How many clicks is my amount?

Divide your amount by the concentration to get mL, then divide by your pen's mL per click. With 10 mg mixed in 2.5 mL (4 mg/mL), 2.92 mg is 0.73 mL: 73 clicks on a 1-unit pen, 146 on a half-unit pen and 36.5 on a 2-unit pen, which has to be rounded to 36 or 37. Enter your own vial, diluent and amount above and the calculator runs the chain for your pen, including the mg each click delivers. On U-100 pens, set the number in the dose window; the click count is only a cross-check.

Is one click one unit?

Not always. Pens built for U-100 cartridges move ½, 1 or 2 units per click depending on the model, so assuming one unit per click can halve or double the volume. Other pens move 0.0075 or 0.0125 mL per step, which is not a U-100 unit at all. Check your pen's manual, then dial 10 and count the clicks: 10 clicks means 1 unit per click, 20 clicks means ½ unit and 5 clicks means 2 units.

What is a half-unit pen?

A pen whose dial moves 0.5 unit (0.005 mL) per click, so it takes two clicks per number in the window. Half units show as a line between two numbers: 24.5 is the line between 24 and 25. Most half-unit pens go up to 30 units per setting; some start at 0.5 unit and others only at 1 unit, which matters for very small readings. Because each click is half the volume, the same amount needs twice as many clicks as on a 1-unit pen, and the mg per click is halved.

My pen's marks aren't U-100 units. What then?

Choose Custom pen, set the window scale to "its own marks", "mg printed on the dial" or "no numbered window", and enter the mL per click from the manual. The result is then shown in clicks and mL, and the U-100 figure appears only as a syringe equivalent. Never dial it on such a pen, because its marks stand for a different volume. Printed mg marks are right only at the concentration the pen was designed for, so the calculator uses your mL per click and your own concentration instead.

How many units are in a 3 mL cartridge?

300 units on the U-100 scale, because 1 unit is 0.01 mL. That is the nominal content. What you can actually use is lower once you allow for the liquid you really loaded, priming and any residual the pen cannot push out. The cartridge panel works this out as full amounts per cartridge and units left over.

What's the most one setting can deliver, and what if I need more?

Every pen has a maximum per setting: commonly 60 or 80 units on 1-unit pens and 30 units on half-unit pens. If your amount needs more, the calculator splits it into the fewest settings, as evenly as possible, so 73 units on a pen with a 60-unit maximum becomes 37 + 36. Each setting is then dialled and delivered separately.

Why doesn't my amount land on a whole click?

Because the volume your amount needs is rarely an exact multiple of the pen's step. The calculator shows the two nearest settings with the mg each one delivers and the % deviation. It highlights the nearer one and leaves the choice to you when your amount is exactly halfway. If the rounding is large, try a different diluent volume: the Plan diluent mode finds volumes that land on whole clicks.

What diluent volume makes clicks come out even?

Use Plan diluent. Enter the amount in your vial, your target amount and your pen, and it lists the diluent volumes at which that amount is a whole number of clicks, plus the deviation if you measure to the nearest 0.1 mL. More diluent gives larger readings, which are proportionally more precise, but the mix still has to fit in the cartridge. These are diluent volumes, not amount suggestions.

Does priming use up the cartridge?

Yes. A 2-unit prime, typical in pen manuals, spends 0.02 mL each time, and priming before every setting adds up over a cartridge. At 4 mg/mL each prime uses 0.08 mg. A new or re-seated cartridge may also need extra priming before the plunger rod reaches the stopper. Set how often you prime and the calculator subtracts it before counting full amounts; it warns you when priming takes more than 15% of the cartridge.

Can I fill a cartridge from a vial mixed with more than 3 mL?

Yes, the arithmetic still works. The concentration comes from the vial (amount ÷ diluent), so it is the same in every cartridge you fill from that mix. One cartridge simply holds a smaller share of the total, and the mix fills more than one cartridge. Transfer losses reduce how much ends up in the cartridge, not the concentration. Enter the liquid actually in the cartridge to see how far it goes.

How do mg, mcg and units relate?

