Concentration, reconstitution, and units
- Distinguish mass, volume, and concentration in research contexts
- Convert between milligrams and micrograms with confidence
- Understand what reconstitution does — and does not — change
- Know why activity units (IU) cannot be converted generically to mass
Three different quantities get confused constantly in peptide research: mass, volume, and concentration. Getting them straight is the foundation of every calculation on this site.
Mass: how much peptide you have
Mass is the amount of compound, measured in milligrams (mg) or micrograms (mcg). One milligram equals 1,000 micrograms — so 5 mg = 5,000 mcg. Peptide vials are sold by mass: a "5 mg vial" contains 5 mg of peptide regardless of anything else.
Volume: how much liquid you add or draw
Volume is liquid space, measured in milliliters (mL). Insulin syringes are marked in "units" — on a U-100 syringe, 100 units equals 1 mL, so each unit is 0.01 mL. Those units measure liquid volume only. They are not a universal measure of anything else, which is why our calculator asks you to pick your syringe scale explicitly.
Concentration: mass divided by volume
Concentration is how much peptide sits in each unit of liquid: mg per mL. Reconstitution — adding diluent to a lyophilized vial — changes concentration, never mass. If you add 2 mL of bacteriostatic water to a 5 mg vial:
5 mg ÷ 2 mL = 2.5 mg/mL
Add more diluent and the mass stays exactly 5 mg while the concentration falls. Every volume you draw then carries a proportional share of that mass — which is the entire basis of dose-volume calculations.
Worked example
With that 2.5 mg/mL solution, drawing 0.2 mL gives you:
2.5 mg/mL × 0.2 mL = 0.5 mg (500 mcg)
The formula is always the same: concentration × volume = mass of peptide in that volume.
A word on units that don't convert
International Units (IU) measure biological activity, not mass. There is no universal IU-to-mg conversion — it is substance-specific and requires an official reference standard for the exact compound. Any tool offering a generic IU conversion without knowing the compound is making the number up, which is why ours refuses to. If a source reports IU, look for that compound's specific conversion in the same document.
Practice this
Open the reconstitution calculator and reproduce the worked example above — the math panel shows every step, so you can check your own arithmetic against the tool's.
Key takeaways.
- Mass (mg/mcg) is amount of compound; volume (mL) is liquid space; concentration (mg/mL) is mass per volume.
- Reconstitution changes concentration, never the total mass of peptide in the vial.
- Syringe "units" measure liquid volume on a specific syringe scale — they are not universal.
- IU measures biological activity and has no universal conversion to mass; generic IU converters are making numbers up.
Check yourself.
4 questions. Answers are checked right here in your browser — retake as many times as you like.
1. Adding more diluent to a reconstituted vial changes:
2. 1 mg equals:
3. A 5 mg vial reconstituted with 1 mL of diluent has a concentration of:
4. Why won't a reputable calculator convert IU to mg generically?