GLP-1 Split Dose Calculator

What this calculator does

It divides a weekly milligram total into two or three equal injections, converts each one into units on a U-100 syringe, and lays the injections out across the week. That is the whole function. It does not decide whether splitting is appropriate for you, and no drug label describes the practice.

per injection = weekly mg ÷ number of injections, then units = (per-injection mg ÷ concentration in mg/mL) × 100

The schedule it builds

Seven does not divide evenly, so the spacing is never perfectly regular. A two-way split lands four days apart, then three days back to the start — Monday and Friday, for example. A three-way split lands two days apart, then three: Monday, Wednesday, Friday. Whatever you choose, the gaps repeat every week, so pick a pattern you can hold indefinitely rather than one that works only until a weekend moves.

Worked example, and the rounding you cannot see

A U-100 syringe is graduated in whole units. Divide by three and the answer usually lands between two graduations, so the syringe silently rounds your arithmetic. At 20 mg/mL, one unit is 0.2 mg — here is what that costs:

The 12.5-unit row is the one to notice. There is no correct answer on that syringe, and whichever way you go, you go the same way every week for the rest of the month.

The failure mode: small doses on concentrated vials

Rounding error is a percentage of the number you are rounding, so it grows as the split gets finer and as the vial gets stronger. At 40 mg/mL, one graduation is 0.4 mg — a third of a 2.5 mg weekly dose is barely two units, and a one-unit misread moves that injection by half. Splitting a low dose from a concentrated vial is the combination that turns a measuring instrument into a rounding instrument.

Two practical consequences. A split schedule needs a concentration low enough that each injection lands on a whole number of units; and it needs more vial entries and more injection sites per week than a once-weekly schedule, each of which is another chance to draw the wrong number on autopilot.

What the label says, and what it does not

Every approved GLP-1 in this class is labelled once weekly. Tirzepatide runs 2.5 mg to 15 mg in 2.5 mg steps with at least four weeks per step; Ozempic escalates to a maximum of 2 mg; Wegovy escalates from 0.25 mg with a further 7.2 mg step permitted. No label describes dividing a weekly dose, so there is no labelled schedule, no labelled missed-dose rule and no labelled titration path for a split regimen.

The pharmacology is worth being precise about. Tirzepatide's half-life is roughly five to six days and steady state arrives at about four weeks, so the drug from each injection is still substantially present when the next one is due. Dividing the weekly dose changes the shape of that curve — lower peaks, higher troughs — rather than the weekly exposure it is built from. Whether a flatter curve feels different is a separate question, and one the trial evidence behind these drugs does not answer, because every trial dosed once weekly.

Every titration step resets the arithmetic

A split schedule is not set once. Tirzepatide moves in 2.5 mg increments with at least four weeks at each step, so each escalation changes the per-injection milligrams, the unit count and the rounding error all at once. A 10 mg week split two ways is 5 mg per injection, or 25 units at 20 mg/mL — exactly on a graduation. Step up to 12.5 mg and the same split becomes 6.25 mg, or 31.3 units, which the syringe cannot hold. Recalculate at every step rather than scaling the old number in your head.

The spacing matters less than it looks. With a half-life around five to six days and injections two to four days apart, each one lands on top of most of the last, so what you feel is the settled pattern after several weeks, not the first altered week.

Where this does not apply

Branded pens cannot be split. Mounjaro, Zepbound, Ozempic and Wegovy pens deliver a fixed dose in one press, and delivering less than the full dose is not recommended. Branded vials are fixed-volume as well: Zepbound single-dose vials are drawn whole from 0.5 mL, and the multi-dose vial is dosed as 0.6 mL. Splitting is a compounded vial-and-syringe practice, it is off-label, and this calculator is arithmetic for people whose prescriber has already agreed to it. Take the per-injection milligrams and units to your prescriber and pharmacist and have them confirmed before the first split week.