Peptide reconstitution calculator
How much bacteriostatic water to add, and how many units to draw for the dose you want. Works in both directions, needs no account, and runs the same code as the TherapyLog app.
A reconstitution calculator answers one question in three steps. Adding water to a vial of dry peptide gives you a concentration; a dose divided by that concentration gives you a volume; and a volume multiplied by a hundred gives you insulin units, because a U-100 syringe has a hundred marks to the millilitre. Everything else is arithmetic around those three lines.
Worked through with real numbers: a 5 mg vial with 2 ml of bacteriostatic water is 5,000 mcg spread through 2 ml, so 2,500 mcg per ml. A 250 mcg dose is 250 ÷ 2,500 = 0.1 ml, and 0.1 ml on a U-100 syringe is 10 units. The same vial in 1 ml of water would be 5,000 mcg/ml, and that same 250 mcg dose would be 5 units instead — half the liquid, the same drug, a different number on the barrel.
That last point is the one worth internalising: the water changes the number you draw, not the dose you get. More diluent means a larger, easier-to-measure draw for the same amount of peptide. Less means a smaller draw, and eventually one too small to measure honestly.
volume to draw (ml) = dose (mcg) ÷ concentration (mcg/ml)
units on a U-100 syringe = volume to draw (ml) × 100
How the math works
Concentration first, because nothing else can be worked out without it. Vial strength is on the label in milligrams; multiply by a thousand to get micrograms, then divide by the millilitres of diluent you added. That is micrograms per millilitre, and it is the number worth writing on the vial in marker once you have it — it does not change until the vial is empty.
Then the draw. Divide the dose you want by the concentration and you have a volume in millilitres. Multiply that by a hundred and you have units on a U-100 syringe, which is what almost every insulin syringe sold is. U-100 means a hundred units per millilitre — the "100" is a concentration marking, not a capacity, which is why a 0.5 ml syringe is still U-100 and still reads to 50.
Two rules the calculator applies that are worth knowing on their own. A practical draw lands on a half-unit and is at least two units: below that, the graduations on an insulin syringe are finer than anyone can read against a meniscus, and a "1 unit" dose is a guess with a number attached. And a draw larger than the barrel is flagged rather than silently split — if your dose needs 130 units, the answer is a different reconstitution, not two injections.
| Draw | On a U-100 (1 ml) | On a U-50 (0.5 ml) | On a 0.3 ml (30 u) |
|---|---|---|---|
| 0.05 ml | 5 units | 5 units | 5 units |
| 0.1 ml | 10 units | 10 units | 10 units |
| 0.25 ml | 25 units | 25 units | over the barrel |
| 0.5 ml | 50 units | 50 units (full) | over the barrel |
| 1 ml | 100 units (full) | over the barrel | over the barrel |
Working backwards from the dose you want
Most calculators only go forwards: you tell them what you mixed, they tell you what to draw. The question people actually have is the other one — I want this dose to be a round number of units, so how much water do I add?
Rearranged, that is the same three lines read from the bottom up. If you want a dose of D micrograms to come out at U units, then the volume per dose is U ÷ 100 millilitres, so the concentration has to be D ÷ (U ÷ 100), and the diluent volume is the vial's total micrograms divided by that concentration. The "work it out" panel in the calculator above does exactly that, and shows every practical fill volume with the concentration and the number of doses per vial for each, so you can pick the one that leaves the least remainder.
It also refuses to answer in two situations, both deliberate: more than about 5 ml of diluent, because most vials do not have the headspace for it, and less than 0.3 ml, because there is not enough liquid to mix and draw from reliably.
Questions people actually ask
How many units is 250 mcg?
There is no answer without the concentration — that is the whole point of the calculation. 250 mcg from a 5 mg vial mixed with 2 ml of bacteriostatic water is 10 units; the same 250 mcg from the same vial mixed with 1 ml is 5 units. Whenever you see "X mcg is Y units" stated flatly, a reconstitution has been assumed and not mentioned.
Does adding more water make the dose weaker?
No. It makes the solution more dilute, so you draw a larger volume to get the same amount of peptide. The dose is however many micrograms end up in you, and that is set by what you draw, not by how much water is in the vial. More water generally makes dosing more accurate, because a bigger draw is easier to measure.
Bacteriostatic water or sterile water?
Bacteriostatic water contains about 0.9% benzyl alcohol, which is what allows a vial to be entered more than once. Plain sterile water has no preservative, so a vial mixed with it is a single-use preparation. If a vial holds several doses over several weeks — which is the normal case for a reconstituted peptide — that difference decides whether the vial is usable on day ten.
What if my dose needs more units than the syringe holds?
Reconstitute with less water. A dose needing 130 units on a U-100 syringe becomes 65 units if you halve the diluent volume, which fits. Splitting one dose across two injections works arithmetically but doubles the injections and the entry points, and it is usually a sign the reconstitution was chosen for the wrong dose.
Can I trust the milligram number on the vial?
The calculator cannot check it, and neither can you without an assay. Everything on this page is arithmetic downstream of the strength you type in; if that figure is wrong the result will be confidently wrong by the same proportion. For a compounded or research-labelled vial, that uncertainty is the largest error term in the whole calculation, and it is worth naming rather than rounding away.
Pre-filled for a specific compound
Same calculator, started at that compound's typical vial sizes, with a milligram-to-units table and the half-life and storage rule the app records: Semaglutide, Tirzepatide, Retatrutide, BPC-157, TB-500.
The app keeps the vial, the concentration and every dose you draw from it, so this arithmetic only has to be done once.
Save this dose to your logThis calculator does the arithmetic you typed and nothing else. It does not know what is actually in your vial, whether the label is accurate, or anything about you. Confirm the vial strength and the diluent volume on your own label before you draw, and take dosing decisions to a qualified provider.