Peptide calculator

How to use it

1. Peptide in the vial. Type the amount printed on the vial label, in mg.
2. Water added. Type the volume of diluent put into the vial, in mL.
3. Amount to measure. Type the amount, then pick mcg or mg beside it. The field starts in mcg.
4. Syringe size. Pick 0.3 mL (30 units), 0.5 mL (50 units) or 1 mL (100 units). All three choices use the U-100 scale.

Results update as you type, and the Calculate button runs them again. Once the first two fields hold numbers, the panel shows the concentration in mg per mL (with mcg per mL beneath it) and the mcg in one unit (0.01 mL). Add the amount to measure and it also shows the volume in mL, the number of units on a U-100 syringe and a bar showing how full the chosen syringe would be. The formula is repeated under the panel with your own numbers. Clear empties the three number fields.

Two checks catch common slips. If the amount to measure is more than the vial holds, you get a message instead of a volume. If the units are more than the chosen syringe holds, the line under the bar says so. Your numbers stay in your browser and never become part of the page address.

The formula

• Concentration (mg/mL) = peptide in the vial (mg) / water added (mL)
• Per unit (mcg) = concentration (mcg/mL) / 100
• Amount in mg = amount in mcg / 1,000 (only when you enter mcg)
• Volume (mL) = amount to measure (mg) / concentration (mg/mL)
• Syringe units = volume (mL) × 100
• Fill (%) = syringe units / units the syringe holds × 100

The factor of 100 is the U-100 scale: 100 units fill 1 mL, so one unit is 0.01 mL.¹² A shortcut gives the same answer in one step: syringe units = amount to measure (mcg) / per unit (mcg).

Worked example

These numbers are arbitrary and show the arithmetic only. They are not a suggested amount for any product.

A vial labeled 10 mg gets 4 mL of water.

1. Concentration = 10 mg / 4 mL = 2.5 mg/mL, which is 2,500 mcg/mL.
2. Per unit = 2,500 mcg/mL / 100 = 25 mcg.
3. The amount to measure is 600 mcg, which is 600 / 1,000 = 0.6 mg.
4. Volume = 0.6 mg / 2.5 mg/mL = 0.24 mL.
5. Syringe units = 0.24 × 100 = 24 units. The shortcut agrees: 600 mcg / 25 mcg = 24.
6. On a 0.3 mL syringe that is 24 of 30 units, 80% full. On a 1 mL syringe it is 24 of 100, 24% full.

The same vial and the same 600 mcg give very different readings at other water volumes:

The amount is identical in every row; only the volume carrying it changes, and with it how forgiving the reading is. A misread of one unit mark is 1/6, about 17%, of a 6-unit volume, but 1/24, about 4%, of a 24-unit volume. At 5 mL the volume fills a 0.3 mL syringe exactly, so a larger amount at that concentration would need a bigger barrel.

Reading the result on a syringe

• Check the scale. The unit math assumes U-100 marks, which most insulin syringes carry.¹ Confirm the scale on the syringe or its package before you rely on a result.
• Check the marks. MedlinePlus puts the small marks on a standard 1 mL insulin syringe 1 unit apart.¹ Smaller barrels can be finer: one maker sells a 0.3 mL barrel with half-unit marks.³
• Match the barrel to the volume. In a 2024 alert on dosing errors with compounded semaglutide drawn from multiple-dose vials, FDA encouraged health care providers and compounders to give patients a syringe sized for the intended dose and to show them how to measure it.²
• Clear the bubbles. Air in the barrel sits on the scale as if it were liquid. MedlinePlus, in its guide to drawing medicine from a vial, says to tap the syringe so bubbles rise, push them back into the vial and check the amount again.⁴

Supplies and guides

The concentration is only as good as the mixing behind it. The reconstitution guide covers the steps, and bacteriostatic water is one of the diluents it describes. Insulin syringes are marked in units; confirm which scale yours carry before you rely on a result. The rest of the lab supplies are on the supplies page.

The other two calculators split this one into parts. The reconstitution calculator stops at the concentration, per mL and per unit, and also works in IU. The units converter handles single conversions, such as mL to units or mcg to mg. Terms such as U-100 and diluent are defined in the glossary.