Write the unit beside the number
A number without a unit is incomplete. Milligrams and micrograms both describe mass, while milliliters describe volume. Keep the unit visible in notes, spreadsheets and calculator inputs.
Convert before comparing
One milligram equals 1,000 micrograms. For example, 2 mg equals 2,000 mcg. The symbols look similar, but overlooking the prefix creates a thousandfold difference. NIST's metric-prefix reference explains the factors represented by milli and micro.
Check a calculation independently
Write the relationship before entering numbers. If a hypothetical sample contains 10 mg in 2 mL, mass divided by volume gives 5 mg/mL. Repeating the calculation in micrograms should give an equivalent result.
Know what the result means
This arithmetic establishes a numerical relationship only. It does not establish identity, stability, compatibility or an appropriate use. Use a second check whenever an input has been copied from another source.
Source
NIST, Metric (SI) Prefixes: https://www.nist.gov/pml/owm/metric-si-prefixes