|---|---|
| 10 mg | 2 ml | 5 mg/ml | 40 units | 80 units | 160 units* |
| 10 mg | 1 ml | 10 mg/ml | 20 units | 40 units | 80 units |
| 20 mg | 2 ml | 10 mg/ml | 20 units | 40 units | 80 units |
| 30 mg | 3 ml | 10 mg/ml | 20 units | 40 units | 80 units |
| 60 mg | 3 ml | 20 mg/ml | 10 units | 20 units | 40 units |
*More than a 1 ml (100-unit) syringe holds. Use less water or a bigger vial.
How much water should you add?
There’s no single right answer. It’s a trade-off:
- More water means each unit holds less peptide, so small amounts are easier to measure accurately. The downside is larger volumes.
- Less water means smaller volumes, but a one-unit misread is a bigger error.
A practical rule: choose the water volume so the amounts you need land between roughly 10 and 50 units on your syringe.
How long does a vial last?
Number of draws = vial mg ÷ amount per draw
A 10 mg vial holds five 2 mg amounts. A 30 mg vial holds 7.5 amounts of 4 mg. Our calculator shows this for any combination, and the price page shows how the price per mg drops on larger vials.
Handling and storage basics
- Let the vial reach room temperature, then add the water slowly down the side of the vial. Don’t squirt it directly onto the powder.
- Swirl gently; don’t shake. Peptides can degrade with aggressive agitation.
- Keep unmixed powder cool and dark. Store reconstituted vials in the fridge (2–8 °C) and use them within the supplier’s stated window.
- Use bacteriostatic water (0.9% benzyl alcohol) for multi-draw vials, not plain sterile water.
- Check the supplier’s batch-specific certificate of analysis before use. Our UK supplier ranking gives extra weight to suppliers that publish independent purity and contaminant testing.
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