This guide explains how prefilled research peptide pens work, the standardised 3 mL / 300-click format used industry-wide, and the underlying maths the calculator on this site uses. Read this if you’ve got a pen in your hand and want to understand exactly what’s happening when you click the dial.
Research and educational purposes only. This guide describes the industry-standard format for research-grade peptide pens. It does not constitute medical advice and is not a recommendation for use in humans.
The 3 mL / 300-click format
Across UK research-grade peptide suppliers, prefilled pens follow a standardised format: 3 millilitres of total solution volume, dispensed across 300 mechanical clicks. Every click of the pen’s dial releases exactly the same volume — 0.01 mL — regardless of which compound is inside.
What changes between products is the concentration of the compound in that 3 mL of liquid. A 30 mg pen contains 30 mg of compound dissolved into the same 3 mL of solution as a 60 mg pen contains 60 mg. So every click of a 60 mg pen delivers twice the compound by mass as every click of a 30 mg pen, even though the volume of liquid is identical.
The basic formula
- mg per click = pen total mg ÷ 300
- clicks needed for your target dose = your target dose mg ÷ mg per click
- doses per pen = pen total mg ÷ your target dose
Worked example: 40 mg retatrutide pen, 2.5 mg target dose
- mg per click = 40 ÷ 300 = 0.133 mg per click
- clicks for 2.5 mg = 2.5 ÷ 0.133 = ≈19 clicks
- doses per pen = 40 ÷ 2.5 = 16 doses
Worked example: 80 mg KLOW stack pen, 1 mg target dose
- mg per click = 80 ÷ 300 = 0.267 mg per click
- clicks for 1 mg = 1 ÷ 0.267 = ≈4 clicks
- doses per pen = 80 ÷ 1 = 80 doses
Common pen-strength reference table
For quick mental reference, here are the per-click amounts for common pen strengths on the 3 mL / 300-click format:
| Pen total mg | mg per click | clicks for 1 mg | clicks for 2.5 mg | clicks for 5 mg |
|---|---|---|---|---|
| 10 mg | 0.033 mg | 30 | 75 | 150 |
| 15 mg | 0.050 mg | 20 | 50 | 100 |
| 20 mg | 0.067 mg | 15 | 37 | 75 |
| 30 mg | 0.100 mg | 10 | 25 | 50 |
| 40 mg | 0.133 mg | 8 | 19 | 38 |
| 50 mg | 0.167 mg | 6 | 15 | 30 |
| 60 mg | 0.200 mg | 5 | 13 | 25 |
| 80 mg | 0.267 mg | 4 | 9 | 19 |
| 100 mg | 0.333 mg | 3 | 8 | 15 |
Why the format is standardised
The 3 mL / 300-click format isn’t an accident — it’s a manufacturing convention that grew up around the original GLP-1 pen designs (Ozempic, Mounjaro and similar licensed prescription pens), which used the same 3 mL cartridge architecture. Research-grade pen manufacturers adopted the same format because:
- The 3 mL cartridge is a widely-available off-the-shelf component
- 300 clicks gives fine enough resolution for typical research-administration amounts
- The maths is easy to do in your head: every 100 mg of pen content = 0.33 mg per click
- The format is consistent across suppliers, making the maths universal
Pen vs vial — when to use which
Use a pen when you want pre-prepared, ready-to-use research administration without the reconstitution step. Pens are particularly common for GLP-1 compounds and pre-mixed stacks. The downside: you’re locked into the manufacturer’s pre-set concentration.
Use a vial (lyophilised powder) when you want control over the final concentration. You add your own bacteriostatic water — choosing how much determines the concentration, which determines how big the per-administration draw volume is. Vials are also typically cheaper per mg of compound than pens.
Frequently asked questions
Are all peptide pens really 3 mL / 300 clicks?
The 3 mL / 300-click format is the industry standard across UK research-grade peptide suppliers and licensed prescription GLP-1 pens. Some legacy or specialty products use different formats — always check the manufacturer’s specification on the pen or accompanying certificate of analysis (COA) before assuming 3 mL / 300 clicks.
What if my pen has fewer or more clicks?
The basic formula still applies, you just change the divisor: mg per click = pen total mg ÷ pen total clicks. For a 240-click pen with 30 mg total: 30 ÷ 240 = 0.125 mg per click. Use the “custom” number input on the universal calculator to enter your specific pen size — the maths still works.
How many doses should a pen last me?
That depends entirely on your per-administration dose and how often you administer. A 40 mg retatrutide pen at 2.5 mg per weekly administration = 16 weeks per pen. The same pen at 4 mg per weekly administration = 10 weeks. The calculator returns “doses per pen” on every calculation — this is the figure to plan supply around.
Does the click count vary by compound?
No. Every click of a 3 mL / 300-click pen releases exactly 0.01 mL of solution, regardless of what’s dissolved in that solution. What varies is the mass of compound per click, which depends on how concentrated the pen is.
Can I get fractional clicks (e.g. half a click)?
Most pen dials are mechanical and only stop at whole-click increments. If your target dose works out to 7.5 clicks, you’ll be rounding to either 7 or 8 clicks in practice. The calculator returns the exact clicks figure including any fractional part so you can see how close your nearest whole-click options are.
Use the calculator
Apply this to your specific pen: visit the universal calculator, pick your compound, switch to pen mode, enter your pen’s total mg and target dose. The calculator shows the maths in plain English alongside the answer.
Compliance: This guide is provided for research and educational purposes only. The maths described is generic and applies to research-grade prefilled peptide pens following the industry-standard 3 mL / 300-click format. The content of this guide does not constitute medical advice or a recommendation for use in humans. Compounds discussed are not licensed as medicines for human use unless explicitly stated. Always consult a qualified healthcare professional. Not for human consumption.