Practical helpers
Peptide dosage & reconstitution calculator
Tell us what’s in the vial and how much water you’re adding. We’ll do the arithmetic and show you exactly where to draw — no mental math, no second-guessing.
Mix your vial
Adding water never changes how much peptide is in the vial — only how diluted it is.
The total printed on the label — e.g. 5 mg.
More water = a bigger, easier-to-measure draw.
Set your dose
Use the dose from your protocol. Remember: 1 mg = 1,000 mcg.
All U-100 — same units on every barrel, finer marks on smaller ones.
Only used to estimate how long a vial lasts.
UNITS · U-100 SCALE
How to use it
Four numbers in, one clear answer out. Everything updates live as you type, so you can nudge the water volume and watch the draw move. Peptides are short chains of amino acids, and more than 80 peptide drugs have reached the market in the century since insulin[1][2] — but nearly all research vials still arrive as dry powder, so the arithmetic below is unavoidable.
Enter what’s in the vial
Type the total peptide printed on the label — 5 mg, 10 mg, whatever it says. Adding water later won’t change this number.
Enter what’s in the vial
Type the total peptide printed on the label — 5 mg, 10 mg, whatever it says. Adding water later won’t change this number.
Enter what’s in the vial
Type the total peptide printed on the label — 5 mg, 10 mg, whatever it says. Adding water later won’t change this number.
Enter what’s in the vial
Type the total peptide printed on the label — 5 mg, 10 mg, whatever it says. Adding water later won’t change this number.
What reconstitution actually means
Peptides arrive as a dry, freeze-dried (lyophilized) powder because removing the water dramatically slows the reactions that would otherwise break them down.[3] That stability is great for shelf life — but it means you can’t measure a dose until you’ve dissolved the powder in liquid.
Reconstitution is that dissolving step. You add bacteriostatic water slowly down the side of the vial, then swirl gently rather than shaking. The moment you choose how much water to add, you’ve fixed the concentration — and therefore every dose you’ll ever draw from that vial.
The one thing to remember
Water changes the concentration, never the amount. A 5 mg vial holds 5 mg whether you add 1 mL or 3 mL — you’re only deciding how spread out it is.
How much water should you add?
There’s no single right answer — it’s a trade-off, and the calculator lets you test both directions in seconds.
More water
Lower concentration, larger draw. Easier to measure accurately — best when your dose is very small.
Less water
Higher concentration, smaller draw. Useful when a dose would otherwise overflow your syringe.
Bacteriostatic water is sterile water with 0.9% benzyl alcohol — a preservative that suppresses microbial growth so a vial can be drawn from repeatedly over its usable life.[4] It isn’t entirely inert: benzyl alcohol can, under some conditions, encourage peptide aggregation, which is part of why gentle swirling and cold storage are the standard advice.[5] Plain sterile water has no preservative, so it’s really only suited to single use.
The three conversions behind it
You never have to do these by hand, but knowing them makes every result easy to sanity-check.
Concentration
peptide (mcg) ÷ water (mL) = mcg per mL
Concentration Draw volume
dose (mcg) ÷ concentration (mcg/mL) = mL
Syringe units
volume (mL) × 100 = units on a U-100 barrel
Reading the syringe
A U-100 insulin syringe holds 100 units per millilitre, so units are just millilitres × 100. A 0.1 mL draw is 10 units; a 0.05 mL draw is 5 units. That much is the same on every barrel.
What differs is the spacing of the marks — and that’s where accuracy is won or lost. Measurement studies consistently find that relative error grows as the measured volume becomes a smaller fraction of the syringe’s capacity, so matching the barrel to your draw genuinely improves precision.[6][7] The calculator flags the smallest barrel that comfortably fits.
| Barrel | Marked every | Best for |
|---|---|---|
| 30-unit · 0.3 mL | Half a unit | Small doses — the most precise option. |
| 50-unit · 0.5 mL | 1 unit | Mid-range draws; a good all-rounder. |
| 100-unit · 1 mL | 2 units | Larger draws that won’t fit a smaller barrel. |
A worked example
Say you’ve got a 5 mg vial and you add 2 mL of bacteriostatic water. That’s 2.5 mg/mL — or 2,500 mcg per mL. Each insulin unit (0.01 mL) therefore holds 25 mcg.
Same vial, same dose — three different water volumes
1 mL
5 mg/mL — twice as concentrated
5 units
2 mL
2.5 mg/mL — the middle ground
10 units
3 mL
≈1.67 mg/mL — more dilute
≈15 units
Nothing about the peptide changed — a 250 mcg dose is still 250 mcg. Only the number you read on the barrel moved.
That 5 mg vial holds 5,000 mcg, so at 250 mcg per injection it gives you 20 full doses. This is exactly the relationship the calculator makes visible, so you can pick a concentration that lands your dose on a clean, easy-to-read mark.
Common questions
There’s no single correct volume — the water sets the concentration, not the dose. More water gives a larger, easier-to-measure draw; less water gives a higher concentration and a smaller draw. Enter your vial size and target dose above, then nudge the water until the draw lands on a clean syringe mark. 1 mL, 2 mL, and 3 mL are the common starting points.
Divide your dose by the concentration to get the draw volume in millilitres, then multiply by 100. A 250 mcg dose from a 2,500 mcg/mL solution is 0.1 mL — which is 10 units on a U-100 syringe. The calculator runs this the moment you type.
Match the barrel to your draw. A 30-unit (0.3 mL) barrel has the finest marks and is most accurate for small doses; 50-unit suits mid-range draws; 100-unit fits the largest. All three use the same U-100 scale, so the unit number is identical — only the precision differs.
The math is identical either way, but sterile water contains no preservative, so a vial mixed with it offers no protection against microbial growth once opened and is really only suited to single use. Bacteriostatic water contains 0.9% benzyl alcohol, which makes multi-dose use over a vial’s life far more practical.
Once reconstituted, a peptide solution is much less stable than the dry powder. Keep it refrigerated and out of the light, and expect a usable window of a few weeks rather than months — dry lyophilized powder is what stays stable long-term. Always follow the guidance for your specific compound.
Because micrograms measure mass and millilitres measure volume — they only connect once you know the concentration. That’s the whole reason reconstitution comes first: once you know how many mcg sit in each mL, dividing your dose by that number gives you the volume.
References
- Therapeutic peptides: historical perspectives, current development trends, and future directions. — Bioorg. Med. Chem., 2018
- Trends in peptide drug discovery. — Nat. Rev. Drug Discov., 202
- Stability of protein pharmaceuticals — why lyophilization preserves peptides. — Pharmaceutical Research, 1989
- Antimicrobial preservatives for protein and peptide formulations: an overview. — Pharmaceutics, 2023
- Effects of benzyl alcohol on aggregation in reconstituted lyophilized formulations. — J. Pharm. Sci., 2005
- Development of guidelines for accurate measurement of small volume parenteral products using syringes. — Hospital Pharmacy, 2021
- Accuracy and reproducibility of low dose insulin administration using pen-injectors and syringes. — Arch. Dis. Child., 1998