Reconstitution Math Made Simple: Units, mg, and mL

Published July 17, 2026 · Medically reviewed content · Updated for 2026

Your pharmacy shipped a vial of lyophilized (freeze-dried) semaglutide with a separate vial of bacteriostatic water. The label says "5 mg semaglutide." Your prescribed dose is 0.5 mg per week. How much water do you add? How much do you draw per injection? How many weeks will the vial last?

If those questions make you nervous, this article is for you. We'll walk through the math step by step, with real examples and a formula you can use every time.

The Core Formula

Every reconstitution calculation uses one fundamental relationship:

Concentration = Total mg in vial ÷ Volume of water added (mL)

And every dose calculation uses the inverse:

Draw volume (mL) = Your dose (mg) ÷ Concentration (mg/mL)

That's it. Two formulas. Everything else is just plugging in numbers.

Step-by-Step Example: 5 mg Vial

Step 1: Know Your Numbers

• Total medication in vial: 5 mg (printed on the label)
• Your prescribed weekly dose: 0.5 mg
• Water volume you'll add: This is either specified by your pharmacy or chosen to create a convenient concentration

Step 2: Add Water and Calculate Concentration

Let's say your pharmacy instructs you to add 2 mL of bacteriostatic water.

Concentration = 5 mg ÷ 2 mL = 2.5 mg/mL

Your vial now contains a solution at 2.5 mg per milliliter.

Step 3: Calculate Your Draw Volume

Your prescribed dose is 0.5 mg per week.

Draw volume = 0.5 mg ÷ 2.5 mg/mL = 0.2 mL per injection

Each week, you draw 0.2 mL from the vial.

Step 4: Calculate How Long the Vial Lasts

Total doses = Total volume ÷ Draw volume = 2 mL ÷ 0.2 mL = 10 doses = 10 weeks

Your 5 mg vial reconstituted with 2 mL of water gives you 10 weeks of 0.5 mg/week doses.

What If You Add More or Less Water?

The amount of water you add changes the concentration — and therefore the draw volume — but it doesn't change the total medication in the vial. Using the same 5 mg vial:

• Add 1 mL: Concentration = 5 mg/mL. Draw for 0.5 mg dose = 0.1 mL. Very small volume — harder to measure accurately.
• Add 2 mL: Concentration = 2.5 mg/mL. Draw = 0.2 mL. Practical and easy to measure.
• Add 2.5 mL: Concentration = 2 mg/mL. Draw = 0.25 mL. Also practical.
• Add 5 mL: Concentration = 1 mg/mL. Draw = 0.5 mL. Easy math, but you now have 5 mL in the vial, which may not fit in a standard 3 mL vial.

Your pharmacy will specify the reconstitution volume based on the vial size, stability data, and the dose range they expect you to draw. Follow their instructions — the volume isn't arbitrary. It's chosen to keep your draw in a range that's measurable with a standard syringe.

Understanding Syringe Markings

The syringe you use should match your draw volume. Most compounded GLP-1 patients use insulin syringes, which are marked in "units" rather than milliliters. The conversion:

1 mL = 100 units on an insulin syringe

So if your draw volume is 0.2 mL, that's 20 units on an insulin syringe. If it's 0.25 mL, that's 25 units. The unit markings on insulin syringes are simply a finer-grained way of measuring milliliters.

Choosing the Right Syringe Size

• 0.3 mL (30-unit) syringe: Best for draw volumes under 0.3 mL. The markings are spaced further apart, making small doses easier to measure accurately. This is the best choice for most compounded GLP-1 doses.
• 0.5 mL (50-unit) syringe: Good for draws between 0.2 and 0.5 mL. Adequate precision for most doses.
• 1 mL (100-unit) syringe: Best for larger draw volumes. Less precise for small doses — each mark represents a larger increment, making it easier to over- or under-draw by a meaningful amount.

Rule of thumb: use the smallest syringe that comfortably fits your draw volume. Smaller syringe = bigger markings = more accurate measurement.

Common Reconstitution Scenarios

Scenario A: 3 mg vial, 1 mL water, 0.25 mg dose

• Concentration: 3 mg ÷ 1 mL = 3 mg/mL
• Draw: 0.25 mg ÷ 3 mg/mL = 0.083 mL ≈ 8 units on an insulin syringe
• Duration: 1 mL ÷ 0.083 mL = 12 doses (12 weeks)

Scenario B: 10 mg vial, 2 mL water, 1.0 mg dose

• Concentration: 10 mg ÷ 2 mL = 5 mg/mL
• Draw: 1.0 mg ÷ 5 mg/mL = 0.2 mL = 20 units
• Duration: 2 mL ÷ 0.2 mL = 10 doses (10 weeks)

Scenario C: 5 mg vial, 2.5 mL water, 1.7 mg dose

• Concentration: 5 mg ÷ 2.5 mL = 2 mg/mL
• Draw: 1.7 mg ÷ 2 mg/mL = 0.85 mL = 85 units
• Duration: 2.5 mL ÷ 0.85 mL = 2.9 doses (approximately 3 weeks)

Notice how higher doses mean fewer doses per vial. A patient on 1.7 mg/week goes through a vial much faster than one on 0.25 mg/week — which is why higher maintenance doses cost more per month even if the per-vial price is the same.

Pre-Mixed Vials: When the Math Is Already Done

Many compounding pharmacies now ship pre-mixed liquid formulations instead of lyophilized powder. With pre-mixed vials, the pharmacy has already reconstituted the medication — the label shows the concentration directly (e.g., "Semaglutide 2.5 mg/mL"), and you only need to calculate your draw volume:

Draw volume = Your dose ÷ Label concentration

Pre-mixed vials eliminate reconstitution entirely but remove the flexibility to choose your own concentration. The pharmacy selects the concentration based on common dose ranges for their patient population.

Mistakes to Avoid

• Adding the wrong amount of water: Using 3 mL instead of 2 mL dilutes the solution by 33%. Every subsequent dose will be 33% under-strength. Measure the reconstitution water carefully.
• Confusing mg with mL: Your dose is prescribed in mg. You draw in mL. They're not the same number unless the concentration is exactly 1 mg/mL. Always calculate.
• Confusing units with mL: "20 units" on an insulin syringe = 0.2 mL. If your pharmacy says "draw 0.2 mL" and you draw 0.2 units, you're getting 1/100th of your dose.
• Mixing up vials: If you have multiple vials (different medications or different concentrations), label them clearly. Drawing from the wrong vial at the wrong concentration produces the wrong dose.
• Not accounting for the vial's total volume when drawing: As you use the vial, the remaining volume decreases. Your last few draws may pull air instead of medication if you're not tracking how much remains.

Frequently Asked Questions