Reconstitution means turning a freeze-dried powder back into a solution you can measure and inject. The arithmetic is genuinely simple — one division and one multiplication — and the complexity people run into is almost entirely about which numbers to trust, not about the maths.
The only equation you need
A vial contains a known mass of peptide. You add a known volume of liquid. Dividing the first by the second gives the concentration, and everything else follows:
- Concentration = milligrams in the vial ÷ millilitres of water added
- Volume to draw = your dose ÷ concentration
- Units on the syringe = volume in millilitres × 100, on a U-100 barrel
A 10 mg vial with 2 mL of water gives 5 mg/mL. A 2 mg dose is 2 ÷ 5 = 0.4 mL, which is 40 units. That is the whole calculation. The calculator does it both directions, including solving the water volume backwards from the units you want to draw.
The water does not change your dose
This is the single most common misunderstanding. Adding more bacteriostatic water does not make the dose weaker. It makes the same dose occupy more volume, so you draw further up the barrel. The peptide in the vial is a fixed mass no matter what you dissolve it in.
Which means the water volume is a convenience decision, not a clinical one. Pick a volume that puts your dose somewhere you can read accurately. Below roughly 5 units, small reading errors become large percentage dose errors. Above the barrel’s capacity, you are splitting into two injections. Between about 10 and 50 units is comfortable.
There is a physical ceiling too: most peptide vials hold around 3 mL, so you cannot add 5 mL regardless of what the arithmetic suggests.
The order of operations
The mechanics are covered in detail by approved drug labels, and their instructions are worth following because they exist for reasons. The tesamorelin Instructions for Use, for instance, walk through a full reconstitution in FDA-approved wording.
- Let a refrigerated vial come to room temperature before adding liquid.
- Wipe both stoppers with alcohol and let them dry.
- Add the diluent slowly, aimed at the glass wall rather than directly onto the powder. This is the step where technique matters most.
- Swirl or roll gently until dissolved. Do not shake — approved labels for these products say so explicitly. Peptides are proteins and agitation can denature them.
- Inspect before drawing. A clear solution with no visible particles is what you want.
Which liquid
Bacteriostatic water contains benzyl alcohol as a preservative, which is what allows repeated withdrawals from the same container. Sterile water contains no preservative and is supplied only in single-dose containers. Some approved products specify one and some the other — tesamorelin’s two formulations differ from each other on exactly this point, which is part of why the label states they are not substitutable.
There is more to this than a one-line answer, including a genuine contraindication. The bacteriostatic water guide covers it.
Does the powder add volume?
Yes, and almost always by an irrelevant amount. Measured displacement for lyophilised powders runs about 0.66 to 0.70 mL per gram of total solids, so 10 mg of solids displaces around 7 microlitres — under three quarters of one unit, finer than the barrel can be read. FDA labels for milligram-scale lyophilisates state the nominal concentration and do not correct for it.
The exception is a vial carrying a heavy bulking agent. The tesamorelin EGRIFTA WR vial holds about 200 mg of total solids, where displacement reaches roughly 9% — which is part of why that label declares 8 mg/mL rather than the 8.9 mg/mL you would get by dividing. When a label states a concentration, use the label’s number rather than your own division. What matters is total solids including excipients, not peptide mass, and vendor peptide vials rarely tell you that figure.
Where the sourced part stops
Everything above is either arithmetic or reproduced from approved labelling. What no source can give you is a dose for a compound that has never been approved — and that is most of what gets reconstituted at home. Our individual peptide pages separate the two: approved drugs get their label’s schedule, and unapproved compounds get the doses that appeared in named studies, tagged with the species they were given to.