Reverse Dose Calculator

Checking a dose you have already drawn

The forward calculators answer "how many units should I draw". This one answers the question you have when the syringe is already in your hand: what dose is actually in it. Enter the units on the barrel and the vial concentration, and the calculator returns the dose in milligrams.

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

The formula reads more plainly if you remember that units are volume. Units ÷ 100 is just millilitres, so the whole calculation is "how many millilitres did I draw, multiplied by how many milligrams sit in each millilitre". A 100-unit syringe holds 1 mL, which is the ceiling on any single draw.

Three moments this is the right tool

Before injecting a loaded syringe. You drew from memory, the number looks slightly off, and you want the milligrams before you commit. After a refill. A new vial arrived and you want to know what the unit count you have drawn for weeks now delivers. When someone hands you a unit count. A forum post, an old note, a friend on the same drug — a number of units means nothing until it meets a concentration.

What the same draw delivers on different vials

One row makes the point better than any warning. Fifty units is 5 mg on a 10 mg/mL vial and 20 mg on a 40 mg/mL vial — a fourfold difference from an identical-looking syringe. There is no such thing as "50 units of tirzepatide". There is only 50 units of a stated concentration.

Notice too that three cells in the right-hand column exceed 15 mg, which is the labelled maximum weekly tirzepatide dose. An answer that lands above the top of the ladder is arithmetic telling you something is wrong with an input — usually the concentration.

What a single unit is worth

One unit is 0.01 mL, so its dose value is the concentration divided by 100. On a 10 mg/mL vial one unit is 0.1 mg; on a 40 mg/mL vial the same one unit is 0.4 mg. The more concentrated the vial, the more each mark on the barrel costs — misreading by two units at 40 mg/mL is 0.8 mg, close to a third of a full 2.5 mg starting dose. This is the trade concentrated vials make: less volume to inject, less room for error in reading it.

The failure this catches

The dangerous case is not a slip of the hand. It is a habit that survives a change it should not have survived. Your prescription is written in milligrams and it does not change when a pharmacy dispenses a stronger vial — but the units that deliver those milligrams do. Drawing the familiar number after a concentration change is the most common route to a large, silent overdose, because everything about the injection looks and feels identical.

Running the reverse conversion on every new vial takes about ten seconds and turns an invisible change into a number you can compare against your prescription.

How to read the answer

Compare the milligrams to the dose written on your prescription — that is the figure that governs, not the unit count. Tirzepatide's labelled doses are 2.5, 5, 7.5, 10, 12.5 and 15 mg once weekly, so an answer far from any of those is worth a call. An unfamiliar number is not automatically an error, because compounded prescriptions are sometimes written at in-between doses deliberately. Either way the person who settles it is your prescriber or pharmacist, not the calculator. If the number does not match, do not inject and do not adjust the draw to make it match.

Other drugs, and where this does not apply

The presets above are the concentrations compounded tirzepatide is commonly supplied at, but the arithmetic is drug-agnostic, because units measure volume and nothing else. Type any concentration into the field and it works: 20 units of a 2.5 mg/mL semaglutide vial is 0.5 mg.

It does not apply to branded pens, which deliver fixed pre-set doses with no units to read. Nor to branded single-dose vials, where the entire 0.5 mL is the dose. Zepbound multi-dose vials are a genuine exception in the other direction: they are dosed in 0.6 mL — 60 units — so a 0.5 mL draw from one delivers about 83% of the intended dose.