Retatrutide Dose Calculator

What this tool calculates, and what it cannot

It does two pieces of arithmetic. It turns a powder mass and a water volume into a concentration, and it turns a milligram figure and that concentration into units on a U-100 syringe. It does not tell you what dose to use, because no such figure exists for retatrutide.

Retatrutide is an investigational compound. No regulator has approved it, so there is no prescribing information, no titration schedule, no maintenance dose and no maximum dose. It is not available through legitimate prescription channels. Anyone quoting a standard dose is quoting a trial protocol or a seller, not a label — and a trial dose exists inside a monitored study with eligibility criteria and safety oversight, none of which travels with a number copied out of it.

Reconstitution: the water volume decides everything

A lyophilised vial contains a mass of powder and no volume. You create the concentration when you add bacteriostatic water:

concentration (mg/mL) = powder in mg ÷ water in mL, then units = (dose in mg ÷ concentration) × 100

The second formula is the standard U-100 identity — 100 units = 1 mL, so one unit is 0.01 mL regardless of what is dissolved in it. Units measure volume; only the concentration converts that volume into drug. Which means the water you chose, not the vial you bought, sets what every mark on the syringe is worth.

The failure mode: identical syringes, different doses

Rows one and four of that table are different vials that produce the same 20 units for the same 2 mg — the arithmetic works out. Rows three and four are the same vial, and the only difference is a millilitre of water, yet 20 units on one is 4 mg and on the other is 2 mg. Nothing about the syringe, the vial or the injection distinguishes them.

This is why a units figure quoted anywhere — a forum post, a spreadsheet, a message from someone using "the same vial" — is meaningless without the reconstitution volume attached to it. It is also why the reconstitution volume should be written on the vial in permanent marker at the moment you add the water, not reconstructed from memory three weeks later.

The trade-off hidden in the water volume

Adding more water buys measuring resolution and costs injection volume. One graduation on a U-100 syringe is 0.01 mL, so at 5 mg/mL a single unit is worth 0.05 mg, while at 20 mg/mL the same graduation is worth 0.2 mg — four times coarser. Run it the other way and the cost appears: a 4 mg dose is 0.8 mL at 5 mg/mL, a large volume to put under the skin at once, and 0.2 mL at 20 mg/mL.

The practical consequence is that concentration is not a free choice. Reconstitute too strong and small amounts land between graduations, where a one-unit misread moves the amount by a fifth. Reconstitute too weak and the volume becomes impractical, and a multi-dose vial runs out sooner. Neither end of that trade-off is corrected by the calculator; it only tells you where you have landed.

What the calculator does not verify

The output is arithmetic on numbers you typed. Nothing in it confirms that the vial holds the milligrams printed on it, that the contents are what the label claims, that the powder is sterile, or that the concentration you built is stable. For an approved product those questions are answered by a regulator and a manufacturing licence. For an unapproved compound they are answered by whoever sold it.

What an approved label gives you that this does not

It is worth seeing the size of the gap. An approved tirzepatide label specifies a starting dose of 2.5 mg once weekly, 2.5 mg increments with at least four weeks between them, defined maintenance doses of 5, 10 and 15 mg, a 15 mg maximum, a rule for missed doses, and exact presentations. Semaglutide labels do the same, escalating from 0.25 mg to a defined ceiling. Every one of those numbers is the output of trials and regulatory review.

For retatrutide, every one of those lines is blank. That is not a gap this calculator, or any calculator, fills.

Where the arithmetic applies

The mg-to-units conversion is the same for any peptide reconstituted from powder, so the formula holds regardless of the compound. It does not apply to branded pens or vials of approved GLP-1 medicines, which arrive already in solution at a fixed dose and require no reconstitution and no conversion. If you have a prescription, the number you need is on the label and in your prescriber's instructions, and those are what any calculation here should be checked against.