GLP-1 Split Dose Calculator: Units and Weekly Dose Delivered

1. Choose the medication and enter your weekly dose in mg.
2. Choose 2 or 3 injections a week. The calculator lays the days out as described below; tap a day to move a shot, or set each shot’s mg for an uneven split.
3. Enter what is in your vial: the mg/mL, the amount in a volume as the label prints it ("10 mg / 0.5 mL" is 20 mg/mL), or the powder and water for a vial you mix.
4. Enter the date of your first split shot, and the time if you want reminders at that hour.
5. Read the units for each shot, the weekly total the rounded draws deliver, the hours between shots, and how much the modelled low rises against one weekly shot. Add the next four weeks to your calendar.

The result gives the mg in each injection, the units to draw, the exact unit count before rounding, the weekly total as drawn, the dates of each shot and the change in the modelled level. With the liquid in the vial, it counts the draws that fall before a compounded vial must be thrown away, 28 days after the first draw (FDA, concerns with unapproved GLP‑1 drugs). embecta offers half-unit marks on its 0.3 mL barrel (embecta, Ultra-Fine syringes), so the calculator offers them only there.

Retatrutide appears in the list for the arithmetic only. It has no approved label, and Lilly registered pre-approval expanded access for it in June 2026 (ClinicalTrials.gov NCT07629401). For a powder vial, the retatrutide dose calculator works out the concentration from the water added.

The calculator runs four steps, all arithmetic:

1. mg per injection = weekly dose in mg ÷ number of injections.
2. Units = mg per injection ÷ concentration in mg/mL × 100, because a U-100 syringe is marked so that 100 units fill 1 mL, which makes 1 unit 0.01 mL (FDA dosing-error alert).
3. Units to draw = that number rounded to the nearest whole unit. On a syringe marked every 2 units the nearest mark can differ, so until you say how yours is marked the calculator shows both readings when they differ.
4. Weekly dose as drawn = units drawn ÷ 100 × concentration × number of injections.

For example, tirzepatide 7.5 mg a week split into 2 injections from a 24 mg/mL vial: each injection is 7.5 ÷ 2 = 3.75 mg, which is 3.75 ÷ 24 × 100 = 15.6 units. Rounded, that is 16 units, so each injection delivers 16 ÷ 100 × 24 = 3.84 mg and the week adds up to 7.68 mg, 2.4% more than 7.5 mg.

Rounding can move the weekly total further as the split gets finer and the dose gets smaller. The table uses a 20 mg/mL tirzepatide vial, where each unit holds 0.2 mg, and the calculator’s rounding.

At 12.5 units the calculator rounds up to 13, so each injection carries 0.1 mg more than planned and the week 0.2 mg more. The weekly total as drawn shows that sum, and the reverse dose calculator turns any unit count on your syringe back into mg.

How much one unit holds depends only on the concentration, so a stronger vial makes every rounding step bigger.

At 40 mg/mL, a third of a 2.5 mg weekly dose is 0.83 mg, or 2.1 units. The syringe gets 2 units, 0.8 mg, and misreading it by one unit changes that injection by half. Syringe markings differ with barrel size, and FDA encourages prescribers and compounders to supply a syringe size suited to the dose (FDA dosing-error alert). The calculator has no concentration shortcuts, so enter the strength printed on your vial.

Seven days do not divide evenly, so the calculator spaces injections round(7 ÷ number of injections) days apart and lets the last gap close the week. Two injections land 4 days apart, then 3 (Monday and Friday). Three injections land 2, 2, then 3 days apart (Monday, Wednesday and Friday).

A 2-day gap is shorter than the 3 days (72 hours) the tirzepatide labels require between two weekly doses when you change your injection day (Zepbound prescribing information, section 2.3). It is also exactly 48 hours, and the US Ozempic label asks for more than that (Ozempic prescribing information, section 2.1). The UK Ozempic and Wegovy SmPCs ask for at least 3 days, more than 72 hours (Ozempic SmPC and Wegovy SmPC, section 4.2). All of these gaps were written for moving a once-weekly dose, and no label gives one for a split schedule.

Tirzepatide goes up in 2.5 mg steps, each held at least 4 weeks (Zepbound prescribing information, section 2.1), so every step changes the mg per injection, the units and the rounding at once. At 20 mg/mL, 10 mg split in 2 is 25 units, exactly on a mark. At 12.5 mg the same split is 31.25 units, which no syringe mark shows. Run the calculator again at each step instead of scaling the old unit count.

Splitting keeps the weekly amount the same and trims the peaks and troughs. How much it trims, and what the published evidence says about doing it, is in splitting a GLP‑1 dose; the GLP‑1 plotter draws the once-weekly curve that arithmetic starts from.

The calculator is for a vial drawn with a syringe. The Zepbound KwikPen holds 4 fixed doses, one taken weekly as a single injection, and its Instructions for Use say not to transfer Zepbound from the pen into a syringe (Zepbound KwikPen Instructions for Use). Lilly Canada says it cannot recommend splitting the KwikPen into smaller doses because this has not been studied (Eli Lilly Canada, Zepbound KwikPen FAQ).

Other branded presentations are also made for fixed doses. Zepbound single-dose pens and vials hold 0.5 mL, each multi-dose vial dose is 0.6 mL (Zepbound prescribing information, section 3), and Ozempic pens and single-dose syringes deliver set doses between 0.25 mg and 2 mg (Ozempic prescribing information, section 3).

Among the Zepbound, Mounjaro, Ozempic and Wegovy labels, only the UK Wegovy SmPC gives one weekly dose as several injections. Its 7.2 mg dose can be three 2.4 mg injections given one after the other on the same day, at least 5 cm apart (Wegovy SmPC, section 4.2).