MEPX
Chapter 6 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 6 of 10

Estimating percentages

Discounts, tips and growth.

Anchor on ten

Ten percent is a decimal shift; everything else is built from it. Fifteen percent is ten plus half of ten, and twenty is ten doubled.

  • A discount of 30 percent leaves 70 percent, which is often the easier calculation.
  • Two successive 20 percent cuts are 36 percent off, not 40.

Growth over time

The rule of 70: divide 70 by the percentage growth to get the doubling time. Seven percent a year doubles in a decade, which is worth knowing for savings and for costs.