What information do I need before I use a peptide calculator?

Three numbers: the vial size in milligrams from the label, the volume of bacteriostatic water you added during reconstitution, and the target dose you intend to research with. The calculator cannot supply any of these — it only computes from what you enter.

Do I enter milligrams or micrograms into a peptide calculator?

Vial size is entered in milligrams, matching the label. Target dose is usually entered in micrograms, since research doses are commonly discussed in that unit. Mixing the two up is the most common input error, since 1 mg equals 1000 mcg.

What does the calculator's output actually mean?

The first output is concentration — how much peptide is in each milliliter of the reconstituted solution. The second is units to draw on a U-100 insulin syringe, which holds 100 units per milliliter. Both numbers follow directly from the three inputs.

Can a peptide calculator tell me how much water to add?

Some calculators, including PeptCalc's, include a reverse mode for this. Instead of entering a water volume and getting units, you enter your usual target dose and the calculator returns a water volume that lands that dose on a clean syringe mark.

How to Use a Peptide Calculator

By PeptCalc Research — Medically reviewed by David Mansour, MD — Last reviewed July 20, 2026

A peptide calculator turns three numbers you already have — vial size, water volume, and target dose — into two numbers you need at the bench: concentration and syringe units. Knowing how to use a peptide calculator well is less about the tool itself and more about getting the right numbers into it, in the right units, in the right order. This walkthrough covers each step and works through a full example. It is written for research and educational purposes only and is not medical advice.

What you need before you start

Have three things in front of you before you open the calculator:

The vial label — the peptide mass in milligrams, exactly as printed (e.g. 5 mg or 10 mg)
The water volume you added — how much bacteriostatic water went into that vial during reconstitution, in milliliters
The target dose — the amount you intend to research with, usually expressed in micrograms

If any one of these is wrong when it goes in, the calculator will still return a confident, cleanly-formatted answer — it just won't be the right one. The tool checks the arithmetic, not the label, which is the main thing to keep in mind when learning how to use a peptide calculator correctly.

Step 1: enter the vial size

Type in the milligram amount printed on the vial, not the volume of liquid it will eventually hold. A 10 mg vial stays a 10 mg vial whether you add 1 mL or 3 mL of water — the mass doesn't change, only the concentration does once water is added. Read the label twice if the print is small; this is the single number every later calculation depends on.

Step 2: enter the water volume

Enter the exact volume of bacteriostatic water you drew into the vial, in milliliters. This is the number that sets concentration, so it has to match what you actually did at the bench, not a number from a chart for a different vial size. If you haven't reconstituted yet and are still deciding on a volume, this is also where a reverse-mode calculator is useful — instead of fixing the water volume, you can fix your usual dose and let the calculator suggest a volume that lands on a round unit mark.

Step 3: enter the target dose

This is where a mg/mcg mix-up most often creeps in. Vial size is in milligrams; target dose is almost always in micrograms, and 1 mg equals 1000 mcg. Entering "250" when the calculator expects micrograms but you were thinking in milligrams will produce a unit count that's off by a factor of a thousand. Check which unit the input field is asking for before typing the number in.

Reading the output

Once the three inputs are in, the calculator returns two numbers, in a fixed order.

Concentration comes first: vial mass divided by water volume. A 5 mg vial mixed with 1 mL of bacteriostatic water is 5 mg/mL, or 5,000 mcg/mL. Add 2 mL instead and the same vial becomes 2.5 mg/mL — less concentrated, and a larger volume to draw for the same dose.

Units to draw comes second, based on a standard U-100 insulin syringe, which measures 100 units per milliliter. The calculator divides concentration by 100 to get micrograms per unit, then divides your target dose by that figure.

Worked example. Take a 5 mg vial reconstituted with 1 mL of bacteriostatic water: 5 mg ÷ 1 mL = 5 mg/mL = 5,000 mcg/mL. Divide by 100 units/mL: 5,000 ÷ 100 = 50 mcg per unit. A 200 mcg target dose is 200 ÷ 50 = 4 units. A 300 mcg dose from that same vial is 300 ÷ 50 = 6 units.

Vial size	Water added	Concentration	Dose	Units to draw
5 mg	1 mL	5,000 mcg/mL	200 mcg	4 units
5 mg	1 mL	5,000 mcg/mL	300 mcg	6 units

When the result isn't a whole number

Not every combination of vial size, water volume, and dose divides evenly. A calculator might return 4.7 units instead of a clean 5, since target doses in research protocols are rarely chosen to match a specific vial's arithmetic. Some people draw to the nearest marked line the syringe barrel actually shows; others go back to Step 2 and change the water volume slightly so the same dose lands on a round number instead. Neither approach is wrong — a small water-volume adjustment just shifts concentration enough that the division comes out even, which is the entire premise behind a reverse-mode calculator. What matters is being consistent about which approach you use for a given vial, so the number drawn stays the same from one session to the next.

Checking the math by hand

You don't have to take the output on faith. The free peptide calculator is running the same two divisions described above — concentration equals vial mass divided by water volume, and units equal target dose divided by mcg-per-unit. A closer breakdown of that arithmetic, including why it's two fixed steps rather than a complex formula, is covered in how a peptide calculator works. Re-running the numbers by hand takes under a minute and catches the rare case where a typo slipped into an input field.

When the numbers don't look right

A few checks before assuming the calculator is wrong:

Re-read the vial label. A misread 10 mg vial entered as 5 mg will halve every downstream number.
Confirm the water volume matches what's actually in the vial, not a volume from memory or a different batch.
Check the unit field for the dose. If the result is off by roughly a factor of 1,000, an mg/mcg mix-up is the likely cause.
Confirm the syringe barrel is U-100. A non-standard syringe, such as a 0.3 mL "micro" barrel, may not share the same 100-units-per-mL scale, and entering its reading into a U-100 calculation will misstate the dose.

A peptide dosage chart for common vial-and-water combinations is a useful cross-check if the calculator's output doesn't match what a reference table shows for the same setup.

That five-step sequence — label, water volume, dose, output, cross-check — is really all there is to how to use a peptide calculator well. Use the peptide dosage calculator directly with your own vial size, water volume, and target dose to get concentration and units in one step, including reverse mode for solving backward from a target dose to a water volume. For the underlying arithmetic in more detail, see how a peptide calculator works.