Converting mg, µg and mL: the Basic Calculations of a Peptide Solution
Milligrams, micrograms, millilitres and concentration: the step-by-step method to go from a mass to a volume without a factor-of-1,000 error.
Contents
Most handling errors on a peptide solution do not come from a clumsy gesture but from a misread unit. Milligrams, micrograms, millilitres: three units, two quantities (mass and volume), and one link between them, the concentration. Here is the method, step by step.
Mass units
- 1 mg (milligram) = 1,000 µg (micrograms).
- The microgram is written µg or mcg: same unit.
- A "10 mg" vial therefore contains 10,000 µg of peptide.
Confusing mg and µg means being wrong by a factor of 1,000: the most serious and most frequent error.
Volume units
- 1 mL (millilitre) = 1,000 µL (microlitres).
- On a U-100 syringe, 1 unit = 0.01 mL (see reading a U-100 syringe).
Concentration, the link between mass and volume
After reconstitution, the concentration is calculated as follows:
Concentration (mg/mL) = vial mass (mg) ÷ solvent volume (mL)
Example: 10 mg dissolved in 2 mL gives 5 mg/mL, i.e. 5,000 µg/mL.
From a mass to a volume
Volume (mL) = target mass ÷ concentration
Example with the previous solution (5 mg/mL): for 0.25 mg (250 µg), the volume is 0.25 ÷ 5 = 0.05 mL, i.e. 5 units on a U-100 syringe.
A four-step method
1. Write down the vial mass and its unit. 2. Write down the volume of solvent added, in mL. 3. Calculate the concentration, keeping the units in the calculation. 4. Convert the target mass into the same unit as the concentration before dividing.
Keeping the units on every line of the calculation is enough to avoid most errors: if the result is not in mL, a conversion has been missed.
Checking your result
- A surprising order of magnitude must lead you to redo the calculation, never to correct it "by eye".
- Our reconstitution calculator applies exactly these formulas: it serves as a double check, not as a recommendation.
- The full logic of choosing the solvent volume is in our article on the methodology of dilution calculation.
To learn more
- HPLC purity: how to read a certificate of analysis (it states the quantity actually measured in the vial)
Research Use Only (RUO). These calculations concern the preparation of solutions in a research laboratory. They constitute neither a recommendation for use in humans or animals nor a dosing indication.