1 mg = 1000 mcg. A pen unit is a volume mark (on the U-100 scale 1 unit = 0.01 mL), so its mg value depends entirely on your concentration: mg per unit = mg/mL × 0.01. At 4 mg/mL, 1 unit is 0.04 mg (40 mcg). IU is something else again: it measures biological activity and has no universal mg factor, so the calculator keeps IU vials in IU unless you switch on the somatropin conversion (see the next question). The classic error is dialling the mg figure as units: at 4 mg/mL, 3 units instead of 73 deliver 0.12 mg, not 2.92 mg.

Can I use this for HGH / somatropin in IU?

Yes. Somatropin vials are often labelled in IU (some kits state mg), so choose the unit printed on your vial, or pick the HGH preset. For somatropin only, a fixed pharmacopoeial convention links IU and mass: 1 mg ≈ 3 IU. Tick "Somatropin (HGH): convert IU ⇄ mg" (the HGH preset does it for you) and you can enter your target in IU, mg or mcg; every result then also shows its equivalent in the other unit. Check your kit, and change the factor only if it states a different one. HCG, HMG and other IU products have no fixed mg factor, so leave the conversion off for them and work in IU only.

Pen vs syringe: same math?

Up to the volume, yes: amount ÷ concentration = mL, and on the U-100 scale mL × 100 = units, the same figure the reconstitution calculator shows for an insulin syringe. A pen adds limits a syringe doesn't have: the volume has to be a whole number of clicks, one setting can't exceed the pen's maximum, priming uses liquid, and the cartridge holds a fixed volume. Those are what this calculator handles on top.

Can I use this for prefilled medicine pens, or to decide how much to use?

No to both. This tool is for cartridges you fill yourself. Prefilled medicine pens are fixed-amount, must not be refilled, and their makers advise against counting clicks and publish no volume per click. The calculator also never suggests an amount: it only converts the numbers you enter into mL, units and clicks. Helica Labs products are for laboratory research only.

Free research tools

Peptide Pen Click Calculator

Free·No sign-up·Instant

Convert an amount in mg, mcg or IU into the setting for your reusable cartridge pen — dose-window units on U-100 pens, clicks and mL on any pen — or turn a click count back into mg. Every result shows the mg per click and a clicks ↔ mg table for your pen and mix. The calculator only works with the numbers you enter. For research use only.

Enter the amount in your vial, the diluent and your target amount to see the result.

Confirm your pen's step above to see the click count.

Delivers

Diluent volumes that land on whole clicks

Enter the amount in your vial, your target amount and your pen. Each row is a diluent volume at which your amount is an exact whole number of clicks, plus the deviation if you measure it to the nearest 0.1 mL.

Confirm your pen's step above to see the click count.

These are diluent volumes, not amount suggestions. More diluent gives larger readings, and larger readings are proportionally more precise.

1 click =
1 window unit =

Confirm your pen's step above to see the click count.

Pen makers tell users to set the amount by reading the window, not by counting clicks. The click count is only a cross-check.

Volume
Concentration
U-100

Which setting will you use?

Clicks	Window units	mL	Amount
Can't be dialled on this pen

Confirm your pen's step above to see the click count.

Arithmetic only — not a recommendation.

Research use only: arithmetic, not advice

This calculator converts numbers you enter into volume, pen units and clicks. It does not suggest amounts and is not medical advice. Helica Labs products are sold for laboratory research only.

Reusable pens are labelled by their makers for specific insulin cartridges, and pen accuracy testing is done with the maker's original cartridges. Accuracy with other cartridges or liquids has not been established. Check your pen's step size, scale and maximum in its manual, set the number in the window rather than counting clicks, and confirm it before use.

How to use the pen click calculator

Enter the amount in your vial (mg or IU) and the diluent you added, in mL — or pick a peptide preset to fill in the vial amount.
Enter your target amount in mg, mcg or IU. The field starts empty: the calculator never suggests an amount.
Choose your pen type. If your pen isn't listed, choose Custom pen and enter its window scale and step from the manual.
Dial 10 on your pen, check that the window shows 10, and tick the confirmation to see click counts.
Read the dose-window setting, the mg it delivers, the mg per click and the clicks ↔ mg table. Use Clicks → mg to turn a reading back into mg.
Adjust priming, the liquid in the cartridge and the residual to see how many full amounts the cartridge holds.

