Dosage calculator

Your vial changed. Your unit count has to change too.

Your prescription has not changed. The vial has. When a compounding pharmacy switches concentration, the same milligrams come out at a different mark on the syringe — and carrying the old number across is where most compounded dosing goes wrong. This gives you the new reading and says plainly when the change is a big one.

Select your drug

Prescribed dose (mg)

Old vial concentration (mg/mL)

New vial concentration (mg/mL)

Your dose stays the same

1 mg

Old vial (2.5 mg/mL)

4units

= 0.400 mL drawn

New vial (5 mg/mL)

2units

= 0.200 mL drawn

Visual comparison

Old

40 u

New

20 u

Change

-2 units(-50%)

Concentration went from 2.5 to 5 mg/mL (2.0× more concentrated)

Read the label on every vial. The concentration is printed there. Where it differs from the vial before it, the syringe reading has to be worked out again before you draw.

Why concentration transitions are dangerous

More people get their dose wrong this way than any other. The vial changes strength, the unit count does not follow, and nobody notices. It falls out of the arithmetic:

units = (dose in mg × 100) ÷ concentration in mg/mL

Concentration sits on the bottom of that fraction, so a stronger vial needs fewer units to deliver the same milligrams. Draw the old number out of a stronger vial and the dose goes up with it — often to double, sometimes further [1].

Example: semaglutide 2.5 → 5 mg/mL

Someone on 1.0 mg of semaglutide a week, drawing to 40 units, with a vial at 2.5 mg/mL. Their pharmacy sends 5 mg/mL next time. Draw to 40 again and:

(40 × 5) ÷ 100 = 2.0 mg — twice what was prescribed.

What the syringe should read instead:

(1.0 × 100) ÷ 5 = 20 units — half of what it read before.

The calculator runs that for you, and it says so out loud in two cases: when the concentration has moved by more than a factor of two, and when the new reading will not fit on the syringe — a standard U-100 stops at 100 units [1].

Common concentration transitions

Semaglutide, 2.5 to 5 mg/mL. A move from the weaker vial to the stronger one. Whatever you were drawing, halve it.
Semaglutide, 5 to 10 mg/mL. Common at the top of the ladder — 1.7 to 2.4 mg — where the point is to shrink the volume being injected. Halved again.
Tirzepatide, 10 to 20 mg/mL. At 10 to 15 mg the weaker vial pushes the reading past what a syringe holds, so the stronger one keeps it under 100 units. Halved.
Any move to a weaker vial. It runs the other way: a lower concentration means more units for the same dose. Should that carry the reading past 100 units, it takes two draws.

Important safety disclaimer

This page does arithmetic and gives no medical advice. Read the concentration off every new vial before you draw from it. Where it differs from the last one, work the units out again here, and have your prescriber or the compounding pharmacy confirm the new number before anything is injected.

References

1.Becton, Dickinson and Company (BD). BD Ultra-Fine Insulin Syringes — Product Specifications and U-100 Standard. BD Diabetes Care Product Documentation. 2024. https://www.bd.com/en-us/products/diabetes/diabetes-injection/insulin-syringes
2.Novo Nordisk Inc. WEGOVY (semaglutide) injection — US Prescribing Information. FDA Approved Labeling. 2025. https://www.accessdata.fda.gov/drugsatfda_docs/label/2025/215256s026lbl.pdf
3.Novo Nordisk Inc. OZEMPIC (semaglutide) injection — US Prescribing Information. FDA Approved Labeling. 2025. https://www.accessdata.fda.gov/drugsatfda_docs/label/2025/209637s035,209637s037lbl.pdf
4.Eli Lilly and Company. ZEPBOUND (tirzepatide) injection — US Prescribing Information. FDA Approved Labeling. 2025. https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/217806s002lbl.pdf
5.Eli Lilly and Company. MOUNJARO (tirzepatide) injection — US Prescribing Information. FDA Approved Labeling. 2025. https://www.accessdata.fda.gov/drugsatfda_docs/label/2025/215866s039lbl.pdf
6.U.S. Food and Drug Administration. Compounded Drug Products — 503A and 503B Outsourcing Facility Information for Patients. FDA Drug Compounding Resources. 2024. https://www.fda.gov/drugs/human-drug-compounding/compounding-and-fda-questions-and-answers

Related tools and research

GLP-1 unit ↔ mg converter — milligrams to units and back, at any concentration.
GLP-1 reverse dose calculator — go the other way, from units on the syringe to milligrams.
GLP-1 split dose calculator — break a weekly dose into more than one injection.
GLP-1 reconstitution calculator — how much bacteriostatic water a dry vial needs.
GLP-1 dose plotter — what each rung of the ladder does to blood levels.
Compounded semaglutide bioequivalence — what a 503A or a 503B is actually required to test for.

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