Peptide pen calculator
For a reconstituted peptide loaded into a reusable 3 ml pen. Enter how many mg are in the cartridge, how much liquid it holds and your dose: the calculator returns mcg per unit, mcg per click, the clicks to dial and how many doses the cartridge gives once priming is counted.
A U-100 pen moves 0.01 ml per unit, so 10 mg in a 3 ml cartridge is 33.3 mcg per unit. On a pen that dials one unit per click (HumaPen Savvio, NovoPen 6) that is 33.3 mcg per click; on a half-unit pen (HumaPen Luxura HD, NovoPen Echo) a click is 16.7 mcg.
250 mcg is 7.5 units, which a one-unit pen cannot dial exactly. Fills that land on a whole click: 2 ml (5 clicks), 2.4 ml (6 clicks), 2.8 ml (7 clicks).
How the pen math works
- Concentration. Divide the peptide by the liquid: 10 mg in 3 ml is 3.33 mg/ml, or 3,333 mcg/ml.
- mcg per unit. A U-100 cartridge holds 100 units per ml, so one unit is 0.01 ml: 3,333 mcg/ml ÷ 100 = 33.3 mcg per unit. The shortcut is 10 × mg ÷ ml.
- Units for the dose. 250 mcg ÷ 33.3 mcg = 7.5 units (0.075 ml).
- Clicks. A one-unit pen can dial 7 clicks (233 mcg) or 8 clicks (267 mcg), both about 7% off. A half-unit pen dials 15 clicks for exactly 250 mcg. On a one-unit pen, change the fill instead: 10 mg in 2 ml is 50 mcg per unit and 250 mcg becomes exactly 5 clicks.
The last step is the one syringe calculators skip. A syringe can be drawn to any line; a pen dose can only be a whole number of clicks, so the liquid volume is worth choosing around the dose. The calculator lists the fills between 1 and 3 ml, in 0.1 ml steps, that land the dose exactly on a click.
mcg per unit by cartridge strength
mcg per unit for common peptide amounts and fill volumes. On a one-unit pen this is also mcg per click; on a half-unit pen, halve it.
| Peptide in cartridge | 1 ml | 1.5 ml | 2 ml | 3 ml |
|---|---|---|---|---|
| 2 mg | 20 mcg | 13.3 mcg | 10 mcg | 6.7 mcg |
| 5 mg | 50 mcg | 33.3 mcg | 25 mcg | 16.7 mcg |
| 10 mg | 100 mcg | 66.7 mcg | 50 mcg | 33.3 mcg |
| 15 mg | 150 mcg | 100 mcg | 75 mcg | 50 mcg |
| 20 mg | 200 mcg | 133.3 mcg | 100 mcg | 66.7 mcg |
| 30 mg | 300 mcg | 200 mcg | 150 mcg | 100 mcg |
More liquid means fewer mcg per click, which gives finer steps but more volume per injection. At 3 ml, a 2 mg cartridge moves 6.7 mcg per click, fine enough to dial almost any dose; a 30 mg cartridge moves 100 mcg per click, so small doses round badly.
Pen dial steps and priming, from the manufacturers
What the dose window counts depends on the pen. These figures come from each manufacturer's instructions for use for their own U-100, 3 ml insulin cartridges.
| Pen | Per click | Dose range | Prime per injection |
|---|---|---|---|
| HumaPen Savvio (Lilly) | 1 unit · 0.01 ml | 1–60 units | 2 units, every injection |
| HumaPen Luxura HD (Lilly) | ½ unit · 0.005 ml | 0.5–30 units | 2 units, every injection |
| NovoPen 6 (Novo Nordisk) | 1 unit · 0.01 ml | 1–60 units | 2-unit flow check |
| NovoPen Echo (Novo Nordisk) | ½ unit · 0.005 ml | 0.5–30 units | 2-unit flow check |
| Unbranded “peptide pens” | Usually 1 unit; check | 60 or 80 units, per listing | No standard; prime until liquid shows |
Unbranded pens sold for 3 ml cartridges do not publish instructions for use the way Lilly and Novo Nordisk do. Check yours before trusting a click count: dial 10 clicks and confirm the window reads 10, not 5.