Clicks vs units: know your pen

A click is one mechanical step of the dial, and it has no fixed mg value. On a pen built for U-100 cartridges each number in the dose window is 0.01 mL, but pens move ½, 1 or 2 units per click. The same 0.73 mL is therefore 146 clicks on a half-unit pen, 73 clicks on a 1-unit pen and 36.5 clicks on a 2-unit pen — which cannot be dialled exactly. Some pens are not on the U-100 scale at all: their marks can be 0.0075 or 0.0125 mL per step, or show mg figures that are right at only one concentration. That is why the calculator asks for your pen type, has you confirm its step by dialling 10, and shows the mg (or IU) that one click carries in your own mix. A pen "unit" is a volume mark, not an IU.

How the math works

Take a 10 mg vial mixed with 2.5 mL of diluent. The concentration is 10 ÷ 2.5 = 4 mg/mL. A target of 2.92 mg needs 2.92 ÷ 4 = 0.73 mL, which is 73 units on the U-100 scale (1 unit = 0.01 mL). On a pen that moves 1 unit per click that is 73 clicks, and each click carries 0.04 mg (40 mcg). If the pen tops out at 60 units per setting, 73 will not fit in one setting, so the calculator splits it as evenly as possible: one setting of 37 units and one of 36 (37 + 36 = 73). On a 2-unit pen the same volume is 36.5 clicks, exactly halfway between 36 clicks (2.88 mg, −1.37%) and 37 clicks (2.96 mg, +1.37%), so the choice is yours. Clicks → mg runs the chain backwards: clicks × mL per click × concentration = mg.

Priming and cartridge limits

Most pen manuals have you prime before use — typically 2 units, repeated until liquid appears at the needle tip — and a new or re-seated cartridge may need extra priming before the plunger rod reaches the stopper. Priming uses liquid: at 4 mg/mL every 2-unit prime spends 0.08 mg. The calculator subtracts priming (before each setting, once per cartridge or never), any extra first-cartridge priming and any unusable residual from the liquid in the cartridge, then counts how many full amounts fit and how many units are left over. Pens also differ at the end of a cartridge: some stop at the units left, others let you dial past them and deliver only what remains. "Units left" stops assume a full factory cartridge, so they may be wrong in a cartridge you filled yourself. If you mix more than one cartridge holds, the concentration stays the same — the mix simply fills more cartridges.

Why this isn't for prefilled medicine pens

This tool is for cartridges you fill yourself. Prefilled medicine pens are fixed-amount devices that must not be refilled. Their makers advise against counting clicks and publish no volume per click, and pens with the same printed strength can hold different concentrations. A click chart for such a pen would be guesswork, so the calculator offers none. Follow the instructions that come with the pen.

Frequently asked questions

The answers use one example mix: 10 mg in 2.5 mL (4 mg/mL). Use the calculator for your own numbers.

