Peptide dose calculator
Enter your vial strength, the bacteriostatic water you added, and your dose. The calculator returns the exact number of units to draw, the volume in millilitres, and the resulting concentration. It supports U-100 insulin syringes in all three barrel sizes, U-40 syringes, and tuberculin syringes marked in millilitres — the same options as the Bo app.
Calculator
Check the result against the markings on your own syringe and your prescriber's instructions before drawing. This tool does the arithmetic; it does not decide your dose.
How much water should I add?
This is a readability decision, not a dosing one. The vial holds a fixed amount of compound no matter how much water goes in, so the only thing the volume changes is where your dose lands on the syringe. Aim for a number that is easy to read and hard to misdraw — most people target 10 units or more per dose — while staying inside the barrel's capacity.
Worked the other way round: to put a 0.25 mg dose at the 20 unit mark on a U-100 syringe you need 0.2 mL to contain 0.25 mg, which is 1.25 mg/mL, so a 5 mg vial wants 4 mL of water.
The formula
- Concentration = vial mg ÷ water mL → mg per mL
- Volume = dose mg ÷ concentration → mL
- Units = volume mL × units per mL (100 for U-100, 40 for U-40)
Round only the final number. Rounding the concentration first introduces error that compounds.
U-100 vs U-40 vs tuberculin
| Syringe | Units per mL | Common capacity | 0.2 mL reads as |
|---|---|---|---|
| U-100 insulin | 100 | 1 mL, 0.5 mL, 0.3 mL | 20 units |
| U-40 insulin | 40 | 1 mL | 8 units |
| Tuberculin | marked in mL, not units | 1 mL | 0.2 mL |
The three barrel sizes of a U-100 syringe are all the same concentration marking; they differ only in how much they hold, and a narrower barrel makes small doses easier to read accurately. Mixing up U-100 and U-40 is a 2.5x error in either direction, which is why the barrel print matters more than the box.
Common questions
How many units is 0.25 mg of semaglutide?
There is no fixed answer, because it depends on the reconstitution. At 2.5 mg/mL (a 5 mg vial in 2 mL) it is 10 units on U-100. At 1.25 mg/mL (the same vial in 4 mL) the same 0.25 mg dose is 20 units. The dose is identical; only the reading changes.
What if my result is more than the syringe holds?
Either an input is wrong or the vial needs less water. Recheck the numbers rather than splitting one dose across two injections.
Does bacteriostatic water expire once the vial is mixed?
Bacteriostatic water contains benzyl alcohol as a preservative and its manufacturer labelling commonly specifies discarding it 28 days after first puncture. Follow the label on the product you actually have and your prescriber's storage guidance.
Do this once instead of every week
Bo stores your vial, water volume and dose, then shows the unit number on every dose reminder, logs each shot, rotates your injection sites and models what is still in your system between doses. It also scans meals and answers peptide questions in plain language.
Educational information only. Bo is a tracking app, not a medical device, and nothing here is medical advice, a dosing recommendation, or a substitute for your prescriber. Half-lives are population averages from the sources cited on each row; individual clearance varies with body weight, kidney and liver function, and other medication.