Abstract
- Fermi problems are about making a rough estimate when exact data is unknown.
- The trick is to break big questions into smaller factors we can guess.
- By working in powers of 10 and combining factors, over- and under-estimates usually balance out, leading to a reasonable answer.a
Fermi Anchors
- Some common numbers to act as entry points for making a reasonable estimation
Time / trading
- Seconds/day ≈ 86,400 (≈10⁵)
- Minutes/day ≈ 1,440
- Trading days/year ≈ 252 (≈250)
- Weeks/year ≈ 52
- Work hours/year (1 FTE) ≈ 2,000
Scale of people & money
- World pop ≈ 8×10⁹
- US pop ≈ 3.3×10⁸
- Chicago (city) ≈ 2.7×10⁶
- NYC ≈ 8–9×10⁶
- US GDP ≈ 2.5×10¹³ USD
- World GDP ≈ 1×10¹⁴ USD
Physics-lite
- g ≈ 10 m/s²
- c ≈ 3×10⁸ m/s
- atm ≈ 10⁵ Pa
- Earth radius ≈ 6.4×10³ km, circumference ≈ 4×10⁴ km
Everyday
- Walking speed ≈ 1.4 m/s (≈5 km/h)
- 1 L water ≈ 1 kg
- Byte math: 1 GB ≈ 10⁹ bytes
Logs / constants (mental math)
- π≈3, e≈2.7, ln(10)≈2.3, log₂10≈3.32, ln2≈0.693
Estimation
- Answer with an interval [low, high]. You score when the truth lands inside; narrower intervals score better if they still cover the truth
- Strategy beats guessing: start wider when unsure, then tighten as your decomposition stabilizes. Match the confidence level requested (often ~90–95%)