Mass, Volume, and Concentration Math for Laboratory Records
Laboratory concentration math is easier to audit when mass, liquid volume, concentration, and aliquot volume are kept as separate quantities with explicit units.
Scope: These are neutral unit-conversion formulas for non-clinical laboratory records. They do not select a target amount, recommend a protocol, or provide human-use instructions.
Keep the four quantities separate
- Total mass — for example, milligrams (mg)
- Total liquid volume — for example, milliliters (mL)
- Concentration — mass divided by volume, such as mg/mL
- Aliquot volume — a measured portion of the total liquid, such as microliters (µL)
Core formulas
Concentration (mg/mL) = total mass (mg) ÷ total volume (mL)
Aliquot volume (mL) = selected aliquot mass (mg) ÷ concentration (mg/mL)
Microliters (µL) = milliliters (mL) × 1,000
Worked laboratory example
If a record lists 10 mg total mass in 4 mL total liquid volume, the concentration is 2.5 mg/mL. If an approved laboratory plan calls for a 0.25 mg analytical aliquot, the corresponding volume is 0.1 mL, or 100 µL. This arithmetic verifies a user-supplied plan; it does not choose the amount.
Common recordkeeping errors
- Converting mg to µg without recording the conversion factor
- Confusing total container volume with aliquot volume
- Dropping units from intermediate steps
- Using more significant figures than the measurements support
- Assuming the labeled mass has been independently verified
Audit checklist
Write the original values, units, conversion factors, formula, unrounded result, final rounded result, instrument used, operator, and date. A second reviewer should be able to reproduce the result from the record alone.
Reference
Peptide Simplified editorial team. Last reviewed July 29, 2026.