Peptide Calculator

How the math works, with one worked example

Take a 5 mg vial of BPC-157 and add 2 mL of bacteriostatic water. The vial now holds 5 mg in 2 mL, so the concentration is 2.5 mg per mL. Because a milligram is 1,000 micrograms, that is 2,500 mcg per mL, and because a U-100 syringe divides each millilitre into 100 units, each unit holds 25 mcg.

If your dose is 250 mcg, you need 250 divided by 25, which is 10 units. Written as volume, that is 0.1 mL. If your dose is 500 mcg, you need 20 units. A 5,000 mcg vial at 250 mcg per dose holds 20 doses, so at once a day it lasts 20 days.

Now change one thing. Add 1 mL of water instead of 2. The concentration doubles to 5 mg per mL, each unit holds 50 mcg, and the same 250 mcg dose is 5 units. You did not change the dose. You changed how much liquid carries it. That is the whole idea, and it is why the question “how much water should I add” has no single answer: you pick the water so the draw lands somewhere you can read.

How much bacteriostatic water should I add?

Enough that your dose lands between about 10 and 40 units on the syringe you own, on a printed mark, and little enough that the vial can hold it. That rule comes from two places. The first is the syringe-accuracy literature: in studies of insulin syringes, draws under 5 units, and draws under a tenth of the barrel, carried errors above 10 percent, and a 2024 hospital study measured a median error of 30 percent when nurses targeted 2 units. The second is practical: a 1 mL syringe holds 100 units, so a draw that computes to 120 units does not fit.

The calculator above does this for you. Enter the vial and the dose, and the “how much water” panel lists the volumes that put your dose on a printed mark, inside the accurate part of the barrel, with the ones closest to the middle of the barrel first. Most microgram doses want 2 to 5 mL. Most milligram doses want 1 to 2 mL. Large vials of GLP-1 peptides often want a concentration of exactly 10 mg per mL because it makes every dose easy: 10 units per milligram.

Two constraints the volume panel already respects: bacteriostatic water is sold in 30 mL multi-dose vials (Hospira's label lists 0.9 percent benzyl alcohol as the preservative), and research peptide vials are usually 2 mL, 3 mL or 10 mL glass. If the panel suggests 5 mL and your vial is a small one, take the next option down.

Reading your syringe: the three sizes and the U-40 trap

Insulin syringes come in three sizes and all of them are U-100, which means 100 units per millilitre. The size changes what fits and how fine the marks are, not what a unit means.

• 0.3 mL (30 units). Usually marked in half units. Best for draws under 30 units and the only size where a 2.5-unit draw sits on a printed line.
• 0.5 mL (50 units). Marked in single units. The everyday choice for draws between 10 and 50 units.
• 1 mL (100 units). Marked in 2-unit steps. Needed for draws above 50 units; less precise for small ones because odd numbers fall between lines.

The trap is the U-40 syringe, sold for veterinary insulin. Its marks are 40 units per millilitre, so a “10 unit” draw on a U-40 syringe is 0.25 mL, two and a half times the volume of 10 units on a U-100 syringe. Every syringe on this page assumes U-100, and every result shows millilitres next to units so you can check. That practice is not ours. The Institute for Safe Medication Practices recommends expressing injectable doses in both units and volume after finding that most syringe-related insulin overdoses involved a non-insulin 1 mL syringe and a tenfold error.

Pre-mixed pharmacy vials: read the label, not the forum

Compounded semaglutide and tirzepatide usually arrive already in solution, with the concentration printed on the label as milligrams per millilitre. Use the pre-mixed mode above and enter that number. The label may state your dose in millilitres; if your syringe reads in units, multiply by 100. A label that says 1.1 mL means 110 units. One of the most common questions in patient forums is a person whose label says 1.1 mL and whose online calculator says 11 units. The label is right and the calculator had the wrong concentration.

Two more label habits worth knowing. Some pharmacies state concentration per 0.5 mL rather than per millilitre; double it before you enter it. And when you change pharmacies, the concentration usually changes even though the dose does not. The “new concentration” mode converts your old draw to the new vial without you touching the milligrams.

The mistakes this calculator is built to catch

• Entering milligrams as micrograms. 0.25 mg is 250 mcg. Enter 0.25 in a microgram field and you get a draw of zero. The dose field shows both units so the mistake is visible.
• The 10-units-equals-1-mg rule. It is only true at exactly 10 mg per mL. The results flag when you are at that concentration, and only then.
• Wrong amount of water. You meant to add 2 mL and added 3. The milligrams did not change; the units scale by three halves. The fix mode does that arithmetic and shows whether the corrected draw still fits your syringe.
• Draws that do not fit. 5 mg of TB-500 in 2 mL puts a 2.5 mg dose at 100 units, the whole barrel. The result says so and suggests less water, not two injections.
• Vials that outlast the 28-day ceiling. A 10 mg vial at 200 mcg a night is 50 doses. USP Chapter 797 treats a punctured multiple-dose vial as good for 28 days. The days-of-supply figure turns amber when you cross it.
• Blend vials. A 10 mg vial that is 5 mg of one peptide and 5 mg of another. Switch on the blend option and each draw is split into its components.

Storage after mixing

Keep reconstituted vials refrigerated at 2 to 8 degrees Celsius, out of light, and do not freeze them. The 28-day figure that appears everywhere is the USP Chapter 797 rule for any punctured multiple-dose container: it is a sterility ceiling, not a statement about how long a particular peptide keeps its potency. For compounded prescriptions the beyond-use date printed on the pharmacy label is the number that counts. The printable label button above puts the mix date, the discard date and your draw on one card for the vial.

Starting points by peptide, with the evidence labelled

Each peptide below has its own page with vial sizes, the dose figures that exist and where they come from, worked unit tables and the same calculator pre-set for that vial. The evidence label is the important column. Where an FDA label exists, we quote it. Where the only figures are forum protocols, we say so and do not pre-fill a dose.

Frequently asked questions

Sources

1. FDA guidance: content of premarket notification submissions for piston syringes (insulin syringes scaled in units, U-100 labelling)
2. ISMP Canada: dose confusion when switching between insulin delivery devices (unit markings apply only to U-100 products)
3. Review of insulin syringe accuracy: error above 10 percent below 5 units and at low fractions of nominal capacity (PMC8114303)
4. 2024 hospital study of small-volume insulin draws: median error 30 percent at a 2-unit target (PMC11291810)
5. Ro and Jones, Patient Safety 2026: syringe-related insulin overdoses, 73.8 percent involved a non-insulin 1 mL syringe, 95.2 percent were tenfold
6. ISMP: express doses in both units and volume
7. Bacteriostatic Water for Injection, USP: prescribing information (DailyMed)
8. USP General Chapter <797>: multiple-dose containers, 28 days after initial puncture
9. Egrifta SV and Egrifta WR (tesamorelin) prescribing information: labelled reconstitution volumes and daily doses
10. Zepbound (tirzepatide) prescribing information
11. Wegovy (semaglutide) prescribing information

How this page was made

We read the eleven calculators that rank for these searches, several hundred forum threads and video comments where people describe the mistakes they actually made, and the syringe-accuracy and medication-safety literature cited above. The tool was then built around the errors people make rather than the arithmetic alone, which is why it shows millilitres beside units, refuses draws that do not fit, and flags draws too small to measure. The FormBlends Editorial Standards Team checked every figure on this page against the sources listed. Dose figures for peptides without an approved label are marked as forum protocols and are not pre-filled.