Metric Mass Prefixes: kg, g, mg, µg, ng and pg

Peptide quantities span about nine orders of magnitude, from grams in synthesis down to picograms in analytical detection. The metric prefixes make those numbers manageable, provided each step of 1000 is counted correctly.

The prefix ladder

Going down the ladder (mg → µg), multiply by 1000. Going up (µg → mg), divide by 1000. A quick check: smaller units give bigger numbers.

Writing micrograms clearly

The SI symbol is µg, with the Greek letter mu. When µ is not available, “ug” is a common substitute in plain text, and some fields write “mcg”. All three mean the same thing. The real risk is a “µ” that turns into “m” when text is copied, printed or handwritten, silently turning micrograms into milligrams. Check the unit whenever a number has been re-typed.

• Write a leading zero: 0.5 mg, not .5 mg.
• Leave a space between the number and the unit, and do not pluralise units: 5 mg, not 5mgs.
• Prefer a unit that avoids long strings of zeros: 25 µg instead of 0.000025 g.

What a balance can weigh

A balance’s readability is not the smallest amount you can weigh well.

• Analytical balances typically read to 0.1 mg (100 µg).
• Semi-micro balances read to 0.01 mg, and microbalances to 0.001 mg (1 µg).

Relative uncertainty grows quickly near the limit. Weighing 0.3 mg on a 0.1 mg balance can easily be 30% out. Labs usually set a minimum weight for each balance from its measured repeatability. Below that, weigh more and dilute.

Common mistakes

• Converting in the wrong direction and ending up a million-fold out after two steps.
• Mixing µg/mL and mg/mL in the same calculation.
• Reading a vial label in mg as µg, or the reverse.