Peptide calculator: reconstitution, then units

Powder in a vial, water going in, a dose coming out. Three sums, in order. This page is written for people filling a prescription — a compounded GLP-1, most often.

Reconstitution → unitsU-100 syringe

Vial strength mg

Water you added mL

Your dose mg

Syringe

—

Fill in all three boxes.

Common setups:

The three sums

	Sum	Example
Concentration	vial mg ÷ water mL	10 ÷ 2 = 5 mg/mL
Volume per dose	dose mg ÷ concentration	2.5 ÷ 5 = 0.5 mL
Units to draw	volume mL × 100	0.5 × 100 = 50 units

The ×100 in the last line is the definition of a U-100 syringe: one millilitre is a hundred units. It's the only place the syringe enters the arithmetic at all.

How much water should I add?

Whatever your pharmacy told you. If they didn't, the drug doesn't care — 5 mg is 5 mg in any volume — so pick the amount that puts your dose somewhere comfortable to measure.

Same 10 mg vial, same 2.5 mg dose, different mixing water.
Water	Concentration	Units per dose	How it feels
0.5 mL	20 mg/mL	12.5	Tight. One mark out is an 8 % error.
1 mL	10 mg/mL	25	Fine.
2 mL	5 mg/mL	50	Comfortable. Easy to read.
4 mL	2.5 mg/mL	100	Fills the whole barrel. No room to titrate up.

The trap is that last row. More water makes today's dose easier to measure and can leave you with nowhere to go when the dose doubles. Mix for the dose you'll be on in three months, not the one you're on this week.

Peptide dosing chart

The same arithmetic, worked out ahead of time. First, what each vial becomes with each amount of water:

Concentration in mg/mL after reconstitution.
Vial	+ 1 mL water	+ 2 mL water	+ 3 mL water
5 mg	5	2.5	1.67
10 mg	10	5	3.33
15 mg	15	7.5	5
20 mg	20	10	6.67
30 mg	30	15	10
60 mg	60	30	20

Then, what a dose is in units at each of those concentrations:

Units on a U-100 syringe. Red: won't fit in one syringe, or under 4 units and hard to measure.
Dose	2.5 mg/mL	5 mg/mL	10 mg/mL	20 mg/mL
0.25 mg	10	5	2.5	1.25
0.5 mg	20	10	5	2.5
1 mg	40	20	10	5
2.5 mg	100	50	25	12.5
5 mg	200 ✗	100	50	25
10 mg	400 ✗	200 ✗	100	50

For the two drugs this site is really about, the full ladders are worked out with the syringe drawn at every step: tirzepatide · semaglutide. For any other vial, mg to units takes a printed concentration directly.

Practical bits

Bacteriostatic water, not sterile water. Bacteriostatic contains benzyl alcohol, which is what lets a vial be entered more than once. Sterile water is single-use.
Down the side of the glass, not onto the powder. Aim the stream at the vial wall and let it run down.
Swirl, don't shake. Give it a minute to dissolve on its own.
Cloudy or full of bits after it's dissolved is a stop sign. Call the pharmacy rather than injecting it.
Once it's mixed, it lives in the fridge and it has an expiry. Your pharmacy's beyond-use date is the one that counts.

This is the general version. If you know which drug you're on, the charts are quicker: tirzepatide · semaglutide.