MEPX
Chapter 4 of 10All chapters
  1. 1Why estimate
  2. 2Anchors worth knowing
  3. 3Fermi estimates
  4. 4Rounding well
  5. 5Sanity checks
  6. 6Estimating percentages
  7. 7Estimating time
  8. 8Making big numbers real
  9. 9Where estimates go wrong
  10. 10Getting better

Chapter 4 of 10

Rounding well

Simplify without drifting.

One significant figure

Round every number to a single digit and do the arithmetic in your head. 312 times 48 becomes 300 times 50, which is 15,000 against a true 14,976.

  • Round one factor up and the other down to keep the error small.
  • Note which way you rounded so you know if the answer is high or low.

Powers of ten

Track the zeros separately from the digits. Thinking in 3 times ten to the eighth removes most large-number mistakes.