GLP-1 reconstitution calculator: water, concentration, units
For planning and record-keeping only — not medical advice. Your dose and schedule are set by your prescriber.
A 5 mg semaglutide vial mixed with 2 mL of bacteriostatic water gives 2.5 mg/mL, and at that strength a 0.25 mg dose is 10 units on a U-100 syringe. This calculator works the water volume out for you — either from a volume you already have in mind, or backwards from the number of units you want to draw.
One page, two directions. The units calculator starts from a concentration that already exists and tells you what to draw; this page starts from a vial with nothing in it yet and tells you what concentration to make. Pick the mode that matches the question you actually have.
Typical vials
0 mg/mL after reconstitution
Standard semaglutide dose steps at this concentration
| Dose | Units (U-100) | Volume |
|---|
0 mL of bacteriostatic water
That makes 0 mg/mL, so a 0 mg dose lands on 0 units. The vial then holds about 0 doses of that size.
That is an unusually large volume, re-check the numbers — vials this size are normally reconstituted with a few millilitres at most.
The dose you entered is larger than everything the vial contains, so no water volume can produce it.
Enter positive numbers only — zero or negative values can't be used here.
Your pharmacy or protocol sets the concentration — confirm these numbers with the pharmacist who dispensed the vial.
mg/mL = vial mg ÷ water mL
water mL = (vial mg × units per dose) ÷ (dose mg × 100)
KeepAlong — GLP-1 shot tracker & planning tools
There is no reconstitution or dose math inside the KeepAlong app — Apple's guideline 1.4.2 rules dosage calculators out, so that stays here on the web. The app logs every injection, reminds you on shot day, and remembers which vial and which dilution you are on right now.
The app is launching soon. The web tools on this site are free — no sign-up needed.
Semaglutide: common vial sizes
Compounded semaglutide is usually dispensed as 2, 5, or 10 mg of powder in a vial, and the same vial can become several different concentrations depending on the water. The five dose steps in the columns below are the standard ladder — the titration schedule planner turns those steps into calendar dates. If you log doses in both milligrams and units, the guide to tracking semaglutide covers how the two lines fit in one weekly record.
| Vial | Water | Concentration | 0.25 mg | 0.5 mg | 1 mg | 1.7 mg | 2.4 mg |
|---|---|---|---|---|---|---|---|
| 2 mg | 1 mL | 2 mg/mL | 12.5 units | 25 units | 50 units | 85 units | — |
| 2 mg | 2 mL | 1 mg/mL | 25 units | 50 units | 100 units | 170 units* | — |
| 5 mg | 1 mL | 5 mg/mL | 5 units | 10 units | 20 units | 34 units | 48 units |
| 5 mg | 2 mL | 2.5 mg/mL | 10 units | 20 units | 40 units | 68 units | 96 units |
| 10 mg | 1 mL | 10 mg/mL | 2.5 units | 5 units | 10 units | 17 units | 24 units |
| 10 mg | 2 mL | 5 mg/mL | 5 units | 10 units | 20 units | 34 units | 48 units |
| 10 mg | 4 mL | 2.5 mg/mL | 10 units | 20 units | 40 units | 68 units | 96 units |
* More than 100 units — that volume doesn't fit one 1 mL U-100 syringe. A dash marks a dose bigger than everything the vial contains.
Two rows make the point on their own: a 5 mg vial with 1 mL and a 10 mg vial with 2 mL both land on 5 mg/mL and produce identical syringe marks. The number printed on the vial is not the concentration, and it never was — the water volume is.
