Educational only — not medical advice. This calculator does arithmetic on numbers you type in. It does not set, recommend or change any dose. Your dose comes only from your prescribing clinician and your prescription label. Medically reviewed by Dr. Taimoor Asghar.
Read the numbers off your own pen carton or pharmacy label. Nothing here is pre-filled with a drug's price or a particular pen's contents, because pens differ by product and presentation — and a number copied from another product's carton is exactly how pen mistakes happen.
Enter the pen's total content and your dose per injection, then calculate.
On a GLP-1 pen, the big number is usually not your dose. The total content of the pen and the dose delivered by one injection are two different numbers, and confusing them is a well-recognised source of error. A multi-dose pen holds several injections' worth of medicine; a single-dose pen or syringe holds exactly one. That is why the calculator asks for both numbers separately and cross-checks them: if you also enter the injections per pen printed on your carton, it compares that figure with total content divided by dose per injection and flags any mismatch for you to resolve with your pharmacist, rather than silently picking one.
Worked logic, in plain words: injections per pen equals total pen content divided by dose per injection, rounded down to whole injections, because a fraction of an injection is not a usable dose. Days supplied equals injections per pen divided by injections per week, multiplied by 7. Weeks supplied is the same figure divided by 7. Pens needed for 4 weeks (28 days) equals 28 divided by days supplied — rounded up, because you cannot buy part of a pen. Any leftover medicine smaller than one full dose is reported as leftover, not counted as an extra injection. If you enter a known injections-per-pen figure, the calculator uses that figure for the days-supplied result and still shows what the division gives, so a discrepancy is visible instead of hidden.
For the brand-name injections covered in the companion titration planner, the presentations differ. Wegovy injection, for example, is supplied as prefilled single-dose pens (0.25 mg/0.5 mL, 0.5 mg/0.5 mL, 1 mg/0.5 mL, 1.7 mg/0.75 mL, 2.4 mg/0.75 mL and 7.2 mg/0.75 mL for Wegovy HD), single-dose syringes at the same doses, and a single-patient-use FlexTouch pen delivering 2.4 mg per injection from a 9.6 mg/3 mL pen — that is, 4 doses per pen. Ozempic injection, by contrast, is supplied as single-patient-use multi-dose pens: 2 mg/3 mL delivering 0.25 mg or 0.5 mg per injection, 4 mg/3 mL delivering 1 mg per injection, and 8 mg/3 mL delivering 2 mg per injection. Tirzepatide single-dose pens (Mounjaro and Zepbound) deliver one dose each, and Mounjaro also has a multi-dose KwikPen presentation. The practical point is simple: never assume injections per pen from another product. For a single-dose pen the answer is 1 by definition; for a multi-dose pen, read the carton. This calculator accepts either case because it takes your numbers, not a built-in assumption.
Days supplied here is pure calendar arithmetic: how many days the injections in one pen last at your entered injection frequency. It is not an expiry date, an in-use storage limit, or a promise that the pen will still be usable on the last day. Pens have in-use periods and storage conditions in their instructions for use — for example, an in-use pen kept beyond its permitted period must not be used even if medicine remains — and those limits can cut the usable period shorter than the arithmetic. The same applies to a dose change mid-pen: if your clinician changes your dose, the remaining injections in the current pen must be recalculated at the new dose, and for many presentations a new pen strength is prescribed instead. Treat this calculator as a planning aid for refills and budgeting, and the instructions for use and your pharmacist as the authority on whether a particular pen is still usable.
The most useful output for most people is pens per 4 weeks, because prescriptions and pharmacy repeats usually run in 4-week blocks for weekly injections. If one pen covers 28 days at your dose, you need one pen per 4-week block; if it covers 14 days, you need two. Rounding up matters here: needing 1.2 pens per block means ordering 2, not hoping the fifth of a pen stretches. Equally, a leftover figure is not waste to be “used up” by taking a smaller injection — a partial amount is not a prescribed dose. If the numbers do not divide evenly, ask your pharmacist what to do with the remainder; do not improvise a dose from it.
Never share a pen with another person, even if the needle is changed — the labels warn against this because of infection risk. Use a new needle for each injection where the instructions require it, rotate injection sites, and inspect the medicine before use: do not use it if it looks discoloured or contains particles, and follow your product's own instructions on what the solution should look like (Ozempic should appear clear and colourless; tirzepatide clear and colourless to slightly yellow). These are educational tools, not medical advice; dosing changes only with the prescribing clinician; and if you think you have taken too much, contact your prescriber or local poison service or seek emergency care promptly, as your product's medication guide directs. Compounded products are not covered here: they are not FDA-approved, have no FDA-approved pen presentation to calculate from, and any “pen duration” claimed for them is not a label figure.
Because this calculator trusts whatever you type, spend thirty seconds checking the three inputs against the carton before you act on the output. Confirm that the total content figure is the whole pen, not the dose, and that the dose per injection is the dose your prescriber wrote, not a neighbouring strength on the same carton. If the pen is a single-dose pen, the carton figure should be 1 injection per pen; if the calculator's division gives a different number for a pen you believe is single-dose, one of the inputs is wrong — stop and ask your pharmacist rather than averaging the two figures. The same applies in reverse: if a multi-dose pen's carton figure and the division disagree, do not pick the larger number because it gives more days. A mismatch is information, not an inconvenience, and resolving it with the pharmacist takes less time than dealing with a missed or incorrect dose later. Keep the carton until the pen is finished, so the numbers can be re-checked if a dose changes mid-pen.
Sources checked 24 October 2026. If a label has since been revised, the revised label governs. This site carries no affiliate links and no links to sellers or telehealth sign-ups for compounded products.