How many clicks is my amount?: Divide your amount by the concentration to get mL, then divide by your pen's mL per click. With 10 mg mixed in 2.5 mL (4 mg/mL), 2.92 mg is 0.73 mL: 73 clicks on a 1-unit pen, 146 on a half-unit pen and 36.5 on a 2-unit pen, which has to be rounded to 36 or 37. Enter your own vial, diluent and amount above and the calculator runs the chain for your pen, including the mg each click delivers. On U-100 pens, set the number in the dose window; the click count is only a cross-check.
Is one click one unit?: Not always. Pens built for U-100 cartridges move ½, 1 or 2 units per click depending on the model, so assuming one unit per click can halve or double the volume. Other pens move 0.0075 or 0.0125 mL per step, which is not a U-100 unit at all. Check your pen's manual, then dial 10 and count the clicks: 10 clicks means 1 unit per click, 20 clicks means ½ unit and 5 clicks means 2 units.
What is a half-unit pen?: A pen whose dial moves 0.5 unit (0.005 mL) per click, so it takes two clicks per number in the window. Half units show as a line between two numbers: 24.5 is the line between 24 and 25. Most half-unit pens go up to 30 units per setting; some start at 0.5 unit and others only at 1 unit, which matters for very small readings. Because each click is half the volume, the same amount needs twice as many clicks as on a 1-unit pen, and the mg per click is halved.
My pen's marks aren't U-100 units. What then?: Choose Custom pen, set the window scale to "its own marks", "mg printed on the dial" or "no numbered window", and enter the mL per click from the manual. The result is then shown in clicks and mL, and the U-100 figure appears only as a syringe equivalent. Never dial it on such a pen, because its marks stand for a different volume. Printed mg marks are right only at the concentration the pen was designed for, so the calculator uses your mL per click and your own concentration instead.
How many units are in a 3 mL cartridge?: 300 units on the U-100 scale, because 1 unit is 0.01 mL. That is the nominal content. What you can actually use is lower once you allow for the liquid you really loaded, priming and any residual the pen cannot push out. The cartridge panel works this out as full amounts per cartridge and units left over.
What's the most one setting can deliver, and what if I need more?: Every pen has a maximum per setting: commonly 60 or 80 units on 1-unit pens and 30 units on half-unit pens. If your amount needs more, the calculator splits it into the fewest settings, as evenly as possible, so 73 units on a pen with a 60-unit maximum becomes 37 + 36. Each setting is then dialled and delivered separately.
Why doesn't my amount land on a whole click?: Because the volume your amount needs is rarely an exact multiple of the pen's step. The calculator shows the two nearest settings with the mg each one delivers and the % deviation. It highlights the nearer one and leaves the choice to you when your amount is exactly halfway. If the rounding is large, try a different diluent volume: the Plan diluent mode finds volumes that land on whole clicks.
What diluent volume makes clicks come out even?: Use Plan diluent. Enter the amount in your vial, your target amount and your pen, and it lists the diluent volumes at which that amount is a whole number of clicks, plus the deviation if you measure to the nearest 0.1 mL. More diluent gives larger readings, which are proportionally more precise, but the mix still has to fit in the cartridge. These are diluent volumes, not amount suggestions.
Does priming use up the cartridge?: Yes. A 2-unit prime, typical in pen manuals, spends 0.02 mL each time, and priming before every setting adds up over a cartridge. At 4 mg/mL each prime uses 0.08 mg. A new or re-seated cartridge may also need extra priming before the plunger rod reaches the stopper. Set how often you prime and the calculator subtracts it before counting full amounts; it warns you when priming takes more than 15% of the cartridge.
Can I fill a cartridge from a vial mixed with more than 3 mL?: Yes, the arithmetic still works. The concentration comes from the vial (amount ÷ diluent), so it is the same in every cartridge you fill from that mix. One cartridge simply holds a smaller share of the total, and the mix fills more than one cartridge. Transfer losses reduce how much ends up in the cartridge, not the concentration. Enter the liquid actually in the cartridge to see how far it goes.
How do mg, mcg and units relate?: 1 mg = 1000 mcg. A pen unit is a volume mark (on the U-100 scale 1 unit = 0.01 mL), so its mg value depends entirely on your concentration: mg per unit = mg/mL × 0.01. At 4 mg/mL, 1 unit is 0.04 mg (40 mcg). IU is something else again: it measures biological activity and has no universal mg factor, so the calculator keeps IU vials in IU unless you switch on the somatropin conversion (see the next question). The classic error is dialling the mg figure as units: at 4 mg/mL, 3 units instead of 73 deliver 0.12 mg, not 2.92 mg.
Can I use this for HGH / somatropin in IU?: Yes. Somatropin vials are often labelled in IU (some kits state mg), so choose the unit printed on your vial, or pick the HGH preset. For somatropin only, a fixed pharmacopoeial convention links IU and mass: 1 mg ≈ 3 IU. Tick "Somatropin (HGH): convert IU ⇄ mg" (the HGH preset does it for you) and you can enter your target in IU, mg or mcg; every result then also shows its equivalent in the other unit. Check your kit, and change the factor only if it states a different one. HCG, HMG and other IU products have no fixed mg factor, so leave the conversion off for them and work in IU only.
Pen vs syringe: same math?: Up to the volume, yes: amount ÷ concentration = mL, and on the U-100 scale mL × 100 = units, the same figure the reconstitution calculator shows for an insulin syringe. A pen adds limits a syringe doesn't have: the volume has to be a whole number of clicks, one setting can't exceed the pen's maximum, priming uses liquid, and the cartridge holds a fixed volume. Those are what this calculator handles on top.
Can I use this for prefilled medicine pens, or to decide how much to use?: No to both. This tool is for cartridges you fill yourself. Prefilled medicine pens are fixed-amount, must not be refilled, and their makers advise against counting clicks and publish no volume per click. The calculator also never suggests an amount: it only converts the numbers you enter into mL, units and clicks. Helica Labs products are for laboratory research only.

For research use only. Verify all calculations against current literature and use sterile technique.