Doses per cartridge: priming costs more than you think
A 3 ml U-100 cartridge holds 300 units. Lilly's HumaPen instructions say to prime with 2 units before every injection and Novo Nordisk's NovoPen instructions call for a 2-unit flow check, so each injection uses the dose plus 2 units. With an 8-unit dose that is 300 ÷ 10 = 30 injections instead of 37, about a fifth of the cartridge spent on priming. Small doses lose proportionally more: a 4-unit dose gets 50 injections instead of 75.
Plan for fewer still. Lilly notes that a new cartridge can take several primes before liquid appears, and both brands' pens stop at the units left in the cartridge, so the final dose usually has to be finished from a new cartridge.
Before you load a pen
- Labelling. Lilly's HumaPen Savvio and Luxura HD are labelled “for use only with Lilly 3 mL insulin cartridges”, and NovoPens for Novo Nordisk PenFill cartridges. Loading a refilled cartridge with anything else is outside that labelling, and the sterility of a hand-filled cartridge has not been validated by anyone.
- One person per pen. Both manufacturers say never to share a pen, even with a new needle.
- Storage. Treat a loaded cartridge like the reconstituted vial it came from: refrigerated, and used within the same window. See peptide storage and shelf life.
- Units are volume, not potency. A pen unit is 0.01 ml, exactly like a unit on a U-100 insulin syringe. It is not an IU; for IU conversions use the dose converter.
FAQ
How many mcg is one click on a peptide pen?
mcg per unit = 10 × mg in the cartridge ÷ ml of liquid. 10 mg in 3 ml is 33.3 mcg per unit: 33.3 mcg per click on a one-unit pen, 16.7 mcg per click on a half-unit pen.
How many units are in a 3 ml pen cartridge?
300 units at U-100. With a 2-unit prime before every injection fewer reach the injection site: an 8-unit dose gives 30 injections, not 37.
How many clicks is 250 mcg on a peptide pen?
With 10 mg in 3 ml, 250 mcg is 7.5 units: 15 clicks on a half-unit pen, while a one-unit pen can only give 7 clicks (233 mcg) or 8 clicks (267 mcg). With 10 mg in 2 ml, 250 mcg is exactly 5 clicks.
How much water should go into a peptide pen cartridge?
Choose the volume that puts the dose on a whole number of clicks: ml = mg × 10 × clicks ÷ dose in mcg. For a 5 mg vial and a 100 mcg dose, 2 ml gives 25 mcg per unit and the dose is exactly 4 clicks. A standard cartridge holds up to 3 ml.
Is a pen unit the same as an insulin syringe unit?
Yes for U-100 devices: both are 0.01 ml. Neither is an IU, which measures biological activity and differs by compound. The reconstitution calculator does the same math for syringes.
Can I count clicks on an Ozempic pen with this calculator?
No. Prefilled GLP-1 pens such as Ozempic dial mg, not units, and Novo Nordisk's instructions say not to set the dose by counting clicks. This calculator only applies to pens whose window reads units.
Sources
- Lilly, HumaPen Savvio instructions for use: 1 to 60 units, dialled one unit at a time; prime before every injection; will not dial more than the units left.
- Lilly, HumaPen Luxura HD instructions for use: 0.5 to 30 units, dialled one-half unit at a time; dial 2 units to prime before every injection.
- Novo Nordisk, NovoPen 6 and NovoPen Echo Plus user guide: NovoPen 6 dials 1 to 60 units in 1-unit increments; select 2 units for the flow check.
- Novo Nordisk, NovoPen Echo instructions for use: 0.5 to 30 units in 0.5-unit increments, PenFill 3 mL cartridges at 100 units/mL.
- Novo Nordisk, Ozempic prescribing information and instructions for use: “Do not set the dose by counting the number of clicks.”
Related tools
Skip the math — let the app handle it
Peptide Protocol remembers each vial, shows the dose for every injection, tracks what is left and rotates injection sites for you.
Get the iPhone app →