Fermi estimates: assumptions, units and a worked answer
Break the question into two or three quantities you can estimate. Multiply or divide only after checking the units.
Write an explicit assumption
For breaths in a day, choose an assumed breaths-per-minute rate, then multiply by minutes and hours. This is a mathematical exercise, not a personal medical measurement.
Round first
A rough calculation such as 50 × 25 × 2 cubic metres is easier to audit than an unexplained large number. Convert cubic metres to litres only at the end.
Compare with the explanation
A Fermi answer can depend on assumptions. Read the worked calculation after the reveal and identify which assumption caused the largest ratio error.