Name the quantity before calculating
Mass, volume and concentration are different quantities. A vial label in milligrams describes a mass basis; millilitres describe volume; a concentration expresses an amount relative to volume. A calculation is only interpretable when its inputs and units are clear.
IUPAC distinguishes amount of substance from mass, and uses amount concentration for amount of substance per volume. Converting a mass concentration to a molar concentration requires the appropriate molar mass. IUPAC Gold Book — Amount of substance. IUPAC Gold Book — Amount-of-substance concentration.
A compact unit reference
| Symbol | Quantity | Relationship |
|---|---|---|
| mg | Mass | 1 mg = 1,000 µg |
| µg | Mass | 1 µg = 0.001 mg |
| mL | Volume | 1 mL = 0.001 L |
| mg/mL | Mass concentration | Mass divided by final solution volume |
| µM | Amount concentration | 1 µM = 1 µmol/L |
These relationships do not specify an experimental concentration or a human dose. This page is a units reference for laboratory records, not an instruction to prepare, inject or administer any product.
Fictional arithmetic examples
If a teaching example contains 6 mg of a defined solute in a final volume of 3 mL, the nominal mass concentration is 2 mg/mL. The important phrase is final volume: record the quantity your procedure actually uses rather than assuming a volume from an informal description.
For a second fictional example, a defined compound with molar mass 2,000 g/mol at 1 mg/mL corresponds to 1 g/L ÷ 2,000 g/mol = 0.0005 mol/L, or 500 µM. This is dimensional arithmetic, not a recommended concentration. The compound is hypothetical and does not represent a specific catalogue product.
Check the inputs before trusting the answer
- Does the mass mean nominal labelled mass, measured content or another defined quantity?
- Is the final volume documented in the intended units?
- Is the molar mass appropriate to the stated identity/form?
- Have micrograms and milligrams been kept distinct?
- Is the answer labelled as nominal or measured?
A precise calculator cannot repair ambiguous input. If net content or identity is not established, carry that uncertainty into the record instead of hiding it behind several decimal places. See purity versus net content for a related distinction.
A useful calculation record
Keep the starting values, units, formula, result and source of each input. Add the operator, date and procedure reference where your laboratory requires them. A second reader should be able to reproduce the number without asking what an unlabeled “5” meant.
For a spreadsheet, place values and units in labelled columns. Avoid combining all information in a single free-text cell. Keep the calculation separate from the acceptance decision: obtaining a numerical result does not establish that the material or concentration is suitable for an experiment.
Interpret calculators as tools, not protocols
Use an approved method to decide what needs calculating. This guide does not select solvents, prescribe preparation volumes, assign stability or advise on administration. The sample-record template helps keep the arithmetic attached to the right sample and batch.
Sources and scope
Sources support the specific concepts cited above; they do not endorse this shop or certify its stock. Check the original documentation for your material and laboratory purpose. About this library and its limitations.
A FEW MORE ANSWERS
Common questions
Are mg and mL interchangeable?
No. One measures mass and the other volume. A concentration relationship is needed to connect them.
Can I convert mg/mL to µM without knowing the compound?
You need the appropriate molar mass and a defined basis for the mass concentration.