Peptide pen calculator

Why a pen counts in units, not mg

Reusable pens were built for insulin, which is sold at 100 units/mL. The dial measures volume on that scale: one unit is 0.01 mL and a 3 mL cartridge holds 300 units. Lilly's KwikPen manual describes exactly that: 3 mL, 300 units, 100 units/mL. Put a peptide in the cartridge instead and the dial still measures volume. It has no idea what is dissolved in it.

So the only number that turns a dial reading into a dose is the concentration of what you put in. 10 mg in 2 mL is 50 mcg per unit. The same 10 mg in 3 mL is about 33 mcg per unit. Dial 5 units on both pens and you get two different doses. That is why a click chart written for one pen and strength never transfers to a cartridge you filled yourself, including the GLP-1 click tables on this site.

One click is not always one unit

• 1-unit pens. Lilly's HumaPen Savvio and the KwikPen dial in steps of 1 unit, up to 60 units per injection.
• Half-unit pens. Novo Nordisk's NovoPen Echo marks half-units between the numbers and stops at 30 units per injection.
• Pens that dial in mg. Branded growth hormone pens skip units altogether. Norditropin FlexPro 10 mg/1.5 mL dials from 0.05 to 4.0 mg in 0.05 mg steps. For those, read the dose straight off the pen. The HGH reconstitution page covers the vial-and-syringe route.
• Unbranded “peptide pens”. Their paperwork varies. If yours does not say what one click delivers, you do not know your dose.

Lilly's manual makes a point worth repeating: do not dial by counting clicks, because that is how a wrong dose gets dialed. Read the number in the dose window. The calculator shows clicks only so you can see how the dial and the dose relate.

Priming is where the doses go

Insulin pen manuals tell users to prime before every injection: dial 2 units, point the needle up and press until a drop appears. Lilly and Novo Nordisk both use 2 units. It clears air from the new needle so the dose that follows is the dose you dialed. Branded GH pens differ. Norditropin FlexPro asks for a flow check before the first injection from each new pen rather than every time.

Two units is a small share of a 40-unit insulin dose. On a 5-unit peptide dose it is 40% extra. It comes out of the cartridge every time:

The practical consequence is arithmetic, not advice: a more dilute cartridge means a bigger dial volume per dose, so the fixed prime becomes a smaller share. It also means fewer doses fit in the 3 mL, so the trade has two sides. The calculator shows both.

Worked examples

The same arithmetic the calculator runs, on a 1-unit pen with a 2-unit prime and a 300-unit (3 mL) scale.

The doses are there to show the arithmetic. They are not dose recommendations. The first row only fills 2 mL of the cartridge, so the count comes from 200 units rather than 300.

Things the arithmetic cannot tell you

• Whether the cartridge is sterile. Pen manuals cover factory-filled cartridges. Moving a mixed peptide from a vial into an empty cartridge is a step no pen manufacturer documents. Every transfer is a chance to contaminate it. The peptide pen guide covers what that means in practice.
• How long the solution stays good. A reconstituted peptide is usually treated as good for about 28 days refrigerated. The real limit varies by compound. See the storage guide. If the calculator says the cartridge lasts longer than that, the calendar wins.
• Whether your pen is accurate. Novo Nordisk's quick guide has a function check: dispense 20 units into the needle cap and look at how much comes out. If it delivers too much or too little twice in a row, Novo Nordisk says not to use the pen.

Frequently asked questions

How do I work out how many units to dial on a peptide pen?

Divide the dose by the concentration to get the volume in mL, then multiply by 100. That works on a pen calibrated for 100 units/mL cartridges, where one unit on the dial is 0.01 mL. Example: 10 mg in 2 mL is 5 mg/mL, so 250 mcg is 0.05 mL, which is 5 units.

Is one click on a pen always one unit?

No. Many reusable insulin-style pens dial in 1-unit steps and some dial in half-units. Branded growth hormone pens such as Norditropin FlexPro dial in mg instead. Lilly's KwikPen manual tells users to read the number in the dose window rather than count clicks, because counting is how a wrong dose gets dialed.

How much does priming waste?

Insulin pen manuals from Lilly and Novo Nordisk describe a 2-unit prime before each injection. On a 5-unit dose that is 7 units drawn from the cartridge for every 5 delivered, so a 3 mL cartridge (300 units) gives 42 doses instead of 60. The smaller the dose, the larger the share priming takes.

How many doses are in a 3 mL pen cartridge?

A 3 mL cartridge on a 100 units/mL scale holds 300 units. Divide 300 by the units you dial plus the units you prime each time. At 10 units per dose with a 2-unit prime that is 25 doses, not 30.

Can I use a GLP-1 click chart for a peptide pen?

Not unless the concentration matches. A click chart is only valid for the pen and strength it was written for. A cartridge you filled yourself has whatever concentration your mixing produced, so work the units out from that concentration rather than borrowing another pen's table.

Sources

• Eli Lilly. Humalog 100 units/ml KwikPen user manual (electronic medicines compendium): 3 mL holds 300 units, 1-unit steps up to 60 units, a 2-unit prime before each injection, read the dose window rather than counting clicks.
• Novo Nordisk. How to use NovoPen Echo (quick guide): half-unit steps, 30-unit maximum, a 2-unit flow test before each injection, the 20-unit function check.
• Novo Nordisk. Norditropin FlexPro 10 mg/1.5 ml package leaflet (electronic medicines compendium): 0.05 mg steps up to 4.0 mg, flow check before the first injection from a new pen.
• Lilly Medical. Which pen should be used for Humalog cartridges: the HumaPen Savvio and its 1-unit steps.

Related tools

• How to use a peptide pen – loading, priming, needles and storage.
• Peptide reconstitution calculator – the vial-and-syringe version of this arithmetic.
• GLP-1 pen click chart – clicks for branded Ozempic, Mounjaro and Zepbound pens.
• HGH reconstitution calculator – IU to insulin units for growth hormone.
• Vial longevity calculator – how long a vial lasts and what each dose costs.
• Peptide nasal spray calculator – sprays per dose and bottle life.
• Beyond-use date calculator – when an in-use cartridge or pen has to go.