Tirzepatide: common vial sizes
Tirzepatide vials are larger, typically 10, 20, or 30 mg, because the dose steps themselves are ten times bigger than semaglutide's. The practical consequence is that low water volumes produce very concentrated solutions where a whole dose sits inside a few syringe marks. The guide to tracking tirzepatide covers what to write down when one vial spans several weeks.
| Vial | Water | Concentration | 2.5 mg | 5 mg | 7.5 mg | 10 mg | 12.5 mg | 15 mg |
|---|---|---|---|---|---|---|---|---|
| 10 mg | 1 mL | 10 mg/mL | 25 units | 50 units | 75 units | 100 units | — | — |
| 20 mg | 1 mL | 20 mg/mL | 12.5 units | 25 units | 37.5 units | 50 units | 62.5 units | 75 units |
| 20 mg | 2 mL | 10 mg/mL | 25 units | 50 units | 75 units | 100 units | 125 units* | 150 units* |
| 30 mg | 1 mL | 30 mg/mL | 8.3 units | 16.7 units | 25 units | 33.3 units | 41.7 units | 50 units |
| 30 mg | 1.5 mL | 20 mg/mL | 12.5 units | 25 units | 37.5 units | 50 units | 62.5 units | 75 units |
* More than 100 units — that volume doesn't fit one 1 mL U-100 syringe. A dash marks a dose bigger than everything the vial contains.
At 30 mg/mL a 2.5 mg starting dose is 8.3 units, which sits between two marks on a U-100 syringe and is hard to read accurately. That is the practical argument people weigh when they pick a volume: fewer marks per dose means less room to measure, more marks means a bulkier injection. The trade-off is real, but the decision is not this page's to make.
What this calculator does not decide
It does not set your dose. The milligrams you type in come from a prescription, and nothing on this page suggests raising, lowering, holding, or spacing them. Every one of those is a prescribing decision, and the arithmetic here is indifferent to which number you put in the box.
It does not recommend a concentration either. Both directions of the tool answer a question you bring to it: you supply the water volume, or you supply the units you want, and the arithmetic follows. Which concentration is right for your vial is a pharmacy decision, and the answer is on your paperwork, not in a calculator.
It says nothing about sterile technique, mixing method, or where a vial came from. Those are outside the scope of a number-crunching page, and getting them from a web tool would be a bad idea regardless of what the tool said.
And it does not tell you how long a reconstituted vial stays usable. That window is a beyond-use date set by the pharmacy that prepared it and printed on the label you were given. Storage windows published for factory-filled pens describe a different product in a different container and do not transfer to a vial you mixed yourself. If the label has no beyond-use date, that is a question for the pharmacy, not for arithmetic.
What the calculator does hand you is a concentration and a units figure — and those two numbers are worth writing down every time the vial changes, because a new dilution silently changes every syringe mark that follows. Keeping that record next to the injection dates is the job of the GLP-1 shot tracker app.
Frequently asked questions
How much bacteriostatic water do you add to a 5 mg semaglutide vial?
Whatever volume your pharmacy or protocol specifies — the water sets the concentration, not the dose. The two common choices: 1 mL gives 5 mg/mL, so a 0.25 mg dose is 5 units; 2 mL gives 2.5 mg/mL and the same dose is 10 units. Both deliver identical medication.
How do you get a round number of units per dose?
Work backwards from the units. Water mL = vial mg × units per dose ÷ (dose mg × 100). For a 10 mg tirzepatide vial, a 2.5 mg dose and a 25-unit target: 10 × 25 ÷ (2.5 × 100) = 1 mL, which makes 10 mg/mL. Confirm the volume with the pharmacy that dispensed the vial.
Does the concentration change how much medication you get?
No. A 0.5 mg dose is 0.5 mg whether you draw 10 units at 5 mg/mL or 20 units at 2.5 mg/mL. Concentration only changes where the plunger lands on the syringe. A lower concentration spreads the same dose over more marks, which some people find easier to read.
When should you not use a reconstitution calculator?
When the vial arrives pre-mixed with a concentration on the label, or when you use a pen — there is nothing to reconstitute. Skip it too if the pharmacy already told you the volume: their number wins over any calculator. And stop entirely if the vial contents are unlabeled or unclear.