What Is a Semaglutide Reconstitution Calculator?

A semaglutide reconstitution calculator does the maths for vials you measure yourself. It turns the amount of semaglutide in a vial and the liquid it is mixed with into a concentration. Then it turns your prescribed dose into a volume and a line on your syringe.

You only need it for vials, not pens. Brand-name Ozempic and Wegovy come in pens that deliver a set dose. There is nothing to mix or measure. The calculator is for compounded or pharmacy-prepared vials, where you draw the dose with an insulin syringe.

How to use the Semaglutide Reconstitution Calculator

1. Choose your vial type. Pick “Powder” if you added water, or “Already liquid” if the label shows mg/mL.
2. Enter the vial details. For powder, that is the total mg and the water added. For liquid, it is the mg/mL and the total mL.
3. Pick your prescribed weekly dose, or choose “Other amount”.
4. Pick your syringe type and barrel. Check the pack for U-100 or U-40.
5. Read the results. You get the unit line, a to-scale syringe picture, every dose step in both syringe types, and how much of the vial is left at 28 days.
6. Use the other tabs. “Amount remaining” tracks a vial you have already started. “Compare mixes” shows how different water amounts change your unit line.

How Semaglutide Reconstitution Calculations Work

Every calculation on this page uses the same three steps.

The key idea is that syringe units measure volume, not drug. The same 10 units can hold 0.25 mg in one vial and 1 mg in another. That is why a units number is only safe to use with the exact mix it was worked out for.

Semaglutide Vial Strength and Concentration

“Strength” can mean two different things on a semaglutide label, and mixing them up is a common source of error.

• Total amount (mg). Powder vials show the total, such as 2.5 mg, 5 mg or 10 mg. The concentration depends on how much water you add.
• Concentration (mg/mL). Liquid vials show mg per mL, such as 2.5 mg/mL. The concentration is fixed. Pick “Already liquid” in the calculator.

For comparison, here are the concentrations inside the approved Wegovy single-dose pens:

The approved pens change strength at each dose step. Many vials keep one strength for every step, so the unit line moves instead. Each pen is also single-use. Never try to draw extra doses from a pen with a syringe.

Semaglutide mg/mL Concentration Calculator

To find the concentration of a powder vial, divide the semaglutide by the water added.

Tab 1 of the calculator works this out for any vial size and water amount.

Semaglutide Dose-to-Volume Conversion

Once you know the concentration, divide your dose by it to get the volume.

At 2.5 mg/mL, a 0.25 mg dose is 0.1 mL, a 1 mg dose is 0.4 mL and a 2.4 mg dose is 0.96 mL. Volumes this small are why semaglutide is measured with insulin syringes, not standard ones.

Semaglutide mg to Syringe Units Calculator

Here is each label dose step in U-100 units for common mixes.

A useful rule: at 2.5 mg/mL, every 0.25 mg is 10 units. At 5 mg/mL, every 0.25 mg is 5 units. Strong mixes like 10 mg/mL shrink the starting dose to 2.5 units, which is hard to measure accurately.

U-100 vs. U-40 Syringe Units for Semaglutide

Both syringe types measure the same volume. They just label it differently. A U-100 syringe has 100 lines per mL, so 1 unit = 0.01 mL. A U-40 syringe has 40 lines per mL, so 1 unit = 0.025 mL.

How Vial Concentration Changes the Syringe Volume

More water makes a weaker mix, so each dose takes up more volume and more units. Less water makes a stronger mix, so each dose takes up less. The amount of semaglutide in the vial never changes.

• Too strong and small doses fall to 2 or 3 units, where half a line is a 15 to 25% error.
• Too weak and higher doses may not fit in your syringe.
• A good mix puts every dose step you will use on a whole, easy-to-read line.

Tab 3 compares water amounts for your dose. It includes a column showing whether 2.4 mg would still fit if you stay on the same mix.

How to Calculate the Amount Remaining in a Semaglutide Vial

This is the question most calculators skip. The maths is simple:

But the amount left is only half the answer. The FDA’s advice for multi-dose GLP-1 vials is to discard them within 28 days of first use. So what matters is how many doses you can take before that date. Tab 2 works out both, including when your dose is about to increase.

Semaglutide Reconstitution Calculation Examples

Example 1: 5 mg vial, 2 mL water, 0.25 mg dose

5 ÷ 2 = 2.5 mg/mL. 0.25 ÷ 2.5 = 0.1 mL. On a U-100 syringe, that is 10 units. On a U-40 syringe, it is 4 units.

Example 2: 10 mg vial, 2 mL water, 1 mg dose

10 ÷ 2 = 5 mg/mL. 1 ÷ 5 = 0.2 mL, which is 20 units on a U-100 syringe.

Example 3: a liquid vial labelled 2.5 mg/mL, 1.7 mg dose

1.7 ÷ 2.5 = 0.68 mL, which is 68 units. This does not fit in a 0.5 mL (50-unit) syringe, so a 1 mL syringe is needed. Its lines are 2 units apart, and 68 sits on a line.

Example 4: how much is left?

A 5 mg vial with 2 mL of water, used for three 0.5 mg doses. Used: 1.5 mg. Left: 3.5 mg, which is 1.4 mL. That is 7 more full doses on paper. But the fourth dose falls on day 21, and the vial must be discarded by day 28. So only 1 more dose can be used, and about 3 mg would be thrown away.

Common Semaglutide Reconstitution Calculation Errors

1. Treating units as mg. In 2024, the FDA reported compounded semaglutide overdoses of 5 to 20 times, caused by mix-ups between mg, mL and units. In one case, 50 units were taken instead of 5.
2. Copying someone else’s units number. It only works for their exact vial and water amount.
3. Confusing total mg with mg/mL. A “2.5 mg” powder vial and a “2.5 mg/mL” liquid vial are very different.
4. Using a U-40 syringe with U-100 instructions. That gives 2.5 times the dose.
5. Using a syringe far too large for the dose. The FDA noted patients being given syringes much larger than their dose. Small doses are hard to read on a 1 mL barrel.
6. Mixing up mg and mcg. 0.25 mg = 250 mcg.
7. Keeping a vial past 28 days because there is still liquid in it.

Signs of too much semaglutide include severe nausea, vomiting, stomach pain and signs of low blood sugar. If you think you have taken too much, contact a doctor, pharmacist or emergency services. In the US, Poison Control is 1-800-222-1222.

Semaglutide Reconstitution Calculator FAQs

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Sources and further reading

1. Novo Nordisk. Wegovy (semaglutide) prescribing information. novo-pi.com
2. Novo Nordisk. Ozempic (semaglutide) prescribing information. novo-pi.com
3. US Food and Drug Administration. FDA approves higher dose semaglutide (Wegovy HD). March 2026. fda.gov
4. US Food and Drug Administration. FDA clarifies policies for compounders as national GLP-1 supply begins to stabilize. fda.gov
5. US Food and Drug Administration. FDA’s concerns with unapproved GLP-1 drugs used for weight loss. Updated September 2026. fda.gov
6. US Food and Drug Administration. FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products. July 2024. fda.gov

Reviewed and updated: September 2026. We update this page when labels or FDA guidance change.