mg, mcg, mL and syringe units
mg and mcg measure mass. mL measures volume. You need the concentration to convert between them, then the syringe scale to interpret the markings.
| Term | What it means |
|---|---|
| mg | Milligram: a unit of mass. 1 mg equals 1,000 mcg. It does not measure liquid volume. |
| mcg | Microgram: a unit of mass. 1 mcg equals 0.001 mg. |
| mL | Milliliter: a unit of volume. The amount of substance in 1 mL depends on concentration. |
| Concentration | The amount of a substance in a given volume of liquid, such as mg per mL. |
| U-100 syringe units | On a U-100 insulin syringe, the 100-unit mark corresponds to 1 mL. One unit is 0.01 mL. These markings alone do not give a peptide amount in mg or mcg. |
Sources: NIST: metric prefixes; NIST: units of volume; FDA: compounded semaglutide measurement errors
U-100 describes the scale, not just the barrel size. Do not apply this conversion to a different syringe scale. The worked example below shows why the same marking can contain different amounts at different concentrations.
The math, shown
How reconstitution math works
Reconstitution math has two stages. First, divide the amount in the vial by the liquid added to find the concentration. Then divide the target amount by that concentration to find the liquid volume to draw. A U-100 syringe labels 1 mL as 100 units, so the final conversion is always 100 units per millilitre.
Concentration (mg/mL) = vial amount (mg) ÷ diluent (mL)
Draw (mL) = target amount (mg) ÷ concentration (mg/mL)
U-100 draw (units) = draw (mL) × 100
Keep the units consistent before dividing. If the vial is labelled in milligrams and the target amount is in micrograms, convert one side first. One milligram equals 1,000 micrograms. The calculator handles that conversion when you switch between mg and mcg.
Worked example: 5 mg mixed with 2 mL
Start with a 5 mg vial and 2 mL of diluent. Five divided by two gives a concentration of 2.5 mg/mL. Multiply by 1,000 to express that as 2,500 mcg/mL. Because each unit on a U-100 syringe is 0.01 mL, each unit contains 25 mcg at this concentration.
- 1. 5 mg ÷ 2 mL = 2.5 mg/mL
- 2. 2.5 mg/mL × 1,000 = 2,500 mcg/mL
- 3. 2,500 mcg/mL ÷ 100 units/mL = 25 mcg per unit
- 4. 250 mcg ÷ 25 mcg per unit = 10 units
If the same 5 mg vial used 1 mL instead, the concentration would double to 5 mg/mL. The same 250 mcg target would then occupy 5 units rather than 10. The amount in the vial did not change. Only the concentration and draw volume changed.
U-100 syringe units to millilitres
The unit scale is a volume scale. On a U-100 syringe, every unit is 0.01 mL. A 30-unit, 50-unit, and 100-unit U-100 syringe use the same conversion even though their barrels have different capacities.
| U-100 marking | Liquid volume |
|---|---|
| 1 unit | 0.01 mL |
| 5 units | 0.05 mL |
| 10 units | 0.10 mL |
| 25 units | 0.25 mL |
| 50 units | 0.50 mL |
| 100 units | 1.00 mL |
Use the calculator in five checks
Confirm the syringe scale
Check that the syringe is U-100, where 100 units equal 1 mL. A 30-unit, 50-unit, and 100-unit U-100 syringe use the same conversion; only their capacity changes.
Enter the amount in the vial
Use the total amount printed on the vial in milligrams. Do not enter the amount you plan to draw here.
Enter the diluent volume
Use the total millilitres added to the vial. The calculator divides the vial amount by this volume to find the concentration.
Enter your target amount
Enter the amount supplied by your own protocol or clinician, then choose milligrams or micrograms. PepKit does not select this amount for you.
Read the draw in units
The result shows millilitres and U-100 syringe units. It also checks the draw against the selected syringe capacity and shows how many equal draws the vial contains mathematically.
Four errors the result can expose
A result can be mathematically correct while an input is wrong. Before using it, read the vial label again and compare every value with the calculator. One error is mixing milligrams and micrograms. A thousand-fold conversion error will also create a thousand-fold difference in the result.
The second error is confusing syringe capacity with concentration. A 50-unit U-100 syringe and a 100-unit U-100 syringe contain the same volume per unit. The smaller one simply stops at 0.5 mL. The third is carrying an old unit count to a vial mixed with a different amount of liquid. A new concentration needs a new calculation.
The fourth is treating the calculator as a source for the target amount. PepKit leaves that field empty on purpose. The tool converts an amount supplied by the user; it does not decide what that amount should be. If any input or result is unclear, stop and check it with a qualified clinician or pharmacist.
Calculator questions
What does a peptide reconstitution calculator calculate?
It converts a vial amount, diluent volume, and user-supplied target amount into a liquid volume and U-100 syringe-unit reading. It performs concentration and unit arithmetic; it does not choose a dose or protocol.
How many millilitres are in 100 units on a U-100 syringe?
On a U-100 syringe, 100 units equal 1 mL. That makes 50 units equal 0.5 mL, 25 units equal 0.25 mL, 10 units equal 0.1 mL, and 1 unit equal 0.01 mL.
How do I convert milligrams to micrograms?
Multiply milligrams by 1,000 to get micrograms. For example, 5 mg equals 5,000 mcg. Divide micrograms by 1,000 to convert back to milligrams.
Does adding more diluent change the amount in the vial?
No. It changes the concentration and therefore the liquid volume needed for a given target amount. The total amount in the vial stays the same.
Why can the same target amount produce a different syringe reading?
The draw changes when the vial amount or diluent volume changes. More diluent makes the solution less concentrated and increases the number of units needed for the same target amount.
Do 30-unit, 50-unit, and 100-unit U-100 syringes use different math?
No. All U-100 syringes use 100 units per millilitre. Their markings represent the same volume per unit. The smaller syringes simply hold less total liquid.
What should I do if the calculated draw exceeds the syringe capacity?
Do not treat an over-capacity result as an instruction to split or change the dose. Recheck every input and confirm the vial, diluent, target amount, and syringe with a qualified clinician or pharmacist.
Does PepKit recommend peptide doses?
No. The calculator leaves the target amount empty and only evaluates values you enter. It is an arithmetic and record-keeping tool, not medical advice or a dosing recommendation.